void gfrr_c ( ConstSpiceChar * target,
ConstSpiceChar * abcorr,
ConstSpiceChar * obsrvr,
ConstSpiceChar * relate,
SpiceDouble refval,
SpiceDouble adjust,
SpiceDouble step,
SpiceInt nintvls,
SpiceCell * cnfine,
SpiceCell * result )
/*
-Brief_I/O
Variable I/O Description
-------- --- --------------------------------------------------
SPICE_GF_CNVTOL P Convergence tolerance
target I Name of the target body.
abcorr I Aberration correction flag.
obsrvr I Name of the observing body.
relate I Relational operator.
refval I Reference value.
adjust I Adjustment value for absolute extrema searches.
step I Step size used for locating extrema and roots.
nintvls I Workspace window interval count.
cnfine I-O SPICE window to which the search is confined.
result O SPICE window containing results.
-Detailed_Input
target is the name of a target body. The target body is
an ephemeris object; its trajectory is given by
SPK data.
The string `target' is case-insensitive, and leading
and trailing blanks in `target' are not significant.
Optionally, you may supply a string containing the
integer ID code for the object. For example both
"MOON" and "301" are legitimate strings that indicate
the Moon is the target body.
The target and observer define a position vector which
points from the observer to the target; the time derivative
length of this vector is the "range rate" that serves as
the subject of the search performed by this routine.
abcorr indicates the aberration corrections to be applied to
the observer-target state vector to account for
one-way light time and stellar aberration.
Any aberration correction accepted by the SPICE
routine spkezr_c is accepted here. See the header
of spkezr_c for a detailed description of the
aberration correction options. For convenience,
the options are listed below:
"NONE" Apply no correction.
"LT" "Reception" case: correct for
one-way light time using a Newtonian
formulation.
"LT+S" "Reception" case: correct for
one-way light time and stellar
aberration using a Newtonian
formulation.
"CN" "Reception" case: converged
Newtonian light time correction.
"CN+S" "Reception" case: converged
Newtonian light time and stellar
aberration corrections.
"XLT" "Transmission" case: correct for
one-way light time using a Newtonian
formulation.
"XLT+S" "Transmission" case: correct for
one-way light time and stellar
aberration using a Newtonian
formulation.
"XCN" "Transmission" case: converged
Newtonian light time correction.
"XCN+S" "Transmission" case: converged
Newtonian light time and stellar
aberration corrections.
Case and blanks are not significant in the string
`abcorr'.
obsrvr is the name of the observing body. The observing body is
an ephemeris object; its trajectory is given by SPK
data. `obsrvr' is case-insensitive, and leading and
trailing blanks in `obsrvr' are not significant.
Optionally, you may supply a string containing the
integer ID code for the object. For example both "MOON"
and "301" are legitimate strings that indicate the Moon
is the observer.
relate is a relational operator used to define a constraint
on observer-target range rate. The result window found
by this routine indicates the time intervals where
the constraint is satisfied. Supported values of
`relate' and corresponding meanings are shown below:
">" Distance is greater than the reference
value `refval'.
"=" Distance is equal to the reference
value `refval'.
"<" Distance is less than the reference
value `refval'.
"ABSMAX" Distance is at an absolute maximum.
"ABSMIN" Distance is at an absolute minimum.
"LOCMAX" Distance is at a local maximum.
"LOCMIN" Distance is at a local minimum.
The caller may indicate that the region of interest
is the set of time intervals where the quantity is
within a specified distance of an absolute extremum.
The argument `adjust' (described below) is used to
specify this distance.
Local extrema are considered to exist only in the
interiors of the intervals comprising the confinement
window: a local extremum cannot exist at a boundary
point of the confinement window.
Case is not significant in the string `relate'.
refval is the reference value used together with the argument
`relate' to define an equality or inequality to be
satisfied by the range rate between the specified target
and observer. See the discussion of `relate' above for
further information.
The units of `refval' are km/sec.
adjust is a parameter used to modify searches for absolute
extrema: when `relate' is set to "ABSMAX" or "ABSMIN" and
`adjust' is set to a positive value, gfdist_c will find
times when the observer-target range rate is within
`adjust' km/sec of the specified extreme value.
If `adjust' is non-zero and a search for an absolute
minimum `min' is performed, the result window contains
time intervals when the observer-target range rate has
values between `min' and min+adjust.
If the search is for an absolute maximum `max', the
corresponding range is from max-adjust to `max'.
`adjust' is not used for searches for local extrema,
equality or inequality conditions.
step is the step size to be used in the search. `step' must
be short enough for a search using this step size
to locate the time intervals where the specified
range rate function is monotone increasing or
decreasing. However, `step' must not be *too* short, or
the search will take an unreasonable amount of time.
The choice of `step' affects the completeness but not
the precision of solutions found by this routine; the
precision is controlled by the convergence tolerance.
See the discussion of the parameter SPICE_GF_CNVTOL for
details.
`step' has units of TDB seconds.
nintvls is a parameter specifying the number of intervals that
can be accommodated by each of the dynamically allocated
windows used internally by this routine. `nintvls' should
be at least as large as the number of intervals within
the search region on which the specified range rate
function is monotone increasing or decreasing. See
the Examples section below for code examples illustrating
the use of this parameter.
cnfine is a SPICE window that confines the time period over
which the specified search is conducted. `cnfine' may
consist of a single interval or a collection of
intervals.
In some cases the confinement window can be used to
greatly reduce the time period that must be searched
for the desired solution. See the Particulars section
below for further discussion.
See the Examples section below for a code example
that shows how to create a confinement window.
-Detailed_Output
cnfine is the input confinement window, updated if necessary
so the control area of its data array indicates the
window's size and cardinality. The window data are
unchanged.
result is the window of intervals, contained within the
confinement window `cnfine', on which the specified
constraint is satisfied.
If `result' is non-empty on input, its contents will be
discarded before 'gfrr_c' conducts its search.
`result' must be declared with sufficient size to capture
the full set of time intervals within the search region
on which the specified constraint is satisfied.
If the search is for local extrema, or for absolute
extrema with `adjust' set to zero, then normally each
interval of `result' will be a singleton: the left and
right endpoints of each interval will be identical.
If no times within the confinement window satisfy the
constraint, `result' will be returned with a cardinality
of zero.
-Parameters
SPICE_GF_CNVTOL
is the convergence tolerance used for finding endpoints
of the intervals comprising the result window.
SPICE_GF_CNVTOL is used to determine when binary searches
for roots should terminate: when a root is bracketed
within an interval of length SPICE_GF_CNVTOL, the root is
considered to have been found.
The accuracy, as opposed to precision, of roots found
by this routine depends on the accuracy of the input
data. In most cases, the accuracy of solutions will be
inferior to their precision.
SPICE_GF_CNVTOL is declared in the header file SpiceGF.h.
-Exceptions
1) In order for this routine to produce correct results,
the step size must be appropriate for the problem at hand.
Step sizes that are too large may cause this routine to miss
roots; step sizes that are too small may cause this routine
to run unacceptably slowly and in some cases, find spurious
roots.
This routine does not diagnose invalid step sizes, except
that if the step size is non-positive, an error is signaled
by a routine in the call tree of this routine.
2) Due to numerical errors, in particular,
- Truncation error in time values
- Finite tolerance value
- Errors in computed geometric quantities
it is *normal* for the condition of interest to not always be
satisfied near the endpoints of the intervals comprising the
result window.
The result window may need to be contracted slightly by the
caller to achieve desired results. The SPICE window routine
wncond_c can be used to contract the result window.
3) If an error (typically cell overflow) occurs while performing
window arithmetic, the error will be diagnosed by a routine
in the call tree of this routine.
4) If the relational operator `relate' is not recognized, an
error is signaled by a routine in the call tree of this
routine.
5) If the aberration correction specifier contains an
unrecognized value, an error is signaled by a routine in the
call tree of this routine.
6) If 'adjust' is negative, the error SPICE(VALUEOUTOFRANGE) will
signal from a routine in the call tree of this routine.
A non-zero value for 'adjust' when 'relate' has any value other than
"ABSMIN" or "ABSMAX" causes the error SPICE(INVALIDVALUE) to
signal from a routine in the call tree of this routine.
7) If either of the input body names do not map to NAIF ID
codes, an error is signaled by a routine in the call tree of
this routine.
8) If required ephemerides or other kernel data are not
available, an error is signaled by a routine in the call tree
of this routine.
9) If the workspace interval count is less than 1, the error
SPICE(VALUEOUTOFRANGE) will be signaled.
10) If the required amount of workspace memory cannot be
allocated, the error SPICE(MALLOCFAILURE) will be
signaled.
11) If any input string argument pointer is null, the error
SPICE(NULLPOINTER) will be signaled.
12) If any input string argument is empty, the error
SPICE(EMPTYSTRING) will be signaled.
13) If either input cell has type other than SpiceDouble,
the error SPICE(TYPEMISMATCH) is signaled.
-Files
Appropriate kernels must be loaded by the calling program before
this routine is called.
The following data are required:
- SPK data: ephemeris data for target and observer for the
time period defined by the confinement window must be
loaded. If aberration corrections are used, the states of
target and observer relative to the solar system barycenter
must be calculable from the available ephemeris data.
Typically ephemeris data are made available by loading one
or more SPK files via furnsh_c.
In all cases, kernel data are normally loaded once per program
run, NOT every time this routine is called.
-Particulars
This routine determines if the caller-specified constraint condition
on the geometric event (range rate) is satisfied for any time intervals
within the confinement window 'cnfine'. If one or more such time
intervals exist, those intervals are added to the 'result' window.
This routine provides a simpler, but less flexible interface
than does the routine gfevnt_c for conducting the searches for
observer-target range rate value events. Applications that require
support for progress reporting, interrupt handling, non-default step
or refinement functions, or non-default convergence tolerance should
call gfevnt_c rather than this routine.
Below we discuss in greater detail aspects of this routine's
solution process that are relevant to correct and efficient
use of this routine in user applications.
The Search Process
==================
Regardless of the type of constraint selected by the caller, this
routine starts the search for solutions by determining the time
periods, within the confinement window, over which the specified
range rate function is monotone increasing and monotone decreasing.
Each of these time periods is represented by a SPICE window. Having
found these windows, all of the range rate function's local extrema
within the confinement window are known. Absolute extrema then can
be found very easily.
Within any interval of these "monotone" windows, there will be at
most one solution of any equality constraint. Since the boundary
of the solution set for any inequality constraint is contained in
the union of
- the set of points where an equality constraint is met
- the boundary points of the confinement window
the solutions of both equality and inequality constraints can be
found easily once the monotone windows have been found.
Step Size
=========
The monotone windows (described above) are found via a two-step
search process. Each interval of the confinement window is
searched as follows: first, the input step size is used to
determine the time separation at which the sign of the rate of
change of range rate will be sampled. Starting at
the left endpoint of an interval, samples will be taken at each
step. If a change of sign is found, a root has been bracketed; at
that point, the time at which the range rate is zero can be
found by a refinement process, for example, via binary search.
Note that the optimal choice of step size depends on the lengths
of the intervals over which the range rate function is monotone:
the step size should be shorter than the shortest of these
intervals (within the confinement window).
The optimal step size is *not* necessarily related to the lengths
of the intervals comprising the result window. For example, if
the shortest monotone interval has length 10 days, and if the
shortest result window interval has length 5 minutes, a step size
of 9.9 days is still adequate to find all of the intervals in the
result window. In situations like this, the technique of using
monotone windows yields a dramatic efficiency improvement over a
state-based search that simply tests at each step whether the
specified constraint is satisfied. The latter type of search can
miss solution intervals if the step size is longer than the
shortest solution interval.
Having some knowledge of the relative geometry of the target and
observer can be a valuable aid in picking a reasonable step size.
In general, the user can compensate for lack of such knowledge by
picking a very short step size; the cost is increased computation
time.
Note that the step size is not related to the precision with which
the endpoints of the intervals of the result window are computed.
That precision level is controlled by the convergence tolerance.
Convergence Tolerance
=====================
As described above, the root-finding process used by this routine
involves first bracketing roots and then using a search process to
locate them. "Roots" include times when extrema are attained and
times when the geometric quantity function is equal to a reference
value or adjusted extremum. All endpoints of the intervals comprising
the result window are either endpoints of intervals of the confinement
window or roots.
Once a root has been bracketed, a refinement process is used to
narrow down the time interval within which the root must lie.
This refinement process terminates when the location of the root
has been determined to within an error margin called the
"convergence tolerance." The convergence tolerance used by this
routine is set via the parameter SPICE_GF_CNVTOL.
The value of SPICE_GF_CNVTOL is set to a "tight" value so that the
tolerance doesn't limit the accuracy of solutions found by this
routine. In general the accuracy of input data will be the limiting
factor.
The user may change the convergence tolerance from the default
SPICE_GF_CNVTOL value by calling the routine gfstol_c, e.g.
gfstol_c( tolerance value in seconds )
Call gfstol_c prior to calling this routine. All subsequent
searches will use the updated tolerance value.
Searches over time windows of long duration may require use of
larger tolerance values than the default: the tolerance must be
large enough so that it, when added to or subtracted from the
confinement window's lower and upper bounds, yields distinct time
values.
Setting the tolerance tighter than SPICE_GF_CNVTOL is unlikely to be
useful, since the results are unlikely to be more accurate.
Making the tolerance looser will speed up searches somewhat,
since a few convergence steps will be omitted. However, in most
cases, the step size is likely to have a much greater effect
on processing time than would the convergence tolerance.
The Confinement Window
======================
The simplest use of the confinement window is to specify a time
interval within which a solution is sought. However, the
confinement window can, in some cases, be used to make searches
more efficient. Sometimes it's possible to do an efficient search
to reduce the size of the time period over which a relatively
slow search of interest must be performed.
Consider the following example: suppose one wishes to find the
times when the range rate between Io and the Earth attains a global
minimum over some (lengthy) time interval. There is one local
minimum every few days. The required step size for this search
must be smaller than the shortest interval on which the range rate
is monotone increasing or decreasing; this step size will be less
than half the average time between local minima. However, we know
that a global minimum can't occur when the Jupiter-Sun-Earth
angle is greater than 90 degrees. We can use a step size of a
half year to find the time period, within our original time
interval, during which this angle is less than 90 degrees; this
time period becomes the confinement window for our Earth-Io
range rate search. This way we've used a quick (due to the large
step size) search to cut out about half of the search period over
which we must perform a slower search using a small step size.
-Examples
The numerical results shown for these examples may differ across
platforms. The results depend on the SPICE kernels used as
input, the compiler and supporting libraries, and the machine
specific arithmetic implementation.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: standard.tm
This meta-kernel is intended to support operation of SPICE
example programs. The kernels shown here should not be
assumed to contain adequate or correct versions of data
required by SPICE-based user applications.
In order for an application to use this meta-kernel, the
kernels referenced here must be present in the user's
current working directory.
The names and contents of the kernels referenced
by this meta-kernel are as follows:
File name Contents
--------- --------
de421.bsp Planetary ephemeris
pck00009.tpc Planet orientation and
radii
naif0009.tls Leapseconds
\begindata
KERNELS_TO_LOAD = ( 'de421.bsp',
'pck00009.tpc',
'naif0009.tls' )
\begintext
Example:
Determine the time windows from January 1, 2007 UTC to
April 1, 2007 UTC for which the sun-moon range rate satisfies the
relation conditions with respect to a reference value of
0.3365 km/s radians (this range rate known to occur within the
search interval). Also determine the time windows corresponding
to the local maximum and minimum range rate, and the absolute
maximum and minimum range rate during the search interval.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "SpiceUsr.h"
#define MAXWIN 20000
#define TIMFMT "YYYY-MON-DD HR:MN:SC.###"
#define TIMLEN 41
#define NLOOPS 7
int main( int argc, char **argv )
{
/.
Create the needed windows. Note, one window
consists of two values, so the total number
of cell values to allocate is twice
the number of intervals.
./
SPICEDOUBLE_CELL ( result, 2*MAXWIN );
SPICEDOUBLE_CELL ( cnfine, 2 );
SpiceDouble begtim;
SpiceDouble endtim;
SpiceDouble step;
SpiceDouble adjust;
SpiceDouble refval;
SpiceDouble beg;
SpiceDouble end;
SpiceChar begstr [ TIMLEN ];
SpiceChar endstr [ TIMLEN ];
SpiceChar * target = "MOON";
SpiceChar * abcorr = "NONE";
SpiceChar * obsrvr = "SUN";
SpiceInt count;
SpiceInt i;
SpiceInt j;
ConstSpiceChar * relate [NLOOPS] = { "=",
"<",
">",
"LOCMIN",
"ABSMIN",
"LOCMAX",
"ABSMAX",
};
/.
Load kernels.
./
furnsh_c( "standard.tm" );
/.
Store the time bounds of our search interval in
the cnfine confinement window.
./
str2et_c( "2007 JAN 01", &begtim );
str2et_c( "2007 APR 01", &endtim );
wninsd_c ( begtim, endtim, &cnfine );
/.
Search using a step size of 1 day (in units of seconds).
The reference value is .3365 km/s. We're not using the
adjustment feature, so we set 'adjust' to zero.
./
step = spd_c();
adjust = 0.;
refval = .3365;
for ( j = 0; j < NLOOPS; j++ )
{
printf ( "Relation condition: %s \n", relate[j] );
/.
Perform the search. The SPICE window 'result' contains
the set of times when the condition is met.
./
gfrr_c ( target,
abcorr,
obsrvr,
relate[j],
refval,
adjust,
step,
MAXWIN,
&cnfine,
&result );
count = wncard_c( &result );
/.
Display the results.
./
if (count == 0 )
{
printf ( "Result window is empty.\n\n" );
}
else
{
for ( i = 0; i < count; i++ )
{
/.
Fetch the endpoints of the Ith interval
of the result window.
./
wnfetd_c ( &result, i, &beg, &end );
timout_c ( beg, TIMFMT, TIMLEN, begstr );
timout_c ( end, TIMFMT, TIMLEN, endstr );
printf ( "Start time, drdt = %s \n", begstr );
printf ( "Stop time, drdt = %s \n", endstr );
}
}
printf("\n");
}
return( 0 );
}
The program outputs:
Relation condition: =
Start time, drdt = 2007-JAN-02 00:35:19.574
Stop time, drdt = 2007-JAN-02 00:35:19.574
Start time, drdt = 2007-JAN-19 22:04:54.899
Stop time, drdt = 2007-JAN-19 22:04:54.899
Start time, drdt = 2007-FEB-01 23:30:13.428
Stop time, drdt = 2007-FEB-01 23:30:13.428
Start time, drdt = 2007-FEB-17 11:10:46.540
Stop time, drdt = 2007-FEB-17 11:10:46.540
Start time, drdt = 2007-MAR-04 15:50:19.929
Stop time, drdt = 2007-MAR-04 15:50:19.929
Start time, drdt = 2007-MAR-18 09:59:05.959
Stop time, drdt = 2007-MAR-18 09:59:05.959
Relation condition: <
Start time, drdt = 2007-JAN-02 00:35:19.574
Stop time, drdt = 2007-JAN-19 22:04:54.899
Start time, drdt = 2007-FEB-01 23:30:13.428
Stop time, drdt = 2007-FEB-17 11:10:46.540
Start time, drdt = 2007-MAR-04 15:50:19.929
Stop time, drdt = 2007-MAR-18 09:59:05.959
Relation condition: >
Start time, drdt = 2007-JAN-01 00:00:00.000
Stop time, drdt = 2007-JAN-02 00:35:19.574
Start time, drdt = 2007-JAN-19 22:04:54.899
Stop time, drdt = 2007-FEB-01 23:30:13.428
Start time, drdt = 2007-FEB-17 11:10:46.540
Stop time, drdt = 2007-MAR-04 15:50:19.929
Start time, drdt = 2007-MAR-18 09:59:05.959
Stop time, drdt = 2007-APR-01 00:00:00.000
Relation condition: LOCMIN
Start time, drdt = 2007-JAN-11 07:03:58.988
Stop time, drdt = 2007-JAN-11 07:03:58.988
Start time, drdt = 2007-FEB-10 06:26:15.439
Stop time, drdt = 2007-FEB-10 06:26:15.439
Start time, drdt = 2007-MAR-12 03:28:36.404
Stop time, drdt = 2007-MAR-12 03:28:36.404
Relation condition: ABSMIN
Start time, drdt = 2007-JAN-11 07:03:58.988
Stop time, drdt = 2007-JAN-11 07:03:58.988
Relation condition: LOCMAX
Start time, drdt = 2007-JAN-26 02:27:33.766
Stop time, drdt = 2007-JAN-26 02:27:33.766
Start time, drdt = 2007-FEB-24 09:35:07.816
Stop time, drdt = 2007-FEB-24 09:35:07.816
Start time, drdt = 2007-MAR-25 17:26:56.150
Stop time, drdt = 2007-MAR-25 17:26:56.150
Relation condition: ABSMAX
Start time, drdt = 2007-MAR-25 17:26:56.150
Stop time, drdt = 2007-MAR-25 17:26:56.150
-Restrictions
1) The kernel files to be used by this routine must be loaded
(normally using the CSPICE routine furnsh_c) before this
routine is called.
2) This routine has the side effect of re-initializing the
range rate quantity utility package. Callers may themselves
need to re-initialize the range rate quantity utility
package after calling this routine.
-Literature_References
None.
-Author_and_Institution
N.J. Bachman (JPL)
E.D. Wright (JPL)
-Version
-CSPICE Version 1.0.1, 28-FEB-2013 (NJB) (EDW)
Header was updated to discuss use of gfstol_c.
Edit to comments to correct search description.
Edits to Example section, proper description of "standard.tm"
meta kernel.
-CSPICE Version 1.0.0, 26-AUG-2009 (EDW) (NJB)
-Index_Entries
GF range rate search
-&
*/
{ /* Begin gfrr_c */
/*
Local variables
*/
doublereal * work;
static SpiceInt nw = SPICE_GF_NWRR;
SpiceInt nBytes;
/*
Participate in error tracing.
*/
chkin_c ( "gfrr_c" );
/*
Make sure cell data types are d.p.
*/
CELLTYPECHK2 ( CHK_STANDARD, "gfrr_c", SPICE_DP, cnfine, result );
/*
Initialize the input cells if necessary.
*/
CELLINIT2 ( cnfine, result );
/*
Check the input strings to make sure each pointer is non-null
and each string length is non-zero.
*/
CHKFSTR ( CHK_STANDARD, "gfrr_c", target );
CHKFSTR ( CHK_STANDARD, "gfrr_c", abcorr );
CHKFSTR ( CHK_STANDARD, "gfrr_c", obsrvr );
CHKFSTR ( CHK_STANDARD, "gfrr_c", relate );
/*
Check the workspace size; some mallocs have a violent
dislike for negative allocation amounts. To be safe,
rule out a count of zero intervals as well.
*/
if ( nintvls < 1 )
{
setmsg_c ( "The specified workspace interval count # was "
"less than the minimum allowed value of one (1)." );
errint_c ( "#", nintvls );
sigerr_c ( "SPICE(VALUEOUTOFRANGE)" );
chkout_c ( "gfrr_c" );
return;
}
/*
Allocate the workspace. 'nintvls' indicates the maximum number of
intervals returned in 'result'. An interval consists of
two values.
*/
nintvls = 2 * nintvls;
nBytes = ( nintvls + SPICE_CELL_CTRLSZ ) * nw * sizeof(SpiceDouble);
work = (doublereal *) alloc_SpiceMemory( nBytes );
if ( !work )
{
setmsg_c ( "Workspace allocation of # bytes failed due to "
"malloc failure" );
errint_c ( "#", nBytes );
sigerr_c ( "SPICE(MALLOCFAILED)" );
chkout_c ( "gfrr_c" );
return;
}
/*
Let the f2'd routine do the work.
*/
gfrr_( ( char * ) target,
( char * ) abcorr,
( char * ) obsrvr,
( char * ) relate,
( doublereal * ) &refval,
( doublereal * ) &adjust,
( doublereal * ) &step,
( doublereal * ) (cnfine->base),
( integer * ) &nintvls,
( integer * ) &nw,
( doublereal * ) work,
( doublereal * ) (result->base),
( ftnlen ) strlen(target),
( ftnlen ) strlen(abcorr),
( ftnlen ) strlen(obsrvr),
( ftnlen ) strlen(relate) );
/*
De-allocate the workspace.
*/
free_SpiceMemory( work );
/*
Sync the output cell.
*/
if ( !failed_c() )
{
zzsynccl_c ( F2C, result ) ;
}
ALLOC_CHECK;
chkout_c ( "gfrr_c" );
} /* End gfrr_c */
void gfuds_c ( void ( * udfunc ) ( SpiceDouble et,
SpiceDouble * value ),
void ( * udqdec ) ( void ( * udfunc )
( SpiceDouble et,
SpiceDouble * value ),
SpiceDouble et,
SpiceBoolean * isdecr ),
ConstSpiceChar * relate,
SpiceDouble refval,
SpiceDouble adjust,
SpiceDouble step,
SpiceInt nintvls,
SpiceCell * cnfine,
SpiceCell * result )
/*
-Brief_I/O
VARIABLE I/O DESCRIPTION
-------- --- --------------------------------------------------
udfunc I Name of the routine that computes the scalar value
of interest at some time.
udqdec I Name of the routine that computes whether the
current state is decreasing.
relate I Operator that either looks for an extreme value
(max, min, local, absolute) or compares the
geometric quantity value and a number.
refval I Value used as reference for geometric quantity
condition.
adjust I Allowed variation for absolute extremal
geometric conditions.
step I Step size used for locating extrema and roots.
nintvls I Workspace window interval count
cnfine I-O SPICE window to which the search is restricted.
result O SPICE window containing results.
-Detailed_Input
udfunc the name of the external routine that returns the
value of the scalar quantity of interest at time ET.
The calling sequence for "udfunc" is:
udfunc ( et, &value )
where:
et an input double precision value
representing the TDB ephemeris seconds time
at which to determine the scalar value.
value is the value of the geometric quantity
at 'et'.
udqdec the name of the external routine that determines if
the scalar quantity calculated by "udfunc" is decreasing.
The calling sequence:
udqdec ( et, &isdecr )
where:
et an input double precision value representing
the TDB ephemeris seconds time at at which
to determine the time derivative of 'udfunc'.
isdecr a logical variable indicating whether
or not the scalar value returned by udfunc
is decreasing. 'isdecr' returns true if the
time derivative of "udfunc" at 'et' is negative.
relate the scalar string comparison operator indicating
the numeric constraint of interest. Values are:
">" value of scalar quantity greater than some
reference (refval).
"=" value of scalar quantity equal to some
reference (refval).
"<" value of scalar quantity less than some
reference (refval).
"ABSMAX" The scalar quantity is at an absolute
maximum.
"ABSMIN" The scalar quantity is at an absolute
minimum.
"LOCMAX" The scalar quantity is at a local
maximum.
"LOCMIN" The scalar quantity is at a local
minimum.
The caller may indicate that the region of interest
is the set of time intervals where the quantity is
within a specified distance of an absolute extremum.
The argument 'adjust' (described below) is used to
specified this distance.
Local extrema are considered to exist only in the
interiors of the intervals comprising the confinement
window: a local extremum cannot exist at a boundary
point of the confinement window.
relate is insensitive to case, leading and
trailing blanks.
refval is the reference value used to define an equality or
inequality to satisfied by the scalar quantity.
The units of refval are those of the scalar quantity.
adjust the amount by which the quantity is allowed to vary
from an absolute extremum.
If the search is for an absolute minimum is performed,
the resulting window contains time intervals when the
geometric quantity value has values between ABSMIN and
ABSMIN + adjust.
If the search is for an absolute maximum, the
corresponding range is between ABSMAX - adjust and
ABSMAX.
'adjust' is not used for searches for local extrema,
equality or inequality conditions and must have value
zero for such searches.
step the double precision time step size to use in
the search.
'step' must be short enough to for a search using this
step size to locate the time intervals where the
scalar quantity function is monotone increasing or
decreasing. However, 'step' must not be *too* short,
or the search will take an
The choice of 'step' affects the completeness but not
the precision of solutions found by this routine; the
precision is controlled by the convergence tolerance.
See the discussion of the parameter SPICE_GF_CNVTOL for
details.
'step' has units of TDB seconds.
nintvls an integer value specifying the number of intervals in the
the internal workspace array used by this routine. 'nintvls'
should be at least as large as the number of intervals
within the search region on which the specified observer-target
vector coordinate function is monotone increasing or decreasing.
It does no harm to pick a value of 'nintvls' larger than the
minimum required to execute the specified search, but if chosen
too small, the search will fail.
cnfine a double precision SPICE window that confines the time
period over which the specified search is conducted.
cnfine may consist of a single interval or a collection
of intervals.
In some cases the confinement window can be used to
greatly reduce the time period that must be searched
for the desired solution. See the Particulars section
below for further discussion.
See the Examples section below for a code example
that shows how to create a confinement window.
-Detailed_Output
cnfine is the input confinement window, updated if necessary
so the control area of its data array indicates the
window's size and cardinality. The window data are
unchanged.
result is a SPICE window representing the set of time
intervals, within the confinement period, when the
specified geometric event occurs.
If `result' is non-empty on input, its contents
will be discarded before gfuds_c conducts its
search.
-Parameters
None.
-Exceptions
1) In order for this routine to produce correct results,
the step size must be appropriate for the problem at hand.
Step sizes that are too large may cause this routine to miss
roots; step sizes that are too small may cause this routine
to run unacceptably slowly and in some cases, find spurious
roots.
This routine does not diagnose invalid step sizes, except
that if the step size is non-positive, an error is signaled
by a routine in the call tree of this routine.
2) Due to numerical errors, in particular,
- Truncation error in time values
- Finite tolerance value
- Errors in computed geometric quantities
it is *normal* for the condition of interest to not always be
satisfied near the endpoints of the intervals comprising the
result window.
The result window may need to be contracted slightly by the
caller to achieve desired results. The SPICE window routine
wncond_c can be used to contract the result window.
3) If an error (typically cell overflow) occurs while performing
window arithmetic, the error will be diagnosed by a routine
in the call tree of this routine.
4) If the relational operator `relate' is not recognized, an
error is signaled by a routine in the call tree of this
routine.
5) If 'adjust' is negative, the error SPICE(VALUEOUTOFRANGE) will
signal from a routine in the call tree of this routine.
A non-zero value for 'adjust' when 'relate' has any value other than
"ABSMIN" or "ABSMAX" causes the error SPICE(INVALIDVALUE) to
signal from a routine in the call tree of this routine.
6) If required ephemerides or other kernel data are not
available, an error is signaled by a routine in the call tree
of this routine.
7) If the workspace interval count is less than 1, the error
SPICE(VALUEOUTOFRANGE) will be signaled.
8) If the required amount of workspace memory cannot be
allocated, the error SPICE(MALLOCFAILURE) will be
signaled.
9) If any input string argument pointer is null, the error
SPICE(NULLPOINTER) will be signaled.
10) If any input string argument is empty, the error
SPICE(EMPTYSTRING) will be signaled.
11) If either input cell has type other than SpiceDouble,
the error SPICE(TYPEMISMATCH) is signaled.
-Files
Appropriate kernels must be loaded by the calling program before
this routine is called.
If the scalar function requires access to ephemeris data:
- SPK data: ephemeris data for any body over the
time period defined by the confinement window must be
loaded. If aberration corrections are used, the states of
target and observer relative to the solar system barycenter
must be calculable from the available ephemeris data.
Typically ephemeris data are made available by loading one
or more SPK files via furnsh_c.
- If non-inertial reference frames are used, then PCK
files, frame kernels, C-kernels, and SCLK kernels may be
needed.
In all cases, kernel data are normally loaded once per program
run, NOT every time this routine is called.
-Particulars
This routine provides a simpler, but less flexible interface
than does the routine zzgfrel_ for conducting searches for events
corresponding to an arbitrary user defined scalar quantity
function. Applications that require support for progress
reporting, interrupt handling, non-default step or refinement
functions, or non-default convergence tolerance should call
zzgfrel_ rather than this routine.
This routine determines a set of one or more time intervals
within the confinement window when the scalar function
satisfies a caller-specified constraint. The resulting set of
intervals is returned as a SPICE window.
udqdec Default Template
=======================
The user must supply a routine to determine whether sign of the
time derivative of udfunc is positive or negative at 'et'. For
cases where udfunc is numerically well behaved, the user
may find it convenient to use a routine based on the below
template. uddc_c determines the truth of the expression
d (udfunc)
-- < 0
dt
using the library routine uddf_c to numerically calculate the
derivative of udfunc using a three-point estimation. Use
of gfdecr requires only changing the "udfunc" argument
to that of the user provided scalar function passed to gfuds_c
and defining the differential interval size, 'dt'. Please see
the Examples section for an example of gfdecr use.
void gfdecr ( SpiceDouble et, SpiceBoolean * isdecr )
{
SpiceDouble dt = h, double precision interval size;
uddc_c( udfunc, uddf_c, et, dt, isdecr );
return;
}
Below we discuss in greater detail aspects of this routine's
solution process that are relevant to correct and efficient
use of this routine in user applications.
The Search Process
==================
Regardless of the type of constraint selected by the caller, this
routine starts the search for solutions by determining the time
periods, within the confinement window, over which the specified
scalar function is monotone increasing and monotone
decreasing. Each of these time periods is represented by a SPICE
window. Having found these windows, all of the quantity
function's local extrema within the confinement window are known.
Absolute extrema then can be found very easily.
Within any interval of these "monotone" windows, there will be at
most one solution of any equality constraint. Since the boundary
of the solution set for any inequality constraint is the set
of points where an equality constraint is met, the solutions of
both equality and inequality constraints can be found easily
once the monotone windows have been found.
Step Size
=========
The monotone windows (described above) are found using a two-step
search process. Each interval of the confinement window is
searched as follows: first, the input step size is used to
determine the time separation at which the sign of the rate of
change of quantity function will be sampled. Starting at
the left endpoint of an interval, samples will be taken at each
step. If a change of sign is found, a root has been bracketed; at
that point, the time at which the time derivative of the quantity
function is zero can be found by a refinement process, for
example, using a binary search.
Note that the optimal choice of step size depends on the lengths
of the intervals over which the quantity function is monotone:
the step size should be shorter than the shortest of these
intervals (within the confinement window).
The optimal step size is *not* necessarily related to the lengths
of the intervals comprising the result window. For example, if
the shortest monotone interval has length 10 days, and if the
shortest result window interval has length 5 minutes, a step size
of 9.9 days is still adequate to find all of the intervals in the
result window. In situations like this, the technique of using
monotone windows yields a dramatic efficiency improvement over a
state-based search that simply tests at each step whether the
specified constraint is satisfied. The latter type of search can
miss solution intervals if the step size is shorter than the
shortest solution interval.
Having some knowledge of the relative geometry of the targets and
observer can be a valuable aid in picking a reasonable step size.
In general, the user can compensate for lack of such knowledge by
picking a very short step size; the cost is increased computation
time.
Note that the step size is not related to the precision with which
the endpoints of the intervals of the result window are computed.
That precision level is controlled by the convergence tolerance.
Convergence Tolerance
=====================
Once a root has been bracketed, a refinement process is used to
narrow down the time interval within which the root must lie.
This refinement process terminates when the location of the root
has been determined to within an error margin called the
"convergence tolerance." The convergence tolerance used by this
routine is set via the parameter SPICE_GF_CNVTOL.
The value of SPICE_GF_CNVTOL is set to a "tight" value so that the
tolerance doesn't become the limiting factor in the accuracy of
solutions found by this routine. In general the accuracy of input
data will be the limiting factor.
Making the tolerance tighter than SPICE_GF_CNVTOL is unlikely to
be useful, since the results are unlikely to be more accurate.
Making the tolerance looser will speed up searches somewhat,
since a few convergence steps will be omitted. However, in most
cases, the step size is likely to have a much greater affect
on processing time than would the convergence tolerance.
The Confinement Window
======================
The simplest use of the confinement window is to specify a time
interval within which a solution is sought. However, the
confinement window can, in some cases, be used to make searches
more efficient. Sometimes it's possible to do an efficient search
to reduce the size of the time period over which a relatively
slow search of interest must be performed.
-Examples
The numerical results shown for these examples may differ across
platforms. The results depend on the SPICE kernels used as
input, the compiler and supporting libraries, and the machine
specific arithmetic implementation.
Conduct a search on the range-rate of the vector from the Sun
to the Moon. Define a function to calculate the value.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: standard.tm
This meta-kernel is intended to support operation of SPICE
example programs. The kernels shown here should not be
assumed to contain adequate or correct versions of data
required by SPICE-based user applications.
In order for an application to use this meta-kernel, the
kernels referenced here must be present in the user's
current working directory.
\begindata
KERNELS_TO_LOAD = ( 'de414.bsp',
'pck00008.tpc',
'naif0009.tls' )
\begintext
Code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "SpiceUsr.h"
#include "SpiceZfc.h"
#include "SpiceZad.h"
#define MAXWIN 20000
#define TIMFMT "YYYY-MON-DD HR:MN:SC.###"
#define TIMLEN 41
#define NLOOPS 7
void gfq ( SpiceDouble et, SpiceDouble * value );
void gfdecrx ( void ( * udfunc ) ( SpiceDouble et,
SpiceDouble * value ),
SpiceDouble et,
SpiceBoolean * isdecr );
doublereal dvnorm_(doublereal *state);
int main( int argc, char **argv )
{
/.
Create the needed windows. Note, one interval
consists of two values, so the total number
of cell values to allocate is twice
the number of intervals.
./
SPICEDOUBLE_CELL ( result, 2*MAXWIN );
SPICEDOUBLE_CELL ( cnfine, 2 );
SpiceDouble begtim;
SpiceDouble endtim;
SpiceDouble step;
SpiceDouble adjust;
SpiceDouble refval;
SpiceDouble beg;
SpiceDouble end;
SpiceChar begstr [ TIMLEN ];
SpiceChar endstr [ TIMLEN ];
SpiceInt count;
SpiceInt i;
SpiceInt j;
ConstSpiceChar * relate [NLOOPS] = { "=",
"<",
">",
"LOCMIN",
"ABSMIN",
"LOCMAX",
"ABSMAX"
};
printf( "Compile date %s, %s\n\n", __DATE__, __TIME__ );
/.
Load kernels.
./
furnsh_c( "standard.tm" );
/.
Store the time bounds of our search interval in the 'cnfine'
confinement window.
./
str2et_c( "2007 JAN 01", &begtim );
str2et_c( "2007 APR 01", &endtim );
wninsd_c ( begtim, endtim, &cnfine );
/.
Search using a step size of 1 day (in units of seconds). The reference
value is .3365 km/s. We're not using the adjustment feature, so
we set 'adjust' to zero.
./
step = spd_c();
adjust = 0.;
refval = .3365;
for ( j = 0; j < NLOOPS; j++ )
{
printf ( "Relation condition: %s \n", relate[j] );
/.
Perform the search. The SPICE window 'result' contains
the set of times when the condition is met.
./
gfuds_c ( gfq,
gfdecrx,
relate[j],
refval,
adjust,
step,
MAXWIN,
&cnfine,
&result );
count = wncard_c( &result );
/.
Display the results.
./
if (count == 0 )
{
printf ( "Result window is empty.\n\n" );
}
else
{
for ( i = 0; i < count; i++ )
{
/.
Fetch the endpoints of the Ith interval
of the result window.
./
wnfetd_c ( &result, i, &beg, &end );
timout_c ( beg, TIMFMT, TIMLEN, begstr );
timout_c ( end, TIMFMT, TIMLEN, endstr );
printf ( "Start time, drdt = %s \n", begstr );
printf ( "Stop time, drdt = %s \n", endstr );
}
}
printf("\n");
}
kclear_c();
return( 0 );
}
/.
The user defined functions required by GFUDS.
gfq for udfunc
gfdecr for udqdec
./
/.
-Procedure Procedure gfq
./
void gfq ( SpiceDouble et, SpiceDouble * value )
/.
-Abstract
User defined geometric quantity function. In this case,
the range from the sun to the Moon at TDB time 'et'.
./
{
/. Initialization ./
SpiceInt targ = 301;
SpiceInt obs = 10;
SpiceChar * ref = "J2000";
SpiceChar * abcorr = "NONE";
SpiceDouble state [6];
SpiceDouble lt;
/.
Retrieve the vector from the Sun to the Moon in the J2000
frame, without aberration correction.
./
spkez_c ( targ, et, ref, abcorr, obs, state, < );
/.
Calculate the scalar range rate corresponding the
'state' vector.
./
*value = dvnorm_( state );
return;
}
/.
-Procedure gfdecrx
./
void gfdecrx ( void ( * udfunc ) ( SpiceDouble et,
SpiceDouble * value ),
SpiceDouble et,
SpiceBoolean * isdecr )
/.
-Abstract
User defined function to detect if the function derivative
is negative (the function is decreasing) at TDB time 'et'.
./
{
SpiceDouble dt = 10.;
/.
Determine if "udfunc" is decreasing at 'et'.
uddc_c - the GF function to determine if
the derivative of the user defined
function is negative at 'et'.
uddf_c - the SPICE function to numerically calculate the
derivative of 'udfunc' at 'et' for the
interval [et-dt, et+dt].
./
uddc_c( udfunc, et, dt, isdecr );
return;
}
The program outputs:
Relation condition: =
Start time, drdt = 2007-JAN-02 00:35:19.574
Stop time, drdt = 2007-JAN-02 00:35:19.574
Start time, drdt = 2007-JAN-19 22:04:54.899
Stop time, drdt = 2007-JAN-19 22:04:54.899
Start time, drdt = 2007-FEB-01 23:30:13.428
Stop time, drdt = 2007-FEB-01 23:30:13.428
Start time, drdt = 2007-FEB-17 11:10:46.540
Stop time, drdt = 2007-FEB-17 11:10:46.540
Start time, drdt = 2007-MAR-04 15:50:19.929
Stop time, drdt = 2007-MAR-04 15:50:19.929
Start time, drdt = 2007-MAR-18 09:59:05.959
Stop time, drdt = 2007-MAR-18 09:59:05.959
Relation condition: <
Start time, drdt = 2007-JAN-02 00:35:19.574
Stop time, drdt = 2007-JAN-19 22:04:54.899
Start time, drdt = 2007-FEB-01 23:30:13.428
Stop time, drdt = 2007-FEB-17 11:10:46.540
Start time, drdt = 2007-MAR-04 15:50:19.929
Stop time, drdt = 2007-MAR-18 09:59:05.959
Relation condition: >
Start time, drdt = 2007-JAN-01 00:00:00.000
Stop time, drdt = 2007-JAN-02 00:35:19.574
Start time, drdt = 2007-JAN-19 22:04:54.899
Stop time, drdt = 2007-FEB-01 23:30:13.428
Start time, drdt = 2007-FEB-17 11:10:46.540
Stop time, drdt = 2007-MAR-04 15:50:19.929
Start time, drdt = 2007-MAR-18 09:59:05.959
Stop time, drdt = 2007-APR-01 00:00:00.000
Relation condition: LOCMIN
Start time, drdt = 2007-JAN-11 07:03:58.988
Stop time, drdt = 2007-JAN-11 07:03:58.988
Start time, drdt = 2007-FEB-10 06:26:15.439
Stop time, drdt = 2007-FEB-10 06:26:15.439
Start time, drdt = 2007-MAR-12 03:28:36.404
Stop time, drdt = 2007-MAR-12 03:28:36.404
Relation condition: ABSMIN
Start time, drdt = 2007-JAN-11 07:03:58.988
Stop time, drdt = 2007-JAN-11 07:03:58.988
Relation condition: LOCMAX
Start time, drdt = 2007-JAN-26 02:27:33.766
Stop time, drdt = 2007-JAN-26 02:27:33.766
Start time, drdt = 2007-FEB-24 09:35:07.816
Stop time, drdt = 2007-FEB-24 09:35:07.816
Start time, drdt = 2007-MAR-25 17:26:56.150
Stop time, drdt = 2007-MAR-25 17:26:56.150
Relation condition: ABSMAX
Start time, drdt = 2007-MAR-25 17:26:56.150
Stop time, drdt = 2007-MAR-25 17:26:56.150
-Restrictions
1) Any kernel files required by this routine must be loaded
before this routine is called.
-Literature_References
None.
-Author_and_Institution
N.J. Bachman (JPL)
E.D. Wright (JPL)
-Version
-CSPICE Version 1.0.0, 22-FEB-2010 (EDW)
-Index_Entries
GF user defined scalar function search
-&
*/
{ /* Begin gfuds_c */
/*
Local variables
*/
doublereal * work;
static SpiceInt nw = SPICE_GF_NWMAX;
SpiceInt nBytes;
/*
Participate in error tracing.
*/
if ( return_c() )
{
return;
}
chkin_c ( "gfuds_c" );
/*
Make sure cell data types are d.p.
*/
CELLTYPECHK2 ( CHK_STANDARD, "gfuds_c", SPICE_DP, cnfine, result );
/*
Initialize the input cells if necessary.
*/
CELLINIT2 ( cnfine, result );
/*
Check the other input strings to make sure each pointer is non-null
and each string length is non-zero.
*/
CHKFSTR ( CHK_STANDARD, "gfuds_c", relate );
/*
Store the input function pointers so these functions can be
called by the GF adapters.
*/
zzadsave_c ( UDFUNC, (void *)(udfunc) );
zzadsave_c ( UDQDEC, (void *)(udqdec) );
/*
Check the workspace size; some mallocs have a violent
dislike for negative allocation amounts. To be safe,
rule out a count of zero intervals as well.
*/
if ( nintvls < 1 )
{
setmsg_c ( "The specified workspace interval count # was "
"less than the minimum allowed value of one (1)." );
errint_c ( "#", nintvls );
sigerr_c ( "SPICE(VALUEOUTOFRANGE)" );
chkout_c ( "gfuds_c" );
return;
}
/*
Allocate the workspace. 'nintvls' indicates the maximum number of
intervals returned in 'result'. An interval consists of
two values.
*/
nintvls = 2 * nintvls;
nBytes = (nintvls + SPICE_CELL_CTRLSZ ) * nw * sizeof(SpiceDouble);
work = (doublereal *) alloc_SpiceMemory( nBytes );
if ( !work )
{
setmsg_c ( "Workspace allocation of # bytes failed due to "
"malloc failure" );
errint_c ( "#", nBytes );
sigerr_c ( "SPICE(MALLOCFAILED)" );
chkout_c ( "gfuds_c" );
return;
}
/*
Let the f2c'd routine do the work.
We pass the adapter functions, not those provided as inputs,
to the f2c'd routine:
zzadfunc_c adapter for udfunc
zzadqdec_c '' udqdec
*/
(void) gfuds_( ( U_fp ) zzadfunc_c,
( U_fp ) zzadqdec_c,
( char * ) relate,
( doublereal * ) &refval,
( doublereal * ) &adjust,
( doublereal * ) &step,
( doublereal * ) (cnfine->base),
( integer * ) &nintvls,
( integer * ) &nw,
( doublereal * ) work,
( doublereal * ) (result->base),
( ftnlen ) strlen(relate) );
/*
Always free dynamically allocated memory.
*/
free_SpiceMemory( work );
/*
Sync the output cell.
*/
if ( !failed_c() )
{
zzsynccl_c ( F2C, result );
}
ALLOC_CHECK;
chkout_c ( "gfuds_c" );
} /* End gfuds_c */
void gfsubc_c ( ConstSpiceChar * target,
ConstSpiceChar * fixref,
ConstSpiceChar * method,
ConstSpiceChar * abcorr,
ConstSpiceChar * obsrvr,
ConstSpiceChar * crdsys,
ConstSpiceChar * coord,
ConstSpiceChar * relate,
SpiceDouble refval,
SpiceDouble adjust,
SpiceDouble step,
SpiceInt nintvls,
SpiceCell * cnfine,
SpiceCell * result )
/*
-Brief_I/O
Variable I/O Description
-------- --- --------------------------------------------------
SPICE_GF_CNVTOL
P Convergence tolerance.
target I Name of the target body
fixref I Body fixed frame associated with 'target'
method I Name of method type for subpoint calculation
abcorr I Aberration correction flag
obsrvr I Name of the observing body
crdsys I Name of the coordinate system containing 'coord'
coord I Name of the coordinate of interest
relate I Operator that either looks for an extreme value
(max, min, local, absolute) or compares the
coordinate value and refval
refval I Reference value
adjust I Adjustment value for absolute extrema searches
step I Step size used for locating extrema and roots
nintvls I Workspace window interval count
cnfine I-O SPICE window to which the search is restricted
result O SPICE window containing results
-Detailed_Input
target the string name of a target body. Optionally, you may
supply the integer ID code for the object as an
integer string. For example both 'MOON' and '301'
are legitimate strings that indicate the moon is the
target body.
The target and observer define a position vector
that points from the observer to the target.
fixref the string name of the body-fixed, body-centered
reference frame associated with the target body target.
The SPICE frame subsystem must recognize the 'fixref' name.
method the string name of the method to use for the subpoint
calculation. The accepted values for method:
'Near point: ellipsoid' The sub-observer point
computation uses a
triaxial ellipsoid to
model the surface of the
target body. The
sub-observer point is
defined as the nearest
point on the target
relative to the
observer.
'Intercept: ellipsoid' The sub-observer point
computation uses a
triaxial ellipsoid to
model the surface of the
target body. The
sub-observer point is
defined as the target
surface intercept of the
line containing the
observer and the
target's center.
The method string lacks sensitivity to case, embedded, leading
and trailing blanks.
abcorr the string description of the aberration corrections to apply
to the state evaluations to account for one-way light time
and stellar aberration.
This routine accepts the same aberration corrections as does
the SPICE routine SPKEZR. See the header of SPKEZR for a
detailed description of the aberration correction options.
For convenience, the options are listed below:
'NONE' Apply no correction.
'LT' "Reception" case: correct for
one-way light time using a Newtonian
formulation.
'LT+S' "Reception" case: correct for
one-way light time and stellar
aberration using a Newtonian
formulation.
'CN' "Reception" case: converged
Newtonian light time correction.
'CN+S' "Reception" case: converged
Newtonian light time and stellar
aberration corrections.
'XLT' "Transmission" case: correct for
one-way light time using a Newtonian
formulation.
'XLT+S' "Transmission" case: correct for
one-way light time and stellar
aberration using a Newtonian
formulation.
'XCN' "Transmission" case: converged
Newtonian light time correction.
'XCN+S' "Transmission" case: converged
Newtonian light time and stellar
aberration corrections.
The abcorr string lacks sensitivity to case, and to embedded,
leading and trailing blanks.
obsrvr the string naming the observing body. Optionally, you
may supply the ID code of the object as an integer
string. For example, both 'EARTH' and '399' are
legitimate strings to supply to indicate the
observer is Earth.
crdsys the string name of the coordinate system for which the
coordinate of interest is a member.
coord the string name of the coordinate of interest in crdsys.
The supported coordinate systems and coordinate names are:
The supported coordinate systems and coordinate names are:
Coordinate System (CRDSYS) Coordinates (COORD) Range
'RECTANGULAR' 'X'
'Y'
'Z'
'LATITUDINAL' 'RADIUS'
'LONGITUDE' (-Pi,Pi]
'LATITUDE' [-Pi/2,Pi/2]
'RA/DEC' 'RANGE'
'RIGHT ASCENSION' [0,2Pi)
'DECLINATION' [-Pi/2,Pi/2]
'SPHERICAL' 'RADIUS'
'COLATITUDE' [0,Pi]
'LONGITUDE' (-Pi,Pi]
'CYLINDRICAL' 'RADIUS'
'LONGITUDE' [0,2Pi)
'Z'
'GEODETIC' 'LONGITUDE' (-Pi,Pi]
'LATITUDE' [-Pi/2,Pi/2]
'ALTITUDE'
'PLANETOGRAPHIC' 'LONGITUDE' [0,2Pi)
'LATITUDE' [-Pi/2,Pi/2]
'ALTITUDE'
The ALTITUDE coordinates have a constant value
of zero +/- roundoff for ellipsoid targets.
Limit searches for coordinate events in the GEODETIC and
PLANETOGRAPHIC coordinate systems to TARGET bodies with
axial symmetry in the equatorial plane, i.e. equality
of the body X and Y radii (oblate or prolate spheroids).
relate the string or character describing the relational operator
used to define a constraint on the selected coordinate of the
subpoint vector. The result window found by this routine
indicates the time intervals where the constraint is satisfied.
Supported values of relate and corresponding meanings are
shown below:
'>' Separation is greater than the reference
value refval.
'=' Separation is equal to the reference
value refval.
'<' Separation is less than the reference
value refval.
'ABSMAX' Separation is at an absolute maximum.
'ABSMIN' Separation is at an absolute minimum.
'LOCMAX' Separation is at a local maximum.
'LOCMIN' Separation is at a local minimum.
The caller may indicate that the region of interest
is the set of time intervals where the quantity is
within a specified measure of an absolute extremum.
The argument ADJUST (described below) is used to
specify this measure.
Local extrema are considered to exist only in the
interiors of the intervals comprising the confinement
window: a local extremum cannot exist at a boundary
point of the confinement window.
The relate string lacks sensitivity to case, leading
and trailing blanks.
refval the double precision reference value used together with
relate argument to define an equality or inequality to
satisfy by the selected coordinate of the subpoint
vector. See the discussion of relate above for
further information.
The units of refval correspond to the type as defined
by coord, radians for angular measures, kilometers for
distance measures.
adjust a double precision value used to modify searches for
absolute extrema: when 'relate' is set to ABSMAX or ABSMIN and
'adjust' is set to a positive value, gfsubc_c finds times
when the position vector coordinate is within adjust
radians/kilometers of the specified extreme value.
For 'relate' set to ABSMAX, the result window contains
time intervals when the position vector coordinate has
values between ABSMAX - adjust and ABSMAX.
For 'relate' set to ABSMIN, the result window contains
time intervals when the position vector coordinate has
values between ABSMIN and ABSMIN + adjust.
'adjust' is not used for searches for local extrema,
equality or inequality conditions.
step the double precision time step size to use in the search.
step must be short enough for a search using this step
size to locate the time intervals where coordinate function
of the subpoint vector is monotone increasing or
decreasing. However, step must not be *too* short, or
the search will take an unreasonable amount of time.
The choice of step affects the completeness but not
the precision of solutions found by this routine; the
precision is controlled by the convergence tolerance.
step has units of TDB seconds.
nintvls an integer value specifying the number of intervals in the
the internal workspace array used by this routine. 'nintvls'
should be at least as large as the number of intervals
within the search region on which the specified observer-target
vector coordinate function is monotone increasing or decreasing.
It does no harm to pick a value of 'nintvls' larger than the
minimum required to execute the specified search, but if chosen
too small, the search will fail.
cnfine a double precision SPICE window that confines the time
period over which the specified search is conducted.
cnfine may consist of a single interval or a collection
of intervals.
In some cases the confinement window can be used to
greatly reduce the time period that must be searched
for the desired solution. See the Particulars section
below for further discussion.
See the Examples section below for a code example
that shows how to create a confinement window.
-Detailed_Output
cnfine is the input confinement window, updated if necessary
so the control area of its data array indicates the
window's size and cardinality. The window data are
unchanged.
result the SPICE window of intervals, contained within the
confinement window cnfine, on which the specified
constraint is satisfied.
If result is non-empty on input, its contents
will be discarded before gfsubc_c conducts its
search.
result must be declared and initialized with sufficient
size to capture the full set of time intervals
within the search region on which the specified constraint
is satisfied.
If the search is for local extrema, or for absolute
extrema with adjust set to zero, then normally each
interval of result will be a singleton: the left and
right endpoints of each interval will be identical.
If no times within the confinement window satisfy the
constraint, result will be returned with a
cardinality of zero.
-Parameters
SPICE_GF_CNVTOL
is the convergence tolerance used for finding endpoints
of the intervals comprising the result window.
SPICE_GF_CNVTOL is used to determine when binary searches
for roots should terminate: when a root is bracketed
within an interval of length SPICE_GF_CNVTOL; the root is
considered to have been found.
The accuracy, as opposed to precision, of roots found by
this routine depends on the accuracy of the input data.
In most cases, the accuracy of solutions will be inferior
to their precision.
SPICE_GF_CNVTOL has the value 1.0e-6. Units are TDB
seconds.
-Exceptions
1) In order for this routine to produce correct results,
the step size must be appropriate for the problem at hand.
Step sizes that are too large may cause this routine to miss
roots; step sizes that are too small may cause this routine
to run unacceptably slowly and in some cases, find spurious
roots.
This routine does not diagnose invalid step sizes, except
that if the step size is non-positive, an error is signaled
by a routine in the call tree of this routine.
2) Due to numerical errors, in particular,
- Truncation error in time values
- Finite tolerance value
- Errors in computed geometric quantities
it is *normal* for the condition of interest to not always be
satisfied near the endpoints of the intervals comprising the
result window.
The result window may need to be contracted slightly by the
caller to achieve desired results. The SPICE window routine
wncond_c can be used to contract the result window.
3) If an error (typically cell overflow) occurs while performing
window arithmetic, the error will be diagnosed by a routine
in the call tree of this routine.
4) If the relational operator `relate' is not recognized, an
error is signaled by a routine in the call tree of this
routine.
5) If the aberration correction specifier contains an
unrecognized value, an error is signaled by a routine in the
call tree of this routine.
6) If `adjust' is negative, an error is signaled by a routine in
the call tree of this routine.
7) If either of the input body names do not map to NAIF ID
codes, an error is signaled by a routine in the call tree of
this routine.
8) If required ephemerides or other kernel data are not
available, an error is signaled by a routine in the call tree
of this routine.
9) If any input string argument pointer is null, the error
SPICE(NULLPOINTER) will be signaled.
10) If any input string argument is empty, the error
SPICE(EMPTYSTRING) will be signaled.
11) If the workspace interval count 'nintvls' is less than 1, the
error SPICE(VALUEOUTOFRANGE) will be signaled.
12) If the required amount of workspace memory cannot be
allocated, the error SPICE(MALLOCFAILURE) will be
signaled.
-Files
Appropriate SPK and PCK kernels must be loaded by the
calling program before this routine is called.
The following data are required:
- SPK data: the calling application must load ephemeris data
for the targets, observer, and any intermediate objects in
a chain connecting the targets and observer that cover the time
period specified by the window CNFINE. If aberration
corrections are used, the states of target and observer
relative to the solar system barycenter must be calculable
from the available ephemeris data. Typically ephemeris data
are made available by loading one or more SPK files using
FURNSH.
- PCK data: bodies modeled as triaxial ellipsoids must have
semi-axis lengths provided by variables in the kernel pool.
Typically these data are made available by loading a text
PCK file using FURNSH.
- If non-inertial reference frames are used, then PCK
files, frame kernels, C-kernels, and SCLK kernels may be
needed.
Such kernel data are normally loaded once per program
run, NOT every time this routine is called.
-Particulars
This routine provides a simpler, but less flexible interface
than does the routine gfevnt_c for conducting searches for
subpoint position vector coordinate value events.
Applications that require support for progress reporting, interrupt
handling, non-default step or refinement functions, or non-default
convergence tolerance should call gfevnt_c rather than this routine.
This routine determines a set of one or more time intervals
within the confinement window when the selected coordinate of
the subpoint position vector satisfies a caller-specified
constraint. The resulting set of intervals is returned as a SPICE
window.
Below we discuss in greater detail aspects of this routine's
solution process that are relevant to correct and efficient
use of this routine in user applications.
The Search Process
==================
Regardless of the type of constraint selected by the caller, this
routine starts the search for solutions by determining the time
periods, within the confinement window, over which the specified
coordinate function is monotone increasing and monotone
decreasing. Each of these time periods is represented by a SPICE
window. Having found these windows, all of the coordinate
function's local extrema within the confinement window are known.
Absolute extrema then can be found very easily.
Within any interval of these "monotone" windows, there will be at
most one solution of any equality constraint. Since the boundary
of the solution set for any inequality constraint is the set
of points where an equality constraint is met, the solutions of
both equality and inequality constraints can be found easily
once the monotone windows have been found.
Step Size
=========
The monotone windows (described above) are found using a two-step
search process. Each interval of the confinement window is
searched as follows: first, the input step size is used to
determine the time separation at which the sign of the rate of
change of coordinate will be sampled. Starting at
the left endpoint of an interval, samples will be taken at each
step. If a change of sign is found, a root has been bracketed; at
that point, the time at which the time derivative of the coordinate
is zero can be found by a refinement process, for example,
using a binary search.
Note that the optimal choice of step size depends on the lengths
of the intervals over which the coordinate function is monotone:
the step size should be shorter than the shortest of these
intervals (within the confinement window).
The optimal step size is *not* necessarily related to the lengths
of the intervals comprising the result window. For example, if
the shortest monotone interval has length 10 days, and if the
shortest result window interval has length 5 minutes, a step size
of 9.9 days is still adequate to find all of the intervals in the
result window. In situations like this, the technique of using
monotone windows yields a dramatic efficiency improvement over a
state-based search that simply tests at each step whether the
specified constraint is satisfied. The latter type of search can
miss solution intervals if the step size is shorter than the
shortest solution interval.
Having some knowledge of the relative geometry of the target and
observer can be a valuable aid in picking a reasonable step size.
In general, the user can compensate for lack of such knowledge by
picking a very short step size; the cost is increased computation
time.
Note that the step size is not related to the precision with which
the endpoints of the intervals of the result window are computed.
That precision level is controlled by the convergence tolerance.
Convergence Tolerance
=====================
As described above, the root-finding process used by this routine
involves first bracketing roots and then using a search process
to locate them. "Roots" are both times when local extrema are
attained and times when the distance function is equal to a
reference value. All endpoints of the intervals comprising the
result window are either endpoints of intervals of the
confinement window or roots.
Once a root has been bracketed, a refinement process is used to
narrow down the time interval within which the root must lie.
This refinement process terminates when the location of the root
has been determined to within an error margin called the
"convergence tolerance." The convergence tolerance used by this
routine is set by the parameter SPICE_GF_CNVTOL.
The value of SPICE_GF_CNVTOL is set to a "tight" value in the f2c'd
routine so that the tolerance doesn't become the limiting factor
in the accuracy of solutions found by this routine. In general the
accuracy of input data will be the limiting factor.
To use a different tolerance value, a lower-level GF routine such
as gfevnt_c must be called. Making the tolerance tighter than
SPICE_GF_CNVTOL is unlikely to be useful, since the results are unlikely
to be more accurate. Making the tolerance looser will speed up
searches somewhat, since a few convergence steps will be omitted.
However, in most cases, the step size is likely to have a much
greater effect on processing time than would the convergence
tolerance.
The Confinement Window
======================
The simplest use of the confinement window is to specify a time
interval within which a solution is sought. However, the
confinement window can, in some cases, be used to make searches
more efficient. Sometimes it's possible to do an efficient search
to reduce the size of the time period over which a relatively
slow search of interest must be performed.
Practical use of the coordinate search capability would likely
consist of searches over multiple coordinate constraints to find
time intervals that satisfies the constraints. An effective
technique to accomplish such a search is to use the result
window from one search as the confinement window of the next.
Longitude and Right Ascension
=============================
The cyclic nature of the longitude and right ascension coordinates
produces branch cuts at +/- 180 degrees longitude and 0-360
longitude. Round-off error may cause solutions near these branches
to cross the branch. Use of the SPICE routine wncond_c will contract
solution windows by some epsilon, reducing the measure of the
windows and eliminating the branch crossing. A one millisecond
contraction will in most cases eliminate numerical round-off caused
branch crossings.
-Examples
The numerical results shown for these examples may differ across
platforms. The results depend on the SPICE kernels used as
input, the compiler and supporting libraries, and the machine
specific arithmetic implementation.
The example shown below requires a "standard" set of SPICE
kernels. We list these kernels in a meta kernel named 'standard.tm'.
KPL/MK
This meta-kernel is intended to support operation of SPICE
example programs. The kernels shown here should not be
assumed to contain adequate or correct versions of data
required by SPICE-based user applications.
In order for an application to use this meta-kernel, the
kernels referenced here must be present in the user's
current working directory.
The names and contents of the kernels referenced
by this meta-kernel are as follows:
File name Contents
--------- --------
de414.bsp Planetary ephemeris
pck00008.tpc Planet orientation and
radii
naif0008.tls Leapseconds
\begindata
KERNELS_TO_LOAD = ( '/kernels/gen/lsk/naif0008.tls'
'/kernels/gen/spk/de414.bsp'
'/kernels/gen/pck/pck00008.tpc'
)
Example:
Find the time during 2007 for which the subpoint position vector
of the sun on earth in the IAU_EARTH frame lies within a geodetic
latitude-longitude "box" defined as
16 degrees <= latitude <= 17 degrees
85 degrees <= longitude <= 86 degrees
This problem requires four searches, each search on one of the
box restrictions. The user needs also realize the temporal
behavior of latitude greatly differs from that of the longitude. The
sub-observer point latitude varies between approximately 23.44 degrees
and -23.44 degrees during the year. The sub-observer point longitude
varies between -180 degrees and 180 degrees in one day.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "SpiceUsr.h"
#define MAXWIN 100
#define TIMFMT "YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND"
#define STRLEN 64
int main( int argc, char **argv )
{
/.
Create the needed windows. Note, one window
consists of two values, so the total number
of cell values to allocate equals twice
the number of intervals.
./
SPICEDOUBLE_CELL ( result1, 2*MAXWIN );
SPICEDOUBLE_CELL ( result2, 2*MAXWIN );
SPICEDOUBLE_CELL ( result3, 2*MAXWIN );
SPICEDOUBLE_CELL ( result4, 2*MAXWIN );
SPICEDOUBLE_CELL ( cnfine, 2 );
SpiceDouble begtim;
SpiceDouble endtim;
SpiceDouble step;
SpiceDouble adjust;
SpiceDouble refval;
SpiceDouble beg;
SpiceDouble end;
SpiceChar begstr [ STRLEN ];
SpiceChar endstr [ STRLEN ];
SpiceChar * target = "EARTH";
SpiceChar * obsrvr = "SUN";
SpiceChar * fixref = "IAU_EARTH";
SpiceChar * method = "Near point: ellipsoid";
SpiceChar * crdsys = "GEODETIC";
SpiceChar * abcorr = "NONE";
SpiceInt count;
SpiceInt i;
/.
Load kernels.
./
furnsh_c( "standard.tm" );
/.
Store the time bounds of our search interval in
the cnfine confinement window.
./
str2et_c( "2007 JAN 01", &begtim );
str2et_c( "2008 JAN 01", &endtim );
wninsd_c ( begtim, endtim, &cnfine );
/.
Perform four searches to determine the times when the
latitude-longitude box restriction conditions apply to
the subpoint vector.
Perform the searches such that the result window of a search
serves as the confinement window of the subsequent search.
Since the latitude coordinate varies slowly and is well behaved
over the time of the confinement window, search first for the
windows satisfying the latitude requirements, then use that result
as confinement for the longitude search.
./
/.
The latitude varies relatively slowly, ~46 degrees during the
year. The extrema occur approximately every six months.
Search using a step size less than half that value (180 days).
For this example use ninety days (in units of seconds).
./
step = (90.)*spd_c();
adjust = 0.;
{
SpiceChar * coord = "LATITUDE";
SpiceChar * relate = ">";
refval = 16. *rpd_c();
gfsubc_c ( target, fixref,
method, abcorr, obsrvr,
crdsys, coord,
relate, refval,
adjust, step,
MAXWIN,
&cnfine, &result1 );
}
{
SpiceChar * coord = "LATITUDE";
SpiceChar * relate = "<";
refval = 17. *rpd_c();
gfsubc_c ( target, fixref,
method, abcorr, obsrvr,
crdsys, coord,
relate, refval,
adjust, step,
MAXWIN,
&result1, &result2 );
}
/.
Now the longitude search.
./
/.
Reset the stepsize to something appropriate for the 360
degrees in 24 hours domain. The longitude shows near
linear behavior so use a stepsize less than half the period
of twelve hours. Ten hours will suffice in this case.
./
step = (10./24.)*spd_c();
{
SpiceChar * coord = "LONGITUDE";
SpiceChar * relate = ">";
refval = 85. *rpd_c();
gfsubc_c ( target, fixref,
method, abcorr, obsrvr,
crdsys, coord,
relate, refval,
adjust, step,
MAXWIN,
&result2, &result3 );
/.
Contract the endpoints of each window to account
for possible round-off error at the -180/180 degree branch.
A contraction value of a millisecond should eliminate
any round-off caused branch crossing.
./
wncond_c( 1e-3, 1e-3, &result3 );
}
{
SpiceChar * coord = "LONGITUDE";
SpiceChar * relate = "<";
refval = 86. *rpd_c();
gfsubc_c ( target, fixref,
method, abcorr, obsrvr,
crdsys, coord,
relate, refval,
adjust, step,
MAXWIN,
&result3, &result4 );
}
/.
List the beginning and ending points in each interval
if result contains data.
./
count = wncard_c( &result4 );
/.
Display the results.
./
if (count == 0 )
{
printf ( "Result window is empty.\n\n" );
}
else
{
for ( i = 0; i < count; i++ )
{
/.
Fetch the endpoints of the Ith interval
of the result window.
./
wnfetd_c ( &result4, i, &beg, &end );
timout_c ( beg, TIMFMT, STRLEN, begstr );
timout_c ( end, TIMFMT, STRLEN, endstr );
printf ( "Interval %d\n", i + 1);
printf ( "Beginning TDB %s \n", begstr );
printf ( "Ending TDB %s \n\n", endstr );
}
}
kclear_c();
return( 0 );
}
The program outputs:
Interval 1
Beginning TDB 2007-MAY-05 06:14:04.637735 (TDB)
Ending TDB 2007-MAY-05 06:18:04.621908 (TDB)
Interval 2
Beginning TDB 2007-MAY-06 06:13:59.583483 (TDB)
Ending TDB 2007-MAY-06 06:17:59.569239 (TDB)
Interval 3
Beginning TDB 2007-MAY-07 06:13:55.102939 (TDB)
Ending TDB 2007-MAY-07 06:17:55.090299 (TDB)
Interval 4
Beginning TDB 2007-MAY-08 06:13:51.202604 (TDB)
Ending TDB 2007-MAY-08 06:17:51.191583 (TDB)
Interval 5
Beginning TDB 2007-AUG-06 06:23:17.282927 (TDB)
Ending TDB 2007-AUG-06 06:27:17.264009 (TDB)
Interval 6
Beginning TDB 2007-AUG-07 06:23:10.545441 (TDB)
Ending TDB 2007-AUG-07 06:27:10.524926 (TDB)
Interval 7
Beginning TDB 2007-AUG-08 06:23:03.233996 (TDB)
Ending TDB 2007-AUG-08 06:27:03.211889 (TDB)
-Restrictions
1) The kernel files to be used by this routine must be loaded
(normally via the CSPICE routine furnsh_c) before this routine
is called.
2) This routine has the side effect of re-initializing the
coordinate quantity utility package. Callers may
need to re-initialize the package after calling this routine.
-Literature_References
None.
-Author_and_Institution
N.J. Bachman (JPL)
E.D. Wright (JPL)
-Version
-CSPICE Version 1.0.1, 26-AUG-2009, EDW (JPL)
Edit to Example description, replaced "intercept" with
"sub-observer point."
Correction of several typos.
-CSPICE Version 1.0.0, 10-FEB-2009 (NJB) (EDW)
-Index_Entries
GF subpoint coordinate search
-&
*/
{ /* Begin gfsubc_c */
/*
Local variables
*/
doublereal * work;
SpiceInt nBytes;
static SpiceInt nw = SPICE_GF_NWMAX;
/*
Participate in error tracing.
*/
if ( return_c() )
{
return;
}
chkin_c ( "gfsubc_c" );
/*
Make sure cell data types are d.p.
*/
CELLTYPECHK2 ( CHK_STANDARD, "gfsubc_c", SPICE_DP, cnfine, result );
/*
Initialize the input cells if necessary.
*/
CELLINIT2 ( cnfine, result );
/*
Check the input strings to make sure each pointer is non-null
and each string length is non-zero.
*/
CHKFSTR ( CHK_STANDARD, "gfsubc_c", target );
CHKFSTR ( CHK_STANDARD, "gfsubc_c", fixref );
CHKFSTR ( CHK_STANDARD, "gfsubc_c", method );
CHKFSTR ( CHK_STANDARD, "gfsubc_c", abcorr );
CHKFSTR ( CHK_STANDARD, "gfsubc_c", obsrvr );
CHKFSTR ( CHK_STANDARD, "gfsubc_c", crdsys );
CHKFSTR ( CHK_STANDARD, "gfsubc_c", coord );
CHKFSTR ( CHK_STANDARD, "gfsubc_c", relate );
/*
Check the workspace size; some mallocs have a violent
dislike for negative allocation amounts. To be safe,
rule out a count of zero intervals as well.
*/
if ( nintvls < 1 )
{
setmsg_c ( "The specified workspace interval count # was "
"less than the minimum allowed value of one (1)." );
errint_c ( "#", nintvls );
sigerr_c ( "SPICE(VALUEOUTOFRANGE)" );
chkout_c ( "gfposc_c" );
return;
}
/*
Allocate the workspace. 'nintvls' indicates the maximum number of
intervals returned in 'result'. An interval consists of
two values.
*/
nintvls = 2 * nintvls;
nBytes = ( nintvls + SPICE_CELL_CTRLSZ ) * nw * sizeof(SpiceDouble);
work = (doublereal *) alloc_SpiceMemory( nBytes );
if ( !work )
{
setmsg_c ( "Workspace allocation of # bytes failed due to "
"malloc failure" );
errint_c ( "#", nBytes );
sigerr_c ( "SPICE(MALLOCFAILED)" );
chkout_c ( "gfsubc_c" );
return;
}
/*
Let the f2'd routine do the work.
*/
gfsubc_ ( ( char * ) target,
( char * ) fixref,
( char * ) method,
( char * ) abcorr,
( char * ) obsrvr,
( char * ) crdsys,
( char * ) coord,
( char * ) relate,
( doublereal * ) &refval,
( doublereal * ) &adjust,
( doublereal * ) &step,
( doublereal * ) (cnfine->base),
( integer * ) &nintvls,
( integer * ) &nw,
( doublereal * ) work,
( doublereal * ) (result->base),
( ftnlen ) strlen(target),
( ftnlen ) strlen(fixref),
( ftnlen ) strlen(method),
( ftnlen ) strlen(abcorr),
( ftnlen ) strlen(obsrvr),
( ftnlen ) strlen(crdsys),
( ftnlen ) strlen(coord),
( ftnlen ) strlen(relate) );
/*
De-allocate the workspace.
*/
free_SpiceMemory( work );
/*
Sync the output cell.
*/
if ( !failed_c() )
{
zzsynccl_c ( F2C, result ) ;
}
ALLOC_CHECK;
chkout_c ( "gfsubc_c" );
} /* End gfsubc_c */
void gfoclt_c ( ConstSpiceChar * occtyp,
ConstSpiceChar * front,
ConstSpiceChar * fshape,
ConstSpiceChar * fframe,
ConstSpiceChar * back,
ConstSpiceChar * bshape,
ConstSpiceChar * bframe,
ConstSpiceChar * abcorr,
ConstSpiceChar * obsrvr,
SpiceDouble step,
SpiceCell * cnfine,
SpiceCell * result )
/*
-Brief_I/O
VARIABLE I/O DESCRIPTION
--------------- --- -------------------------------------------------
SPICE_GF_CNVTOL P Convergence tolerance.
occtyp I Type of occultation.
front I Name of body occulting the other.
fshape I Type of shape model used for front body.
fframe I Body-fixed, body-centered frame for front body.
back I Name of body occulted by the other.
bshape I Type of shape model used for back body.
bframe I Body-fixed, body-centered frame for back body.
abcorr I Aberration correction flag.
obsrvr I Name of the observing body.
step I Step size in seconds for finding occultation
events.
cnfine I-O SPICE window to which the search is restricted.
result O SPICE window containing results.
-Detailed_Input
occtyp indicates the type of occultation that is to be found.
Note that transits are considered to be a type of
occultation.
Supported values and corresponding definitions are:
"FULL" denotes the full occultation
of the body designated by
`back' by the body designated
by `front', as seen from
the location of the observer.
In other words, the occulted
body is completely invisible
as seen from the observer's
location.
"ANNULAR" denotes an annular
occultation: the body
designated by `front' blocks
part of, but not the limb of,
the body designated by `back',
as seen from the location of
the observer.
"PARTIAL" denotes a partial, non-annular
occultation: the body designated
by `front' blocks part, but not
all, of the limb of the body
designated by `back', as seen
from the location of the
observer.
"ANY" denotes any of the above three
types of occultations:
"PARTIAL", "ANNULAR", or
"FULL".
"ANY" should be used to search
for times when the body
designated by `front' blocks
any part of the body designated
by `back'.
The option "ANY" must be used
if either the front or back
target body is modeled as
a point.
Case and leading or trailing blanks are not
significant in the string `occtyp'.
front is the name of the target body that occults---that is,
passes in front of---the other. Optionally, you may
supply the integer NAIF ID code for the body as a
string. For example both "MOON" and "301" are
legitimate strings that designate the Moon.
Case and leading or trailing blanks are not
significant in the string `front'.
fshape is a string indicating the geometric model used to
represent the shape of the front target body. The
supported options are:
"ELLIPSOID" Use a triaxial ellipsoid model
with radius values provided via the
kernel pool. A kernel variable
having a name of the form
"BODYnnn_RADII"
where nnn represents the NAIF
integer code associated with the
body, must be present in the kernel
pool. This variable must be
associated with three numeric
values giving the lengths of the
ellipsoid's X, Y, and Z semi-axes.
"POINT" Treat the body as a single point.
When a point target is specified,
the occultation type must be
set to "ANY".
At least one of the target bodies `front' and `back' must
be modeled as an ellipsoid.
Case and leading or trailing blanks are not
significant in the string `fshape'.
fframe is the name of the body-fixed, body-centered reference
frame associated with the front target body. Examples
of such names are "IAU_SATURN" (for Saturn) and
"ITRF93" (for the Earth).
If the front target body is modeled as a point, `fframe'
should be left empty or blank.
Case and leading or trailing blanks bracketing a
non-blank frame name are not significant in the string
`fframe'.
back is the name of the target body that is occulted
by---that is, passes in back of---the other.
Optionally, you may supply the integer NAIF ID code
for the body as a string. For example both "MOON" and
"301" are legitimate strings that designate the Moon.
Case and leading or trailing blanks are not
significant in the string `back'.
bshape is the shape specification for the body designated by
`back'. The supported options are those for `fshape'. See
the description of `fshape' above for details.
bframe is the name of the body-fixed, body-centered reference
frame associated with the ``back'' target body.
Examples of such names are "IAU_SATURN" (for Saturn)
and "ITRF93" (for the Earth).
If the back target body is modeled as a point, `bframe'
should be left empty or blank.
Case and leading or trailing blanks bracketing a
non-blank frame name are not significant in the string
`bframe'.
abcorr indicates the aberration corrections to be applied to
the state of each target body to account for one-way
light time. Stellar aberration corrections are
ignored if specified, since these corrections don't
improve the accuracy of the occultation determination.
See the header of the SPICE routine spkezr_c for a
detailed description of the aberration correction
options. For convenience, the options supported by
this routine are listed below:
"NONE" Apply no correction.
"LT" "Reception" case: correct for
one-way light time using a Newtonian
formulation.
"CN" "Reception" case: converged
Newtonian light time correction.
"XLT" "Transmission" case: correct for
one-way light time using a Newtonian
formulation.
"XCN" "Transmission" case: converged
Newtonian light time correction.
Case and blanks are not significant in the string
`abcorr'.
obsrvr is the name of the body from which the occultation is
observed. Optionally, you may supply the integer NAIF
ID code for the body as a string.
Case and leading or trailing blanks are not
significant in the string `obsrvr'.
step is the step size to be used in the search. `step' must
be shorter than any interval, within the confinement
window, over which the specified condition is met. In
other words, `step' must be shorter than the shortest
occultation event that the user wishes to detect; `step'
must also be shorter than the shortest time interval
between two occultation events that occur within the
confinement window (see below). However, `step' must not
be *too* short, or the search will take an unreasonable
amount of time.
The choice of `step' affects the completeness but not the
precision of solutions found by this routine; the
precision is controlled by the convergence tolerance. See
the discussion of the parameter SPICE_GF_CNVTOL for
details.
`step' has units of TDB seconds.
cnfine is a SPICE window that confines the time period over
which the specified search is conducted. `cnfine' may
consist of a single interval or a collection of
intervals.
The endpoints of the time intervals comprising `cnfine'
are interpreted as seconds past J2000 TDB.
See the Examples section below for a code example
that shows how to create a confinement window.
-Detailed_Output
cnfine is the input confinement window, updated if necessary
so the control area of its data array indicates the
window's size and cardinality. The window data are
unchanged.
result is a SPICE window representing the set of time
intervals, within the confinement period, when the
specified occultation occurs.
The endpoints of the time intervals comprising `result'
are interpreted as seconds past J2000 TDB.
If `result' is non-empty on input, its contents
will be discarded before gfoclt_c conducts its
search.
-Parameters
SPICE_GF_CNVTOL
is the convergence tolerance used for finding endpoints
of the intervals comprising the result window.
SPICE_GF_CNVTOL is used to determine when binary searches
for roots should terminate: when a root is bracketed
within an interval of length SPICE_GF_CNVTOL, the root is
considered to have been found.
The accuracy, as opposed to precision, of roots found
by this routine depends on the accuracy of the input
data. In most cases, the accuracy of solutions will be
inferior to their precision.
SPICE_GF_CNVTOL is declared in the header file
SpiceGF.h
-Exceptions
1) In order for this routine to produce correct results,
the step size must be appropriate for the problem at hand.
Step sizes that are too large may cause this routine to miss
roots; step sizes that are too small may cause this routine
to run unacceptably slowly and in some cases, find spurious
roots.
This routine does not diagnose invalid step sizes, except
that if the step size is non-positive, the error
SPICE(INVALIDSTEPSIZE) will be signaled.
2) Due to numerical errors, in particular,
- Truncation error in time values
- Finite tolerance value
- Errors in computed geometric quantities
it is *normal* for the condition of interest to not always be
satisfied near the endpoints of the intervals comprising the
result window.
The result window may need to be contracted slightly by the
caller to achieve desired results. The SPICE window routine
wncond_c can be used to contract the result window.
3) If name of either target or the observer cannot be translated
to a NAIF ID code, the error will be diagnosed by a routine
in the call tree of this routine.
4) If the radii of a target body modeled as an ellipsoid cannot
be determined by searching the kernel pool for a kernel
variable having a name of the form
"BODYnnn_RADII"
where nnn represents the NAIF integer code associated with
the body, the error will be diagnosed by a routine in the
call tree of this routine.
5) If either of the target bodies `front' or `back' coincides with
the observer body `obsrvr', the error will be diagnosed by a
routine in the call tree of this routine.
6) If the body designated by `front' coincides with that
designated by `back', the error will be diagnosed by a routine
in the call tree of this routine.
7) If either of the body model specifiers `fshape' or `bshape'
is not recognized, the error will be diagnosed by a routine
in the call tree of this routine.
8) If both of the body model specifiers `fshape' and `bshape'
specify point targets, the error will be diagnosed by a
routine in the call tree of this routine.
9) If a target body-fixed reference frame associated with a
non-point target is not recognized, the error will be
diagnosed by a routine in the call tree of this routine.
10) If a target body-fixed reference frame is not centered at
the corresponding target body, the error will be
diagnosed by a routine in the call tree of this routine.
11) If the loaded kernels provide insufficient data to
compute any required state vector, the deficiency will
be diagnosed by a routine in the call tree of this routine.
12) If an error occurs while reading an SPK or other kernel file,
the error will be diagnosed by a routine in the call tree
of this routine.
13) If the output SPICE window `result' has insufficient capacity
to contain the number of intervals on which the specified
occultation condition is met, the error will be diagnosed
by a routine in the call tree of this routine.
14) If a point target is specified and the occultation
type is set to a valid value other than "ANY", the
error will be diagnosed by a routine in the call tree
of this routine.
15) Invalid occultation types will be diagnosed by a routine in
the call tree of this routine.
16) Invalid aberration correction specifications will be
diagnosed by a routine in the call tree of this routine.
17) If any input string argument pointer is null, the error
SPICE(NULLPOINTER) will be signaled.
18) If any input string argument, other than `fframe' or `bframe',
is empty, the error SPICE(EMPTYSTRING) will be signaled.
-Files
Appropriate SPICE kernels must be loaded by the calling program
before this routine is called.
The following data are required:
- SPK data: the calling application must load ephemeris data
for the target, source and observer that cover the time
period specified by the window `cnfine'. If aberration
corrections are used, the states of target and observer
relative to the solar system barycenter must be calculable
from the available ephemeris data. Typically ephemeris data
are made available by loading one or more SPK files via
furnsh_c.
- PCK data: bodies modeled as triaxial ellipsoids must have
semi-axis lengths provided by variables in the kernel pool.
Typically these data are made available by loading a text
PCK file via furnsh_c.
- FK data: if either of the reference frames designated by
`bframe' or `fframe' are not built in to the SPICE system,
one or more FKs specifying these frames must be loaded.
Kernel data are normally loaded once per program run, NOT every time
this routine is called.
-Particulars
This routine provides a simpler, but less flexible, interface
than does the CSPICE routine gfocce_c for conducting searches for
occultation events. Applications that require support for
progress reporting, interrupt handling, non-default step or
refinement functions, or non-default convergence tolerance should
call gfocce_c rather than this routine.
This routine determines a set of one or more time intervals
within the confinement window when a specified type of
occultation occurs. The resulting set of intervals is returned as
a SPICE window.
Below we discuss in greater detail aspects of this routine's
solution process that are relevant to correct and efficient
use of this routine in user applications.
The Search Process
==================
The search for occultations is treated as a search for state
transitions: times are sought when the state of the `back' body
changes from "not occulted" to "occulted" or vice versa.
Step Size
=========
Each interval of the confinement window is searched as follows:
first, the input step size is used to determine the time separation
at which the occultation state will be sampled. Starting at the left
endpoint of the interval, samples of the occultation state will be
taken at each step. If a state change is detected, a root has been
bracketed; at that point, the "root"--the time at which the state
change occurs---is found by a refinement process, for example, via
binary search.
Note that the optimal choice of step size depends on the lengths
of the intervals over which the occultation state is constant:
the step size should be shorter than the shortest occultation
duration and the shortest period between occultations, within
the confinement window.
Having some knowledge of the relative geometry of the targets and
observer can be a valuable aid in picking a reasonable step size.
In general, the user can compensate for lack of such knowledge by
picking a very short step size; the cost is increased computation
time.
Note that the step size is not related to the precision with which
the endpoints of the intervals of the result window are computed.
That precision level is controlled by the convergence tolerance.
Convergence Tolerance
=====================
Once a root has been bracketed, a refinement process is used to
narrow down the time interval within which the root must lie. This
refinement process terminates when the location of the root has been
determined to within an error margin called the "convergence
tolerance." The convergence tolerance used by this routine is set
via the parameter SPICE_GF_CNVTOL.
The value of SPICE_GF_CNVTOL is set to a "tight" value so that the
tolerance doesn't limit the accuracy of solutions found by this
routine. In general the accuracy of input data will be the limiting
factor.
To use a different tolerance value, a lower-level GF routine such as
gfocce_c must be called. Making the tolerance tighter than
SPICE_GF_CNVTOL is unlikely to be useful, since the results are
unlikely to be more accurate. Making the tolerance looser will speed
up searches somewhat, since a few convergence steps will be omitted.
However, in most cases, the step size is likely to have a much
greater effect on processing time than would the convergence
tolerance.
The Confinement Window
======================
The simplest use of the confinement window is to specify a time
interval within which a solution is sought.
The confinement window also can be used to restrict a search to
a time window over which required data (typically ephemeris
data, in the case of occultation searches) are known to be
available.
In some cases, the confinement window be used to make searches
more efficient. Sometimes it's possible to do an efficient search
to reduce the size of the time period over which a relatively
slow search of interest must be performed. See the "CASCADE"
example program in gf.req for a demonstration.
-Examples
The numerical results shown for these examples may differ across
platforms. The results depend on the SPICE kernels used as
input, the compiler and supporting libraries, and the machine
specific arithmetic implementation.
1) Find occultations of the Sun by the Moon (that is, solar
eclipses) as seen from the center of the Earth over the month
December, 2001.
Use light time corrections to model apparent positions of Sun
and Moon. Stellar aberration corrections are not specified
because they don't affect occultation computations.
We select a step size of 3 minutes, which means we
ignore occultation events lasting less than 3 minutes,
if any exist.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: standard.tm
This meta-kernel is intended to support operation of SPICE
example programs. The kernels shown here should not be
assumed to contain adequate or correct versions of data
required by SPICE-based user applications.
In order for an application to use this meta-kernel, the
kernels referenced here must be present in the user's
current working directory.
\begindata
KERNELS_TO_LOAD = ( 'de421.bsp',
'pck00008.tpc',
'naif0009.tls' )
\begintext
Example code begins here.
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
/.
Local constants
./
#define TIMFMT "YYYY MON DD HR:MN:SC.###### (TDB)::TDB"
#define MAXWIN 200
#define TIMLEN 41
/.
Local variables
./
SPICEDOUBLE_CELL ( cnfine, MAXWIN );
SPICEDOUBLE_CELL ( result, MAXWIN );
SpiceChar * win0;
SpiceChar * win1;
SpiceChar begstr [ TIMLEN ];
SpiceChar endstr [ TIMLEN ];
SpiceDouble et0;
SpiceDouble et1;
SpiceDouble left;
SpiceDouble right;
SpiceDouble step;
SpiceInt i;
/.
Load kernels.
./
furnsh_c ( "standard.tm" );
/.
Obtain the TDB time bounds of the confinement
window, which is a single interval in this case.
./
win0 = "2001 DEC 01 00:00:00 TDB";
win1 = "2002 JAN 01 00:00:00 TDB";
str2et_c ( win0, &et0 );
str2et_c ( win1, &et1 );
/.
Insert the time bounds into the confinement
window.
./
wninsd_c ( et0, et1, &cnfine );
/.
Select a 3-minute step. We'll ignore any occultations
lasting less than 3 minutes. Units are TDB seconds.
./
step = 180.0;
/.
Perform the search.
./
gfoclt_c ( "any",
"moon", "ellipsoid", "iau_moon",
"sun", "ellipsoid", "iau_sun",
"lt", "earth", step,
&cnfine, &result );
if ( wncard_c(&result) == 0 )
{
printf ( "No occultation was found.\n" );
}
else
{
for ( i = 0; i < wncard_c(&result); i++ )
{
/.
Fetch and display each occultation interval.
./
wnfetd_c ( &result, i, &left, &right );
timout_c ( left, TIMFMT, TIMLEN, begstr );
timout_c ( right, TIMFMT, TIMLEN, endstr );
printf ( "Interval %ld\n"
" Start time: %s\n"
" Stop time: %s\n",
i, begstr, endstr );
}
}
return ( 0 );
}
When this program was executed on a PC/Linux/gcc platform, the
output was:
Interval 0
Start time: 2001 DEC 14 20:10:14.195952 (TDB)
Stop time: 2001 DEC 14 21:35:50.317994 (TDB)
2) Find occultations of Titan by Saturn or of Saturn by
Titan as seen from the center of the Earth over the
last four months of 2008. Model both target bodies as
ellipsoids. Search for every type of occultation.
Use light time corrections to model apparent positions of
Saturn and Titan. Stellar aberration corrections are not
specified because they don't affect occultation computations.
We select a step size of 15 minutes, which means we
ignore occultation events lasting less than 15 minutes,
if any exist.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: gfoclt_ex2.tm
This meta-kernel is intended to support operation of SPICE
example programs. The kernels shown here should not be
assumed to contain adequate or correct versions of data
required by SPICE-based user applications.
In order for an application to use this meta-kernel, the
kernels referenced here must be present in the user's
current working directory.
The names and contents of the kernels referenced
by this meta-kernel are as follows:
File name Contents
--------- --------
de421.bsp Planetary ephemeris
sat288.bsp Satellite ephemeris for
Saturn
pck00008.tpc Planet orientation and
radii
naif0009.tls Leapseconds
\begindata
KERNELS_TO_LOAD = ( 'de421.bsp',
'sat288.bsp',
'pck00008.tpc',
'naif0009.tls' )
\begintext
End of meta-kernel
Example code begins here.
#include <stdio.h>
#include <string.h>
#include "SpiceUsr.h"
int main()
{
/.
Local constants
./
#define TIMFMT "YYYY MON DD HR:MN:SC.###### (TDB)::TDB"
#define MAXWIN 200
#define TIMLEN 41
#define LNSIZE 81
#define NTYPES 4
/.
Local variables
./
SPICEDOUBLE_CELL ( cnfine, MAXWIN );
SPICEDOUBLE_CELL ( result, MAXWIN );
SpiceChar * back;
SpiceChar * bframe;
SpiceChar * front;
SpiceChar * fframe;
SpiceChar line [ LNSIZE ];
SpiceChar * obsrvr;
SpiceChar * occtyp [ NTYPES ] =
{
"FULL",
"ANNULAR",
"PARTIAL",
"ANY"
};
SpiceChar * templt [ NTYPES ] =
{
"Condition: # occultation of # by #",
"Condition: # occultation of # by #",
"Condition: # occultation of # by #",
"Condition: # occultation of # by #"
};
SpiceChar timstr [ TIMLEN ];
SpiceChar title [ LNSIZE ];
SpiceChar * win0;
SpiceChar * win1;
SpiceDouble et0;
SpiceDouble et1;
SpiceDouble finish;
SpiceDouble start;
SpiceDouble step;
SpiceInt i;
SpiceInt j;
SpiceInt k;
/.
Load kernels.
./
furnsh_c ( "gfoclt_ex2.tm" );
/.
Obtain the TDB time bounds of the confinement
window, which is a single interval in this case.
./
win0 = "2008 SEP 01 00:00:00 TDB";
win1 = "2009 JAN 01 00:00:00 TDB";
str2et_c ( win0, &et0 );
str2et_c ( win1, &et1 );
/.
Insert the time bounds into the confinement
window.
./
wninsd_c ( et0, et1, &cnfine );
/.
Select a 15-minute step. We'll ignore any occultations
lasting less than 15 minutes. Units are TDB seconds.
./
step = 900.0;
/.
The observation location is the Earth.
./
obsrvr = "Earth";
/.
Loop over the occultation types.
./
for ( i = 0; i < NTYPES; i++ )
{
/.
For each type, do a search for both transits of
Titan across Saturn and occultations of Titan by
Saturn.
./
for ( j = 0; j < 2; j++ )
{
if ( j == 0 )
{
front = "TITAN";
fframe = "IAU_TITAN";
back = "SATURN";
bframe = "IAU_SATURN";
}
else
{
front = "SATURN";
fframe = "IAU_SATURN";
back = "TITAN";
bframe = "IAU_TITAN";
}
/.
Perform the search. The target body shapes
are modeled as ellipsoids.
./
gfoclt_c ( occtyp[i],
front, "ellipsoid", fframe,
back, "ellipsoid", bframe,
"lt", obsrvr, step,
&cnfine, &result );
/.
Display the results.
./
printf ( "\n" );
/.
Substitute the occultation type and target
body names into the title string:
./
repmc_c ( templt[i], "#", occtyp[i], LNSIZE, title );
repmc_c ( title, "#", back, LNSIZE, title );
repmc_c ( title, "#", front, LNSIZE, title );
printf ( "%s\n", title );
if ( wncard_c(&result) == 0 )
{
printf ( " Result window is empty: "
"no occultation was found.\n" );
}
else
{
printf ( " Result window start, stop times:\n" );
for ( k = 0; k < wncard_c(&result); k++ )
{
/.
Fetch the endpoints of the kth interval
of the result window.
./
wnfetd_c ( &result, k, &start, &finish );
/.
Call strncpy with a length of 7 to include
a terminating null.
./
strncpy ( line, " # #", 7 );
timout_c ( start, TIMFMT, TIMLEN, timstr );
repmc_c ( line, "#", timstr, LNSIZE, line );
timout_c ( finish, TIMFMT, TIMLEN, timstr );
repmc_c ( line, "#", timstr, LNSIZE, line );
printf ( "%s\n", line );
}
}
/.
We've finished displaying the results of the
current search.
./
}
/.
We've finished displaying the results of the
searches using the current occultation type.
./
}
printf ( "\n" );
return ( 0 );
}
When this program was executed on a PC/Linux/gcc platform, the
output was:
Condition: FULL occultation of SATURN by TITAN
Result window is empty: no occultation was found.
Condition: FULL occultation of TITAN by SATURN
Result window start, stop times:
2008 OCT 27 22:08:01.627053 (TDB) 2008 OCT 28 01:05:03.375236 (TDB)
2008 NOV 12 21:21:59.252262 (TDB) 2008 NOV 13 02:06:05.053051 (TDB)
2008 NOV 28 20:49:02.402832 (TDB) 2008 NOV 29 02:13:58.986344 (TDB)
2008 DEC 14 20:05:09.246177 (TDB) 2008 DEC 15 01:44:53.523002 (TDB)
2008 DEC 30 19:00:56.577073 (TDB) 2008 DEC 31 00:42:43.222909 (TDB)
Condition: ANNULAR occultation of SATURN by TITAN
Result window start, stop times:
2008 OCT 19 21:29:20.599087 (TDB) 2008 OCT 19 22:53:34.518737 (TDB)
2008 NOV 04 20:15:38.620368 (TDB) 2008 NOV 05 00:18:59.139978 (TDB)
2008 NOV 20 19:38:59.647712 (TDB) 2008 NOV 21 00:35:26.725908 (TDB)
2008 DEC 06 18:58:34.073268 (TDB) 2008 DEC 07 00:16:17.647040 (TDB)
2008 DEC 22 18:02:46.288289 (TDB) 2008 DEC 22 23:26:52.712459 (TDB)
Condition: ANNULAR occultation of TITAN by SATURN
Result window is empty: no occultation was found.
Condition: PARTIAL occultation of SATURN by TITAN
Result window start, stop times:
2008 OCT 19 20:44:30.326771 (TDB) 2008 OCT 19 21:29:20.599087 (TDB)
2008 OCT 19 22:53:34.518737 (TDB) 2008 OCT 19 23:38:26.250580 (TDB)
2008 NOV 04 19:54:40.339331 (TDB) 2008 NOV 04 20:15:38.620368 (TDB)
2008 NOV 05 00:18:59.139978 (TDB) 2008 NOV 05 00:39:58.612935 (TDB)
2008 NOV 20 19:21:46.689523 (TDB) 2008 NOV 20 19:38:59.647712 (TDB)
2008 NOV 21 00:35:26.725908 (TDB) 2008 NOV 21 00:52:40.604703 (TDB)
2008 DEC 06 18:42:36.100544 (TDB) 2008 DEC 06 18:58:34.073268 (TDB)
2008 DEC 07 00:16:17.647040 (TDB) 2008 DEC 07 00:32:16.324244 (TDB)
2008 DEC 22 17:47:10.776722 (TDB) 2008 DEC 22 18:02:46.288289 (TDB)
2008 DEC 22 23:26:52.712459 (TDB) 2008 DEC 22 23:42:28.850542 (TDB)
Condition: PARTIAL occultation of TITAN by SATURN
Result window start, stop times:
2008 OCT 27 21:37:16.970175 (TDB) 2008 OCT 27 22:08:01.627053 (TDB)
2008 OCT 28 01:05:03.375236 (TDB) 2008 OCT 28 01:35:49.266506 (TDB)
2008 NOV 12 21:01:47.105498 (TDB) 2008 NOV 12 21:21:59.252262 (TDB)
2008 NOV 13 02:06:05.053051 (TDB) 2008 NOV 13 02:26:18.227357 (TDB)
2008 NOV 28 20:31:28.522707 (TDB) 2008 NOV 28 20:49:02.402832 (TDB)
2008 NOV 29 02:13:58.986344 (TDB) 2008 NOV 29 02:31:33.691598 (TDB)
2008 DEC 14 19:48:27.094229 (TDB) 2008 DEC 14 20:05:09.246177 (TDB)
2008 DEC 15 01:44:53.523002 (TDB) 2008 DEC 15 02:01:36.360243 (TDB)
2008 DEC 30 18:44:23.485898 (TDB) 2008 DEC 30 19:00:56.577073 (TDB)
2008 DEC 31 00:42:43.222909 (TDB) 2008 DEC 31 00:59:17.030568 (TDB)
Condition: ANY occultation of SATURN by TITAN
Result window start, stop times:
2008 OCT 19 20:44:30.326771 (TDB) 2008 OCT 19 23:38:26.250580 (TDB)
2008 NOV 04 19:54:40.339331 (TDB) 2008 NOV 05 00:39:58.612935 (TDB)
2008 NOV 20 19:21:46.689523 (TDB) 2008 NOV 21 00:52:40.604703 (TDB)
2008 DEC 06 18:42:36.100544 (TDB) 2008 DEC 07 00:32:16.324244 (TDB)
2008 DEC 22 17:47:10.776722 (TDB) 2008 DEC 22 23:42:28.850542 (TDB)
Condition: ANY occultation of TITAN by SATURN
Result window start, stop times:
2008 OCT 27 21:37:16.970175 (TDB) 2008 OCT 28 01:35:49.266506 (TDB)
2008 NOV 12 21:01:47.105498 (TDB) 2008 NOV 13 02:26:18.227357 (TDB)
2008 NOV 28 20:31:28.522707 (TDB) 2008 NOV 29 02:31:33.691598 (TDB)
2008 DEC 14 19:48:27.094229 (TDB) 2008 DEC 15 02:01:36.360243 (TDB)
2008 DEC 30 18:44:23.485898 (TDB) 2008 DEC 31 00:59:17.030568 (TDB)
-Restrictions
The kernel files to be used by gfoclt_c must be loaded (normally via
the CSPICE routine furnsh_c) before gfoclt_c is called.
-Literature_References
None.
-Author_and_Institution
N. J. Bachman (JPL)
L. S. Elson (JPL)
E. D. Wright (JPL)
-Version
-CSPICE Version 1.0.0, 07-APR-2009 (NJB) (LSE) (EDW)
-Index_Entries
GF occultation search
-&
*/
{ /* Begin gfoclt_c */
/*
Local variables
*/
static const SpiceChar * blankStr = " ";
SpiceChar * bFrameStr;
SpiceChar * fFrameStr;
/*
Participate in error tracing.
*/
if ( return_c() )
{
return;
}
chkin_c ( "gfoclt_c" );
/*
Make sure cell data types are d.p.
*/
CELLTYPECHK2 ( CHK_STANDARD, "gfoclt_c", SPICE_DP, cnfine, result );
/*
Initialize the input cells if necessary.
*/
CELLINIT2 ( cnfine, result );
/*
The input frame names are special cases because we allow the caller
to pass in empty strings. If either of these strings are empty,
we pass a null-terminated string containing one blank character to
the underlying f2c'd routine.
First make sure the frame name pointers are non-null.
*/
CHKPTR ( CHK_STANDARD, "gfoclt_c", bframe );
CHKPTR ( CHK_STANDARD, "gfoclt_c", fframe );
/*
Use the input frame strings if they're non-empty; otherwise
use blank strings for the frame names.
*/
if ( bframe[0] )
{
bFrameStr = (SpiceChar *) bframe;
}
else
{
bFrameStr = (SpiceChar *) blankStr;
}
if ( fframe[0] )
{
fFrameStr = (SpiceChar *) fframe;
}
else
{
fFrameStr = (SpiceChar *) blankStr;
}
/*
Check the other input strings to make sure each pointer is non-null
and each string length is non-zero.
*/
CHKFSTR ( CHK_STANDARD, "gfoclt_c", occtyp );
CHKFSTR ( CHK_STANDARD, "gfoclt_c", front );
CHKFSTR ( CHK_STANDARD, "gfoclt_c", fshape );
CHKFSTR ( CHK_STANDARD, "gfoclt_c", back );
CHKFSTR ( CHK_STANDARD, "gfoclt_c", bshape );
CHKFSTR ( CHK_STANDARD, "gfoclt_c", abcorr );
CHKFSTR ( CHK_STANDARD, "gfoclt_c", obsrvr );
/*
Let the f2c'd routine do the work.
*/
gfoclt_ ( (char *) occtyp,
(char *) front,
(char *) fshape,
(char *) fFrameStr,
(char *) back,
(char *) bshape,
(char *) bFrameStr,
(char *) abcorr,
(char *) obsrvr,
(doublereal *) &step,
(doublereal *) cnfine->base,
(doublereal *) result->base,
(ftnlen ) strlen(occtyp),
(ftnlen ) strlen(front),
(ftnlen ) strlen(fshape),
(ftnlen ) strlen(fframe),
(ftnlen ) strlen(back),
(ftnlen ) strlen(bshape),
(ftnlen ) strlen(bframe),
(ftnlen ) strlen(abcorr),
(ftnlen ) strlen(obsrvr) );
/*
Sync the output result cell.
*/
if ( !failed_c() )
{
zzsynccl_c ( F2C, result );
}
chkout_c ( "gfoclt_c" );
} /* End gfoclt_c */
void gfposc_c ( ConstSpiceChar * target,
ConstSpiceChar * frame,
ConstSpiceChar * abcorr,
ConstSpiceChar * obsrvr,
ConstSpiceChar * crdsys,
ConstSpiceChar * coord,
ConstSpiceChar * relate,
SpiceDouble refval,
SpiceDouble adjust,
SpiceDouble step,
SpiceInt nintvls,
SpiceCell * cnfine,
SpiceCell * result )
/*
-Brief_I/O
Variable I/O Description
-------- --- --------------------------------------------------
SPICE_GF_CNVTOL
P Convergence tolerance.
target I Name of the target body
frame I Name of the reference frame for coordinate calculations
abcorr I Aberration correction flag
obsrvr I Name of the observing body
crdsys I Name of the coordinate system containing COORD
coord I Name of the coordinate of interest
relate I Operator that either looks for an extreme value
(max, min, local, absolute) or compares the
coordinate value and refval
refval I Reference value
adjust I Adjustment value for absolute extrema searches
step I Step size used for locating extrema and roots
nintvls I Workspace window interval count
cnfine I-O SPICE window to which the search is restricted
result O SPICE window containing results
-Detailed_Input
target the string name of a target body. Optionally, you may
supply the integer ID code for the object as an
integer string. For example both 'MOON' and '301'
are legitimate strings that indicate the moon is the
target body.
The target and observer define a position vector
that points from the observer to the target.
frame the string name of the reference frame in which to perform
state look-ups and coordinate calculations.
The SPICE frame subsystem must recognize the 'frame' name.
abcorr the string description of the aberration corrections to apply
to the state evaluations to account for one-way light time
and stellar aberration.
This routine accepts the same aberration corrections as does
the SPICE routine SPKEZR. See the header of SPKEZR for a
detailed description of the aberration correction options.
For convenience, the options are listed below:
'NONE' Apply no correction.
'LT' "Reception" case: correct for
one-way light time using a Newtonian
formulation.
'LT+S' "Reception" case: correct for
one-way light time and stellar
aberration using a Newtonian
formulation.
'CN' "Reception" case: converged
Newtonian light time correction.
'CN+S' "Reception" case: converged
Newtonian light time and stellar
aberration corrections.
'XLT' "Transmission" case: correct for
one-way light time using a Newtonian
formulation.
'XLT+S' "Transmission" case: correct for
one-way light time and stellar
aberration using a Newtonian
formulation.
'XCN' "Transmission" case: converged
Newtonian light time correction.
'XCN+S' "Transmission" case: converged
Newtonian light time and stellar
aberration corrections.
The abcorr string lacks sensitivity to case, and to embedded,
leading and trailing blanks.
obsrvr the string naming the observing body. Optionally, you
may supply the ID code of the object as an integer
string. For example, both 'EARTH' and '399' are
legitimate strings to supply to indicate the
observer is Earth.
crdsys the string name of the coordinate system for which the
coordinate of interest is a member.
coord the string name of the coordinate of interest in crdsys.
The supported coordinate systems and coordinate names are:
Coordinate System (CRDSYS) Coordinates (COORD) Range
'RECTANGULAR' 'X'
'Y'
'Z'
'LATITUDINAL' 'RADIUS'
'LONGITUDE' (-Pi,Pi]
'LATITUDE' [-Pi/2,Pi/2]
'RA/DEC' 'RANGE'
'RIGHT ASCENSION' [0,2Pi)
'DECLINATION' [-Pi/2,Pi/2]
'SPHERICAL' 'RADIUS'
'COLATITUDE' [0,Pi]
'LONGITUDE' (-Pi,Pi]
'CYLINDRICAL' 'RADIUS'
'LONGITUDE' [0,2Pi)
'Z'
'GEODETIC' 'LONGITUDE' (-Pi,Pi]
'LATITUDE' [-Pi/2,Pi/2]
'ALTITUDE'
'PLANETOGRAPHIC' 'LONGITUDE' [0,2Pi)
'LATITUDE' [-Pi/2,Pi/2]
'ALTITUDE'
Limit searches for coordinate events in the GEODETIC and
PLANETOGRAPHIC coordinate systems to TARGET bodies with
axial symmetry in the equatorial plane, i.e. equality
of the body X and Y radii (oblate or prolate spheroids).
relate the string or character describing the relational operator
used to define a constraint on the selected coordinate of the
observer-target vector. The result window found by this routine
indicates the time intervals where the constraint is satisfied.
Supported values of relate and corresponding meanings are
shown below:
'>' Separation is greater than the reference
value refval.
'=' Separation is equal to the reference
value refval.
'<' Separation is less than the reference
value refval.
'ABSMAX' Separation is at an absolute maximum.
'ABSMIN' Separation is at an absolute minimum.
'LOCMAX' Separation is at a local maximum.
'LOCMIN' Separation is at a local minimum.
The caller may indicate that the region of interest
is the set of time intervals where the quantity is
within a specified measure of an absolute extremum.
The argument ADJUST (described below) is used to
specify this measure.
Local extrema are considered to exist only in the
interiors of the intervals comprising the confinement
window: a local extremum cannot exist at a boundary
point of the confinement window.
The relate string lacks sensitivity to case, leading
and trailing blanks.
refval the double precision reference value used together with
relate argument to define an equality or inequality to
satisfy by the selected coordinate of the observer-target
vector. See the discussion of relate above for
further information.
The units of refval correspond to the type as defined
by coord, radians for angular measures, kilometers for
distance measures.
adjust a double precision value used to modify searches for
absolute extrema: when relate is set to ABSMAX or ABSMIN and
adjust is set to a positive value, gfposc_c finds times when the
observer-target vector coordinate is within adjust
radians/kilometers of the specified extreme value.
For relate set to ABSMAX, the result window contains
time intervals when the observer-target vector coordinate has
values between ABSMAX - adjust and ABSMAX.
For relate set to ABSMIN, the result window contains
time intervals when the observer-target vector coordinate has
values between ABSMIN and ABSMIN + adjust.
adjust is not used for searches for local extrema,
equality or inequality conditions.
step the double precision time step size to use in the search.
step must be short enough for a search using this step
size to locate the time intervals where coordinate function
of the observer-target vector is monotone increasing or
decreasing. However, step must not be *too* short, or
the search will take an unreasonable amount of time.
The choice of step affects the completeness but not
the precision of solutions found by this routine; the
precision is controlled by the convergence tolerance.
step has units of seconds.
nintvls an integer value specifying the number of intervals in the
the internal workspace array used by this routine. 'nintvls'
should be at least as large as the number of intervals
within the search region on which the specified observer-target
vector coordinate function is monotone increasing or decreasing.
It does no harm to pick a value of 'nintvls' larger than the
minimum required to execute the specified search, but if chosen
too small, the search will fail.
cnfine a double precision SPICE window that confines the time
period over which the specified search is conducted.
cnfine may consist of a single interval or a collection
of intervals.
In some cases the confinement window can be used to
greatly reduce the time period that must be searched
for the desired solution. See the Particulars section
below for further discussion.
See the Examples section below for a code example
that shows how to create a confinement window.
-Detailed_Output
cnfine is the input confinement window, updated if necessary
so the control area of its data array indicates the
window's size and cardinality. The window data are
unchanged.
result the SPICE window of intervals, contained within the
confinement window cnfine, on which the specified
constraint is satisfied.
If result is non-empty on input, its contents
will be discarded before gfposc_c conducts its
search.
result must be declared and initialized with sufficient
size to capture the full set of time intervals
within the search region on which the specified constraint
is satisfied.
If the search is for local extrema, or for absolute
extrema with adjust set to zero, then normally each
interval of result will be a singleton: the left and
right endpoints of each interval will be identical.
If no times within the confinement window satisfy the
constraint, result will be returned with a
cardinality of zero.
-Parameters
SPICE_GF_CNVTOL
is the convergence tolerance used for finding endpoints
of the intervals comprising the result window.
SPICE_GF_CNVTOL is used to determine when binary searches
for roots should terminate: when a root is bracketed
within an interval of length SPICE_GF_CNVTOL; the root is
considered to have been found.
The accuracy, as opposed to precision, of roots found by
this routine depends on the accuracy of the input data.
In most cases, the accuracy of solutions will be inferior
to their precision.
SPICE_GF_CNVTOL has the value 1.0e-6. Units are TDB
seconds.
-Exceptions
1) In order for this routine to produce correct results,
the step size must be appropriate for the problem at hand.
Step sizes that are too large may cause this routine to miss
roots; step sizes that are too small may cause this routine
to run unacceptably slowly and in some cases, find spurious
roots.
This routine does not diagnose invalid step sizes, except
that if the step size is non-positive, an error is signaled
by a routine in the call tree of this routine.
2) Due to numerical errors, in particular,
- Truncation error in time values
- Finite tolerance value
- Errors in computed geometric quantities
it is *normal* for the condition of interest to not always be
satisfied near the endpoints of the intervals comprising the
result window.
The result window may need to be contracted slightly by the
caller to achieve desired results. The SPICE window routine
wncond_c can be used to contract the result window.
3) If an error (typically cell overflow) occurs while performing
window arithmetic, the error will be diagnosed by a routine
in the call tree of this routine.
4) If the relational operator `relate' is not recognized, an
error is signaled by a routine in the call tree of this
routine.
5) If the aberration correction specifier contains an
unrecognized value, an error is signaled by a routine in the
call tree of this routine.
6) If `adjust' is negative, an error is signaled by a routine in
the call tree of this routine.
7) If either of the input body names do not map to NAIF ID
codes, an error is signaled by a routine in the call tree of
this routine.
8) If required ephemerides or other kernel data are not
available, an error is signaled by a routine in the call tree
of this routine.
9) If any input string argument pointer is null, the error
SPICE(NULLPOINTER) will be signaled.
10) If any input string argument is empty, the error
SPICE(EMPTYSTRING) will be signaled.
11) If the workspace interval count 'nintvls' is less than 1, the
error SPICE(VALUEOUTOFRANGE) will be signaled.
12) If the required amount of workspace memory cannot be
allocated, the error SPICE(MALLOCFAILURE) will be
signaled.
-Files
Appropriate SPK and PCK kernels must be loaded by the
calling program before this routine is called.
The following data are required:
- SPK data: the calling application must load ephemeris data
for the targets, observer, and any intermediate objects in
a chain connecting the targets and observer that cover the time
period specified by the window CNFINE. If aberration
corrections are used, the states of target and observer
relative to the solar system barycenter must be calculable
from the available ephemeris data. Typically ephemeris data
are made available by loading one or more SPK files using
FURNSH.
- PCK data: bodies modeled as triaxial ellipsoids must have
semi-axis lengths provided by variables in the kernel pool.
Typically these data are made available by loading a text
PCK file using FURNSH.
- If non-inertial reference frames are used, then PCK
files, frame kernels, C-kernels, and SCLK kernels may be
needed.
Such kernel data are normally loaded once per program
run, NOT every time this routine is called.
-Particulars
This routine provides a simpler, but less flexible interface
than does the routine gfevnt_c for conducting searches for
observer-target vector coordinate value events. Applications
that require support for progress reporting, interrupt
handling, non-default step or refinement functions, or non-default
convergence tolerance should call gfevnt_c rather than this routine.
This routine determines a set of one or more time intervals
within the confinement window when the selected coordinate of
the observer-target vector satisfies a caller-specified
constraint. The resulting set of intervals is returned as a SPICE
window.
Below we discuss in greater detail aspects of this routine's
solution process that are relevant to correct and efficient
use of this routine in user applications.
The Search Process
==================
Regardless of the type of constraint selected by the caller, this
routine starts the search for solutions by determining the time
periods, within the confinement window, over which the specified
coordinate function is monotone increasing and monotone
decreasing. Each of these time periods is represented by a SPICE
window. Having found these windows, all of the coordinate
function's local extrema within the confinement window are known.
Absolute extrema then can be found very easily.
Within any interval of these "monotone" windows, there will be at
most one solution of any equality constraint. Since the boundary
of the solution set for any inequality constraint is the set
of points where an equality constraint is met, the solutions of
both equality and inequality constraints can be found easily
once the monotone windows have been found.
Step Size
=========
The monotone windows (described above) are found using a two-step
search process. Each interval of the confinement window is
searched as follows: first, the input step size is used to
determine the time separation at which the sign of the rate of
change of coordinate will be sampled. Starting at
the left endpoint of an interval, samples will be taken at each
step. If a change of sign is found, a root has been bracketed; at
that point, the time at which the time derivative of the coordinate
is zero can be found by a refinement process, for example,
using a binary search.
Note that the optimal choice of step size depends on the lengths
of the intervals over which the coordinate function is monotone:
the step size should be shorter than the shortest of these
intervals (within the confinement window).
The optimal step size is *not* necessarily related to the lengths
of the intervals comprising the result window. For example, if
the shortest monotone interval has length 10 days, and if the
shortest result window interval has length 5 minutes, a step size
of 9.9 days is still adequate to find all of the intervals in the
result window. In situations like this, the technique of using
monotone windows yields a dramatic efficiency improvement over a
state-based search that simply tests at each step whether the
specified constraint is satisfied. The latter type of search can
miss solution intervals if the step size is shorter than the
shortest solution interval.
Having some knowledge of the relative geometry of the target and
observer can be a valuable aid in picking a reasonable step size.
In general, the user can compensate for lack of such knowledge by
picking a very short step size; the cost is increased computation
time.
Note that the step size is not related to the precision with which
the endpoints of the intervals of the result window are computed.
That precision level is controlled by the convergence tolerance.
Convergence Tolerance
=====================
As described above, the root-finding process used by this routine
involves first bracketing roots and then using a search process
to locate them. "Roots" are both times when local extrema are
attained and times when the distance function is equal to a
reference value. All endpoints of the intervals comprising the
result window are either endpoints of intervals of the
confinement window or roots.
Once a root has been bracketed, a refinement process is used to
narrow down the time interval within which the root must lie.
This refinement process terminates when the location of the root
has been determined to within an error margin called the
"convergence tolerance." The convergence tolerance used by this
routine is set by the parameter SPICE_GF_CNVTOL.
The value of SPICE_GF_CNVTOL is set to a "tight" value in the f2c'd
routine so that the tolerance doesn't become the limiting factor
in the accuracy of solutions found by this routine. In general the
accuracy of input data will be the limiting factor.
To use a different tolerance value, a lower-level GF routine such
as gfevnt_c must be called. Making the tolerance tighter than
SPICE_GF_CNVTOL is unlikely to be useful, since the results are unlikely
to be more accurate. Making the tolerance looser will speed up
searches somewhat, since a few convergence steps will be omitted.
However, in most cases, the step size is likely to have a much
greater effect on processing time than would the convergence
tolerance.
The Confinement Window
======================
The simplest use of the confinement window is to specify a time
interval within which a solution is sought. However, the
confinement window can, in some cases, be used to make searches
more efficient. Sometimes it's possible to do an efficient search
to reduce the size of the time period over which a relatively
slow search of interest must be performed.
Practical use of the coordinate search capability would likely
consist of searches over multiple coordinate constraints to find
time intervals that satisfies the constraints. An effective
technique to accomplish such a search is to use the result
window from one search as the confinement window of the next.
Longitude and Right Ascension
=============================
The cyclic nature of the longitude and right ascension coordinates
produces branch cuts at +/- 180 degrees longitude and 0-360
longitude. Round-off error may cause solutions near these branches
to cross the branch. Use of the SPICE routine wncond_c will contract
solution windows by some epsilon, reducing the measure of the
windows and eliminating the branch crossing. A one millisecond
contraction will in most cases eliminate numerical round-off caused
branch crossings.
-Examples
The numerical results shown for these examples may differ across
platforms. The results depend on the SPICE kernels used as
input, the compiler and supporting libraries, and the machine
specific arithmetic implementation.
The examples shown below require a "standard" set of SPICE
kernels. We list these kernels in a meta kernel named 'standard.tm'.
KPL/MK
This meta-kernel is intended to support operation of SPICE
example programs. The kernels shown here should not be
assumed to contain adequate or correct versions of data
required by SPICE-based user applications.
In order for an application to use this meta-kernel, the
kernels referenced here must be present in the user's
current working directory.
The names and contents of the kernels referenced
by this meta-kernel are as follows:
File name Contents
--------- --------
de414.bsp Planetary ephemeris
pck00008.tpc Planet orientation and radii
naif0009.tls Leapseconds kernel
earthstns_itrf93_050714.bsp SPK for DSN Station Locations
earth_topo_050714.tf Topocentric DSN stations frame
definitions
earth_000101_080120_071029.bpc High precision earth PCK
\begindata
KERNELS_TO_LOAD = (
'/kernels/gen/lsk/naif0008.tls'
'/kernels/gen/spk/de414.bsp'
'/kernels/gen/pck/pck00008.tpc'
'/kernels/gen/spk/earthstns_itrf93_050714.bsp',
'/kernels/gen/fk/earth_topo_050714.tf',
'/kernels/gen/pck/earth_000101_080120_071029.bpc',
)
Example(1):
Find the time during 2007 for which the latitude of the
Earth-Sun vector in IAU_EARTH frame has the maximum value,
i.e. the latitude of the Tropic of Cancer.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "SpiceUsr.h"
#define MAXWIN 750
#define TIMFMT "YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND"
#define TIMLEN 41
int main( int argc, char **argv )
{
/.
Create the needed windows. Note, one window
consists of two values, so the total number
of cell values to allocate is twice
the number of intervals.
./
SPICEDOUBLE_CELL ( result, 2*MAXWIN );
SPICEDOUBLE_CELL ( cnfine, 2 );
SpiceDouble begtim;
SpiceDouble endtim;
SpiceDouble step;
SpiceDouble adjust;
SpiceDouble refval;
SpiceDouble beg;
SpiceDouble end;
SpiceChar begstr [ TIMLEN ];
SpiceChar endstr [ TIMLEN ];
SpiceChar * relate = "ABSMAX";
SpiceChar * crdsys = "LATITUDINAL";
SpiceChar * coord = "LATITUDE";
SpiceChar * targ = "SUN";
SpiceChar * obsrvr = "EARTH";
SpiceChar * frame = "IAU_EARTH";
SpiceChar * abcorr = "NONE";
SpiceInt count;
SpiceInt i;
/.
Load kernels.
./
furnsh_c( "standard.tm" );
/.
Store the time bounds of our search interval in
the cnfine confinement window.
./
str2et_c( "2007 JAN 01", &begtim );
str2et_c( "2008 JAN 01", &endtim );
wninsd_c ( begtim, endtim, &cnfine );
/.
The latitude varies relatively slowly, ~46 degrees during the
year. The extrema occur approximately every six months.
Search using a step size less than half that value (180 days).
For this example use ninety days (in units of seconds).
./
step = (90.)*spd_c();
adjust = 0.;
refval = 0;
/.
List the beginning and ending points in each interval
if result contains data.
./
gfposc_c ( targ,
frame,
abcorr,
obsrvr,
crdsys,
coord,
relate,
refval,
adjust,
step,
MAXWIN,
&cnfine,
&result );
count = wncard_c( &result );
/.
Display the results.
./
if (count == 0 )
{
printf ( "Result window is empty.\n\n" );
}
else
{
for ( i = 0; i < count; i++ )
{
/.
Fetch the endpoints of the Ith interval
of the result window.
./
wnfetd_c ( &result, i, &beg, &end );
if ( beg == end )
{
timout_c ( beg, TIMFMT, TIMLEN, begstr );
printf ( "Event time: %s\n", begstr );
}
else
{
timout_c ( beg, TIMFMT, TIMLEN, begstr );
timout_c ( end, TIMFMT, TIMLEN, endstr );
printf ( "Interval %d\n", i + 1);
printf ( "From : %s \n", begstr );
printf ( "To : %s \n", endstr );
printf( " \n" );
}
}
}
kclear_c();
return( 0 );
}
The program outputs:
Event time: 2007-JUN-21 17:54:13.166910 (TDB)
Example(2):
A minor modification of the program listed in Example 1; find the
time during 2007 for which the latitude of the Earth-Sun vector
in IAU_EARTH frame has the minimum value, i.e. the latitude of
the Tropic of Capricorn.
Edit the example program, assign:
SpiceChar * relate = "ABSMIN";
The program outputs:
Event time: 2007-DEC-22 06:04:32.630160 (TDB)
Example(3):
Find the time during 2007 for which the Z component of the
Earth-Sun vector in IAU_EARTH frame has value 0, i.e. crosses
the equatorial plane (this also defines a zero latitude).
The search should return two times, one for an ascending
passage and one for descending.
Edit the example program, assign:
SpiceChar * relate = "=";
SpiceChar * crdsys = "RECTANGULAR";
SpiceChar * coord = "Z";
Note, this RELATE operator refers to the REFVAL value,
assigned to 0.D0 for this example.
The program outputs:
Event time: 2007-MAR-21 00:01:25.495120 (TDB)
Event time: 2007-SEP-23 09:46:39.574124 (TDB)
Example(4):
Find the times between Jan 1, 2007 and Jan 1, 2008 corresponding
to the apoapsis on the Moon's orbit around the Earth (note, the
GFDIST routine can also perform this search).
Edit the example program, assign:
This search requires a change in the step size since the Moon's
orbit about the earth (earth-moon barycenter) has a twenty-eight
day period. Use a step size something less than half that value.
In this case, we use twelve days.
SpiceChar * relate = "LOCMAX";
SpiceChar * crdsys = "SPHERICAL";
SpiceChar * coord = "RADIUS";
SpiceChar * targ = "MOON";
SpiceChar * frame = "J2000";
step = 12.*spd_c();
The program outputs:
Event time: 2007-JAN-10 16:26:18.805837 (TDB)
Event time: 2007-FEB-07 12:39:35.078525 (TDB)
Event time: 2007-MAR-07 03:38:07.334769 (TDB)
Event time: 2007-APR-03 08:38:55.222606 (TDB)
Event time: 2007-APR-30 10:56:49.847027 (TDB)
Event time: 2007-MAY-27 22:03:28.857783 (TDB)
Event time: 2007-JUN-24 14:26:23.639351 (TDB)
Event time: 2007-JUL-22 08:43:50.135565 (TDB)
Event time: 2007-AUG-19 03:28:33.538169 (TDB)
Event time: 2007-SEP-15 21:07:13.964698 (TDB)
Event time: 2007-OCT-13 09:52:30.819372 (TDB)
Event time: 2007-NOV-09 12:32:50.070555 (TDB)
Event time: 2007-DEC-06 16:54:31.225504 (TDB)
Example(5):
Find times between Jan 1, 2007 and Jan 1, 2008 when the latitude
(elevation) of the observer-target vector between DSS 17 and the
Moon, as observed in the DSS 17 topocentric (station) frame,
exceeds 83 degrees.
Edit the example program, assign:
This search uses a step size of four hours since the time
for all declination zero-to-max-to-zero passes within
the search window exceeds eight hours.
SpiceChar * relate = ">";
SpiceChar * crdsys = "LATITUDINAL";
SpiceChar * coord = "LATITUDE";
SpiceChar * targ = "MOON";
SpiceChar * obsrvr = "DSS-17";
SpiceChar * frame = "DSS-17_TOPO";
step = (4./24.)*spd_c();
refval = 83. * rpd_c();
The program outputs:
Interval 1
From : 2007-FEB-26 03:18:48.229806 (TDB)
To : 2007-FEB-26 03:31:29.734169 (TDB)
Interval 2
From : 2007-MAR-25 01:12:38.551183 (TDB)
To : 2007-MAR-25 01:23:53.908601 (TDB)
-Restrictions
1) The kernel files to be used by this routine must be loaded
(normally via the CSPICE routine furnsh_c) before this routine
is called.
2) This routine has the side effect of re-initializing the
coordinate quantity utility package. Callers may
need to re-initialize the package after calling this routine.
-Literature_References
None.
-Author_and_Institution
N.J. Bachman (JPL)
E.D. Wright (JPL)
-Version
-CSPICE Version 1.0.1, 26-AUG-2009 (EDW)
Correction of several typos.
-CSPICE Version 1.0.0, 10-FEB-2009 (NJB) (EDW)
-Index_Entries
GF position coordinate search
-&
*/
{ /* Begin gfposc_c */
/*
Local variables
*/
doublereal * work;
SpiceInt nBytes;
static SpiceInt nw = SPICE_GF_NWMAX;
/*
Participate in error tracing.
*/
if ( return_c() )
{
return;
}
chkin_c ( "gfposc_c" );
/*
Make sure cell data types are d.p.
*/
CELLTYPECHK2 ( CHK_STANDARD, "gfposc_c", SPICE_DP, cnfine, result );
/*
Initialize the input cells if necessary.
*/
CELLINIT2 ( cnfine, result );
/*
Check the input strings to make sure each pointer is non-null
and each string length is non-zero.
*/
CHKFSTR ( CHK_STANDARD, "gfposc_c", target );
CHKFSTR ( CHK_STANDARD, "gfposc_c", frame );
CHKFSTR ( CHK_STANDARD, "gfposc_c", abcorr );
CHKFSTR ( CHK_STANDARD, "gfposc_c", obsrvr );
CHKFSTR ( CHK_STANDARD, "gfposc_c", crdsys );
CHKFSTR ( CHK_STANDARD, "gfposc_c", coord );
CHKFSTR ( CHK_STANDARD, "gfposc_c", relate );
/*
Check the workspace size; some mallocs have a violent
dislike for negative allocation amounts. To be safe,
rule out a count of zero intervals as well.
*/
if ( nintvls < 1 )
{
setmsg_c ( "The specified workspace interval count # was "
"less than the minimum allowed value of one (1)." );
errint_c ( "#", nintvls );
sigerr_c ( "SPICE(VALUEOUTOFRANGE)" );
chkout_c ( "gfposc_c" );
return;
}
/*
Allocate the workspace. 'nintvls' indicates the maximum number of
intervals returned in 'result'. An interval consists of
two values.
*/
nintvls = 2 * nintvls;
nBytes = ( nintvls + SPICE_CELL_CTRLSZ ) * nw * sizeof(SpiceDouble);
work = (doublereal *) alloc_SpiceMemory( nBytes );
if ( !work )
{
setmsg_c ( "Workspace allocation of # bytes failed due to "
"malloc failure" );
errint_c ( "#", nBytes );
sigerr_c ( "SPICE(MALLOCFAILED)" );
chkout_c ( "gfposc_c" );
return;
}
/*
Let the f2'd routine do the work.
*/
gfposc_( ( char * ) target,
( char * ) frame,
( char * ) abcorr,
( char * ) obsrvr,
( char * ) crdsys,
( char * ) coord,
( char * ) relate,
( doublereal * ) &refval,
( doublereal * ) &adjust,
( doublereal * ) &step,
( doublereal * ) (cnfine->base),
( integer * ) &nintvls,
( integer * ) &nw,
( doublereal * ) work,
( doublereal * ) (result->base),
( ftnlen ) strlen(target),
( ftnlen ) strlen(frame),
( ftnlen ) strlen(abcorr),
( ftnlen ) strlen(obsrvr),
( ftnlen ) strlen(crdsys),
( ftnlen ) strlen(coord),
( ftnlen ) strlen(relate) );
/*
De-allocate the workspace.
*/
free_SpiceMemory( work );
/*
Sync the output cell.
*/
if ( !failed_c() )
{
zzsynccl_c ( F2C, result ) ;
}
ALLOC_CHECK;
chkout_c ( "gfposc_c" );
} /* End gfposc_c */
void gfpa_c ( ConstSpiceChar * target,
ConstSpiceChar * illmn,
ConstSpiceChar * abcorr,
ConstSpiceChar * obsrvr,
ConstSpiceChar * relate,
SpiceDouble refval,
SpiceDouble adjust,
SpiceDouble step,
SpiceInt nintvls,
SpiceCell * cnfine,
SpiceCell * result )
/*
-Brief_I/O
Variable I/O Description
--------------- --- ------------------------------------------------
SPICE_GF_CNVTOL P Convergence tolerance
target I Name of the target body.
illmn I Name of the illuminating body.
abcorr I Aberration correction flag.
obsrvr I Name of the observing body.
relate I Relational operator.
refval I Reference value.
adjust I Adjustment value for absolute extrema searches.
step I Step size used for locating extrema and roots.
nintvls I Workspace window interval count.
cnfine I-O SPICE window to which the search is confined.
result O SPICE window containing results.
-Detailed_Input
target is the name of a target body. Optionally, you may supply
a string containing the integer ID code for the object.
For example both "MOON" and "301" are legitimate strings
that indicate the Moon is the target body.
Case and leading or trailing blanks are not significant
in the string `target'.
illmn the string name of the illuminating body. This will
normally be "SUN" but the algorithm can use any
ephemeris object
Case and leading or trailing blanks are not significant
in the string `illmn'.
abcorr indicates the aberration corrections to be applied to
the observer-target position vector to account for
one-way light time and stellar aberration.
Any aberration correction accepted by the SPICE
routine spkezr_c is accepted here. See the header
of spkezr_c for a detailed description of the
aberration correction options. For convenience,
the allowed aberation options are listed below:
"NONE" Apply no correction.
"LT" "Reception" case: correct for
one-way light time using a Newtonian
formulation.
"LT+S" "Reception" case: correct for
one-way light time and stellar
aberration using a Newtonian
formulation.
"CN" "Reception" case: converged
Newtonian light time correction.
"CN+S" "Reception" case: converged
Newtonian light time and stellar
aberration corrections.
Note that this routine accepts only reception mode
aberration corrections.
Case and leading or trailing blanks are not significant
in the string `abcorr'.
obsrvr is the name of the observing body. Optionally, you may
supply a string containing the integer ID code for the
object. For example both "MOON" and "301" are legitimate
strings that indicate the Moon is the observer.
Case and leading or trailing blanks are not significant
in the string `obsrvr'.
relate is a relational operator used to define a constraint on
the phase angle. The result window found by
this routine indicates the time intervals where the
constraint is satisfied. Supported values of `relate'
and corresponding meanings are shown below:
">" The phase angle value is greater than the
reference value REFVAL.
"=" The phase angle value is equal to the
reference value REFVAL.
"<" The phase angle value is less than the
reference value REFVAL.
"ABSMAX" The phase angle value is at an absolute
maximum.
"ABSMIN" The phase angle value is at an absolute
minimum.
"LOCMAX" The phase angle value is at a local
maximum.
"LOCMIN" The phase angle value is at a local
minimum.
`relate' may be used to specify an "adjusted" absolute
extremum constraint: this requires the phase angle
to be within a specified offset relative to an
absolute extremum. The argument `adjust' (described
below) is used to specify this offset.
Local extrema are considered to exist only in the
interiors of the intervals comprising the confinement
window: a local extremum cannot exist at a boundary
point of the confinement window.
Case and leading or trailing blanks are not significant
in the string `relate'.
`refval' is the reference value used together with the argument
`relate' to define an equality or inequality to be
satisfied by the phase angle. See the discussion of
`relate' above for further information.
The units of `refval' are radians.
adjust is a parameter used to modify searches for absolute
extrema: when `relate' is set to "ABSMAX" or "ABSMIN"
and `adjust' is set to a positive value, gfpa_c will
find times when the phase angle is within
`adjust' radians of the specified extreme value.
If `adjust' is non-zero and a search for an absolute
minimum `min' is performed, the result window contains
time intervals when the phase angle has values between
`min' and min+adjust.
If the search is for an absolute maximum `max', the
corresponding range is from max-adjust to `max'.
`adjust' is not used for searches for local extrema,
equality or inequality conditions.
step is the step size to be used in the search. `step' must
be shorter than any maximal time interval on which the
specified phase angle function is monotone increasing or
decreasing. That is, if the confinement window is
partitioned into alternating intervals on which the
phase angle function is either monotone increasing or
decreasing, `step' must be shorter than any of these
intervals.
However, `step' must not be *too* short, or the search
will take an unreasonable amount of time.
The choice of `step' affects the completeness but not
the precision of solutions found by this routine; the
precision is controlled by the convergence tolerance.
See the discussion of the parameter SPICE_GF_CNVTOL for
details.
STEP has units of TDB seconds.
nintvls is a parameter specifying the number of intervals that
can be accommodated by each of the dynamically allocated
workspace windows used internally by this routine.
In many cases, it's not necessary to compute an accurate
estimate of how many intervals are needed; rather, the
user can pick a size considerably larger than what's
really required.
However, since excessively large arrays can prevent
applications from compiling, linking, or running
properly, sometimes `nintvls' must be set according to
the actual workspace requirement. A rule of thumb for
the number of intervals needed is
nintvls = 2*n + ( m / step )
where
n is the number of intervals in the confinement
window
m is the measure of the confinement window, in
units of seconds
`step' is the search step size in seconds
cnfine is a SPICE window that confines the time period over
which the specified search is conducted. `cnfine' may
consist of a single interval or a collection of
intervals.
The endpoints of the time intervals comprising `cnfine'
are interpreted as seconds past J2000 TDB.
See the Examples section below for a code example that
shows how to create a confinement window.
-Detailed_Output
cnfine is the input confinement window, updated if necessary so
the control area of its data array indicates the
window's size and cardinality. The window data are
unchanged.
result is the window of intervals, contained within the
confinement window `cnfine', on which the specified
phase angle constraint is satisfied.
The endpoints of the time intervals comprising `result'
are interpreted as seconds past J2000 TDB.
If `result' is non-empty on input, its contents will be
discarded before gfpa_c conducts its search.
-Parameters
SPICE_GF_CNVTOL
is the convergence tolerance used for finding endpoints
of the intervals comprising the result window.
SPICE_GF_CNVTOL is used to determine when binary
searches for roots should terminate: when a root is
bracketed within an interval of length SPICE_GF_CNVTOL,
the root is considered to have been found.
The accuracy, as opposed to precision, of roots found by
this routine depends on the accuracy of the input data.
In most cases, the accuracy of solutions will be
inferior to their precision.
SPICE_GF_CNVTOL is declared in the header file
SpiceGF.h.
-Exceptions
1) In order for this routine to produce correct results,
the step size must be appropriate for the problem at hand.
Step sizes that are too large may cause this routine to miss
roots; step sizes that are too small may cause this routine
to run unacceptably slowly and in some cases, find spurious
roots.
This routine does not diagnose invalid step sizes, except
that if the step size is non-positive, an error is signaled
by a routine in the call tree of this routine.
2) Due to numerical errors, in particular,
- Truncation error in time values
- Finite tolerance value
- Errors in computed geometric quantities
it is *normal* for the condition of interest to not always be
satisfied near the endpoints of the intervals comprising the
result window.
The result window may need to be contracted slightly by the
caller to achieve desired results. The SPICE window routine
wncond_c can be used to contract the result window.
3) If an error (typically cell overflow) occurs while performing
window arithmetic, the error will be diagnosed by a routine
in the call tree of this routine.
4) If the relational operator `relate' is not recognized, an
error is signaled by a routine in the call tree of this
routine.
5) If the aberration correction specifier contains an
unrecognized value, an error is signaled by a routine in the
call tree of this routine.
6) If `adjust' is negative, an error is signaled by a routine in
the call tree of this routine.
7) If either of the input body names do not map to NAIF ID
codes, an error is signaled by a routine in the call tree of
this routine.
8) If required ephemerides or other kernel data are not
available, an error is signaled by a routine in the call tree
of this routine.
9) If the workspace interval count is less than 1, the error
SPICE(VALUEOUTOFRANGE) will be signaled.
10) If the required amount of workspace memory cannot be
allocated, the error SPICE(MALLOCFAILURE) will be
signaled.
11) If the output SPICE window `result' has insufficient capacity to
contain the number of intervals on which the specified geometric
condition is met, the error will be diagnosed by a routine in
the call tree of this routine. If the result window has size
less than 2, the error SPICE(INVALIDDIMENSION) will be signaled
by this routine.
12) If any input string argument pointer is null, the error
SPICE(NULLPOINTER) will be signaled.
13) If any input string argument is empty, the error
SPICE(EMPTYSTRING) will be signaled.
14) If either input cell has type other than SpiceDouble,
the error SPICE(TYPEMISMATCH) is signaled.
15) An error signals from a routine in the call tree of
this routine for any transmit mode aberration correction.
-Files
Appropriate SPK and PCK kernels must be loaded by the calling
program before this routine is called.
The following data are required:
- SPK data: the calling application must load ephemeris data
for the targets, observer, and any intermediate objects in
a chain connecting the targets and observer that cover the
time period specified by the window CNFINE. If aberration
corrections are used, the states of target and observer
relative to the solar system barycenter must be calculable
from the available ephemeris data. Typically ephemeris data
are made available by loading one or more SPK files using
furnsh_c.
Kernel data are normally loaded once per program run, NOT every
time this routine is called.
-Particulars
ILLMN OBS
ILLMN as seen * /
from TARG at | /
ET - LT. | /
>|..../< phase angle
| /
. | /
. | /
. * TARG as seen from OBS
SEP . TARG at ET
. /
/
*
This routine determines if the caller-specified constraint
condition on the geometric event (phase angle) is satisfied for
any time intervals within the confinement window `cnfine'. If one
or more such time intervals exist, those intervals are added
to the `result' window.
This routine provides a simpler, but less flexible interface
than does the routine gfevnt_c for conducting searches for
illuminator-target-observer phase angle value events.
Applications that require support for progress reporting,
interrupt handling, non-default step or refinement functions
should call gfevnt_c rather than this routine.
Below we discuss in greater detail aspects of this routine's
solution process that are relevant to correct and efficient
use of this routine in user applications.
The Search Process
==================
Regardless of the type of constraint selected by the caller, this
routine starts the search for solutions by determining the time
periods, within the confinement window, over which the
phase angle function is monotone increasing and monotone
decreasing. Each of these time periods is represented by a SPICE
window. Having found these windows, all of the phase angle
function's local extrema within the confinement window are known.
Absolute extrema then can be found very easily.
Within any interval of these "monotone" windows, there will be at
most one solution of any equality constraint. Since the boundary
of the solution set for any inequality constraint is contained in
the union of
- the set of points where an equality constraint is met
- the boundary points of the confinement window
the solutions of both equality and inequality constraints can be
found easily once the monotone windows have been found.
Step Size
=========
The monotone windows (described above) are found using a two-step
search process. Each interval of the confinement window is
searched as follows: first, the input step size is used to
determine the time separation at which the sign of the rate of
change of phase angle will be sampled. Starting at
the left endpoint of an interval, samples will be taken at each
step. If a change of sign is found, a root has been bracketed; at
that point, the time at which the time derivative of the
phase angle is zero can be found by a refinement process, for
example, using a binary search.
Note that the optimal choice of step size depends on the lengths
of the intervals over which the phase angle function is monotone:
the step size should be shorter than the shortest of these
intervals (within the confinement window).
The optimal step size is *not* necessarily related to the lengths
of the intervals comprising the result window. For example, if
the shortest monotone interval has length 10 days, and if the
shortest result window interval has length 5 minutes, a step size
of 9.9 days is still adequate to find all of the intervals in the
result window. In situations like this, the technique of using
monotone windows yields a dramatic efficiency improvement over a
state-based search that simply tests at each step whether the
specified constraint is satisfied. The latter type of search can
miss solution intervals if the step size is longer than the
shortest solution interval.
Having some knowledge of the relative geometry of the target,
illumination source, and observer can be a valuable aid in
picking a reasonable step size. In general, the user can
compensate for lack of such knowledge by picking a very short
step size; the cost is increased computation time.
Note that the step size is not related to the precision with which
the endpoints of the intervals of the result window are computed.
That precision level is controlled by the convergence tolerance.
Convergence Tolerance
=====================
As described above, the root-finding process used by this routine
involves first bracketing roots and then using a search process to
locate them. "Roots" include times when extrema are attained and
times when the geometric quantity function is equal to a reference
value or adjusted extremum. All endpoints of the intervals comprising
the result window are either endpoints of intervals of the confinement
window or roots.
Once a root has been bracketed, a refinement process is used to
narrow down the time interval within which the root must lie.
This refinement process terminates when the location of the root
has been determined to within an error margin called the
"convergence tolerance." The convergence tolerance used by this
routine is set via the parameter SPICE_GF_CNVTOL.
The value of SPICE_GF_CNVTOL is set to a "tight" value so that the
tolerance doesn't limit the accuracy of solutions found by this
routine. In general the accuracy of input data will be the limiting
factor.
The user may change the convergence tolerance from the default
SPICE_GF_CNVTOL value by calling the routine gfstol_c, e.g.
gfstol_c( tolerance value in seconds )
Call gfstol_c prior to calling this routine. All subsequent
searches will use the updated tolerance value.
Searches over time windows of long duration may require use of
larger tolerance values than the default: the tolerance must be
large enough so that it, when added to or subtracted from the
confinement window's lower and upper bounds, yields distinct time
values.
Setting the tolerance tighter than SPICE_GF_CNVTOL is unlikely to be
useful, since the results are unlikely to be more accurate.
Making the tolerance looser will speed up searches somewhat,
since a few convergence steps will be omitted. However, in most
cases, the step size is likely to have a much greater effect
on processing time than would the convergence tolerance.
The Confinement Window
======================
The simplest use of the confinement window is to specify a time
interval within which a solution is sought. However, the
confinement window can, in some cases, be used to make searches
more efficient. Sometimes it's possible to do an efficient search
to reduce the size of the time period over which a relatively
slow search of interest must be performed. See the "CASCADE"
example program in gf.req for a demonstration.
-Examples
The numerical results shown for these examples may differ across
platforms. The results depend on the SPICE kernels used as
input, the compiler and supporting libraries, and the machine
specific arithmetic implementation.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: standard.tm
This meta-kernel is intended to support operation of SPICE
example programs. The kernels shown here should not be
assumed to contain adequate or correct versions of data
required by SPICE-based user applications.
In order for an application to use this meta-kernel, the
kernels referenced here must be present in the user's
current working directory.
The names and contents of the kernels referenced
by this meta-kernel are as follows:
File name Contents
--------- --------
de421.bsp Planetary ephemeris
pck00009.tpc Planet orientation and
radii
naif0009.tls Leapseconds
\begindata
KERNELS_TO_LOAD = ( 'de421.bsp',
'pck00009.tpc',
'naif0009.tls' )
\begintext
Example:
Determine the time windows from December 1, 2006 UTC to
January 31, 2007 UTC for which the sun-moon-earth configuration
phase angle satisfies the relation conditions with respect to a
reference value of .57598845 radians (the phase angle at
January 1, 2007 00:00:00.000 UTC, 33.001707 degrees). Also
determine the time windows corresponding to the local maximum and
minimum phase angles, and the absolute maximum and minimum phase
angles during the search interval. The configuration defines the
sun as the illuminator, the moon as the target, and the earth as
the observer.
#include <stdio.h>
#include "SpiceUsr.h"
#define TIMFMT "YYYY MON DD HR:MN:SC.###"
#define NINTVL 5000
#define TIMLEN 41
#define NLOOPS 7
int main()
{
/.
Local variables
./
SpiceChar begstr [ TIMLEN ];
SpiceChar endstr [ TIMLEN ];
SPICEDOUBLE_CELL ( cnfine, 2 );
SPICEDOUBLE_CELL ( result, NINTVL*2 );
SpiceDouble adjust;
SpiceDouble et0;
SpiceDouble et1;
SpiceDouble phaseq;
SpiceDouble refval;
SpiceDouble start;
SpiceDouble step;
SpiceDouble stop;
SpiceInt i;
SpiceInt j;
/.
Define the values for target, observer, illuminator, and
aberration correction.
./
ConstSpiceChar * target = "moon";
ConstSpiceChar * illmn = "sun";
ConstSpiceChar * abcorr = "lt+s";
ConstSpiceChar * obsrvr = "earth";
ConstSpiceChar * relate [NLOOPS] = { "=",
"<",
">",
"LOCMIN",
"ABSMIN",
"LOCMAX",
"ABSMAX",
};
/.
Load kernels.
./
furnsh_c ( "standard.tm" );
/.
Store the time bounds of our search interval in
the confinement window.
./
str2et_c ( "2006 DEC 01", &et0 );
str2et_c ( "2007 JAN 31", &et1 );
wninsd_c ( et0, et1, &cnfine );
/.
Search using a step size of 1 day (in units of seconds).
The reference value is 0.57598845 radians. We're not using the
adjustment feature, so we set ADJUST to zero.
./
step = spd_c();
refval = 0.57598845;
adjust = 0.0;
for ( j = 0; j < NLOOPS; j++ )
{
printf ( "Relation condition: %s\n", relate[j] );
/.
Perform the search. The SPICE window `result' contains
the set of times when the condition is met.
./
gfpa_c ( target, illmn, abcorr, obsrvr,
relate[j], refval, adjust, step,
NINTVL, &cnfine, &result );
/.
Display the results.
./
if ( wncard_c(&result) == 0 )
{
printf ( "Result window is empty.\n\n" );
}
else
{
for ( i = 0; i < wncard_c(&result); i++ )
{
/.
Fetch the endpoints of the Ith interval
of the result window.
./
wnfetd_c ( &result, i, &start, &stop );
phaseq = phaseq_c ( start, target, illmn, obsrvr, abcorr );
timout_c ( start, TIMFMT, TIMLEN, begstr );
printf ( "Start time = %s %16.9f\n", begstr, phaseq );
phaseq = phaseq_c ( stop, target, illmn, obsrvr, abcorr );
timout_c ( stop, TIMFMT, TIMLEN, endstr );
printf ( "Stop time = %s %16.9f\n", endstr, phaseq );
}
printf("\n");
}
}
return ( 0 );
}
The program outputs:
Relation condition: =
Start time = 2006 DEC 02 13:31:34.414 0.575988450
Stop time = 2006 DEC 02 13:31:34.414 0.575988450
Start time = 2006 DEC 07 14:07:55.470 0.575988450
Stop time = 2006 DEC 07 14:07:55.470 0.575988450
Start time = 2006 DEC 31 23:59:59.997 0.575988450
Stop time = 2006 DEC 31 23:59:59.997 0.575988450
Start time = 2007 JAN 06 08:16:25.512 0.575988450
Stop time = 2007 JAN 06 08:16:25.512 0.575988450
Start time = 2007 JAN 30 11:41:32.557 0.575988450
Stop time = 2007 JAN 30 11:41:32.557 0.575988450
Relation condition: <
Start time = 2006 DEC 02 13:31:34.414 0.575988450
Stop time = 2006 DEC 07 14:07:55.470 0.575988450
Start time = 2006 DEC 31 23:59:59.997 0.575988450
Stop time = 2007 JAN 06 08:16:25.512 0.575988450
Start time = 2007 JAN 30 11:41:32.557 0.575988450
Stop time = 2007 JAN 31 00:00:00.000 0.468279091
Relation condition: >
Start time = 2006 DEC 01 00:00:00.000 0.940714974
Stop time = 2006 DEC 02 13:31:34.414 0.575988450
Start time = 2006 DEC 07 14:07:55.470 0.575988450
Stop time = 2006 DEC 31 23:59:59.997 0.575988450
Start time = 2007 JAN 06 08:16:25.512 0.575988450
Stop time = 2007 JAN 30 11:41:32.557 0.575988450
Relation condition: LOCMIN
Start time = 2006 DEC 05 00:16:50.317 0.086121423
Stop time = 2006 DEC 05 00:16:50.317 0.086121423
Start time = 2007 JAN 03 14:18:31.977 0.079899769
Stop time = 2007 JAN 03 14:18:31.977 0.079899769
Relation condition: ABSMIN
Start time = 2007 JAN 03 14:18:31.977 0.079899769
Stop time = 2007 JAN 03 14:18:31.977 0.079899769
Relation condition: LOCMAX
Start time = 2006 DEC 20 14:09:10.392 3.055062862
Stop time = 2006 DEC 20 14:09:10.392 3.055062862
Start time = 2007 JAN 19 04:27:54.600 3.074603891
Stop time = 2007 JAN 19 04:27:54.600 3.074603891
Relation condition: ABSMAX
Start time = 2007 JAN 19 04:27:54.600 3.074603891
Stop time = 2007 JAN 19 04:27:54.600 3.074603891
-Restrictions
1) The kernel files to be used by this routine must be loaded
(normally using the CSPICE routine furnsh_c) before this
routine is called.
-Literature_References
None.
-Author_and_Institution
N.J. Bachman (JPL)
E.D. Wright (JPL)
-Version
-CSPICE Version 1.0.0, 15-JUL-2014 (EDW) (NJB)
-Index_Entries
GF phase angle search
-&
*/
{ /* Begin gfpa_c */
/*
Static local variables
*/
static SpiceInt nw = SPICE_GF_NWPA;
/*
Local variables
*/
doublereal * work;
SpiceInt nBytes;
/*
Participate in error tracing.
*/
if ( return_c() )
{
return;
}
chkin_c ( "gfpa_c" );
/*
Make sure cell data types are d.p.
*/
CELLTYPECHK2 ( CHK_STANDARD, "gfpa_c", SPICE_DP, cnfine, result );
/*
Initialize the input cells if necessary.
*/
CELLINIT2 ( cnfine, result );
/*
Check the input strings to make sure each pointer is non-null
and each string length is non-zero.
*/
CHKFSTR ( CHK_STANDARD, "gfpa_c", target );
CHKFSTR ( CHK_STANDARD, "gfpa_c", illmn );
CHKFSTR ( CHK_STANDARD, "gfpa_c", abcorr );
CHKFSTR ( CHK_STANDARD, "gfpa_c", obsrvr );
CHKFSTR ( CHK_STANDARD, "gfpa_c", relate );
/*
Check the workspace size; some mallocs have a violent
dislike for negative allocation amounts. To be safe,
rule out a count of zero intervals as well.
*/
if ( nintvls < 1 )
{
setmsg_c ( "The specified workspace interval count # was "
"less than the minimum allowed value (1)." );
errint_c ( "#", nintvls );
sigerr_c ( "SPICE(VALUEOUTOFRANGE)" );
chkout_c ( "gfpa_c" );
return;
}
/*
Allocate the workspace.
We have `nw' "doublereal" cells, each having cell size 2*nintvls.
Each cell also has a control area containing SPICE_CELL_CTRLSZ
double precision values.
*/
nintvls = nintvls * 2;
nBytes = ( nintvls + SPICE_CELL_CTRLSZ ) * nw * sizeof(SpiceDouble);
work = (doublereal *) alloc_SpiceMemory( nBytes );
if ( !work )
{
setmsg_c ( "Workspace allocation of # bytes failed due to "
"malloc failure" );
errint_c ( "#", nBytes );
sigerr_c ( "SPICE(MALLOCFAILURE)" );
chkout_c ( "gfpa_c" );
return;
}
/*
Let the f2'd routine do the work.
*/
gfpa_ ( ( char * ) target,
( char * ) illmn,
( char * ) abcorr,
( char * ) obsrvr,
( char * ) relate,
( doublereal * ) &refval,
( doublereal * ) &adjust,
( doublereal * ) &step,
( doublereal * ) (cnfine->base),
( integer * ) &nintvls,
( integer * ) &nw,
( doublereal * ) work,
( doublereal * ) (result->base),
( ftnlen ) strlen(target),
( ftnlen ) strlen(illmn),
( ftnlen ) strlen(abcorr),
( ftnlen ) strlen(obsrvr),
( ftnlen ) strlen(relate) );
/*
De-allocate the workspace.
*/
free_SpiceMemory( work );
/*
Sync the output cell.
*/
if ( !failed_c() )
{
zzsynccl_c ( F2C, result ) ;
}
ALLOC_CHECK;
chkout_c ( "gfpa_c" );
} /* End gfpa_c */