/** Assemble a NMEA GPGSV message and send it out NMEA USARTs.
* NMEA GPGSV message contains GPS satellites in view.
*
* \param n_used Number of satellites currently being tracked.
* \param nav_meas Array of navigation_measurement structs.
* \param soln Pointer to gnss_solution struct.
*/
void nmea_gpgsv(u8 n_used, const navigation_measurement_t *nav_meas,
const gnss_solution *soln)
{
if (n_used == 0)
return;
u8 n_mess = (n_used + 3) / 4;
u8 n = 0;
double az, el;
for (u8 i = 0; i < n_mess; i++) {
NMEA_SENTENCE_START(120);
NMEA_SENTENCE_PRINTF("$GPGSV,%d,%d,%d", n_mess, i+1, n_used);
for (u8 j = 0; j < 4; j++) {
if (n < n_used) {
wgsecef2azel(nav_meas[n].sat_pos, soln->pos_ecef, &az, &el);
NMEA_SENTENCE_PRINTF(",%02d,%02d,%03d,%02d",
nav_meas[n].prn + 1,
(u8)round(el * R2D),
(u16)round(az * R2D),
(u8)round(nav_meas[n].snr)
);
} else {
NMEA_SENTENCE_PRINTF(",,,,");
}
n++;
}
NMEA_SENTENCE_DONE();
}
}
/** Calculate the azimuth and elevation of a satellite from a reference
* position given the satellite almanac.
*
* \param alm Pointer to an almanac structure for the satellite of interest.
* \param t GPS time of week at which to calculate the az/el.
* \param week GPS week number modulo 1024 or pass -1 to assume within one
* half-week of the almanac time of applicability.
* \param ref ECEF coordinates of the reference point from which the azimuth
* and elevation is to be determined, passed as [X, Y, Z], all in
* meters.
* \param az Pointer to where to store the calculated azimuth output.
* \param el Pointer to where to store the calculated elevation output.
*/
void calc_sat_az_el_almanac(almanac_t* alm, double t, s16 week,
double ref[3], double* az, double* el)
{
double sat_pos[3];
calc_sat_state_almanac(alm, t, week, sat_pos, 0);
wgsecef2azel(sat_pos, ref, az, el);
}
/** Assemble an NMEA GPRMC message and send it out NMEA USARTs.
* NMEA RMC contains minimum GPS data
*
* \param nav_meas Pointer to navigation_measurement struct.
* \param soln Pointer to gnss_solution struct
* \param gps_t Pointer to the current GPS Time
*/
void nmea_gprmc(const navigation_measurement_t *nav_meas,
const gnss_solution *soln, const gps_time_t *gps_t)
{
/* NMEA Parameters
* Ex.
* $GPRMC,220516,A,5133.82,N,00042.24,W,173.8,231.8,130694,004.2,W*70
* | | | | | | | | | | | | |
* Command | | Lat N/S | | | | Date Stamp | W/E |
* Time (UTC) | Long W/E | True Course | Cksum
* Validity (A-OK) Speed Variation
* Variation is ignored as we have no way to maintain that information
* currently
*/
time_t unix_t;
struct tm t;
unix_t = gps2time(*gps_t);
gmtime_r(&unix_t, &t);
double frac_s = fmod(gps_t->tow, 1.0);
s16 lat_deg = R2D * (soln->pos_llh[0]);
double lat_min = MINUTES(soln->pos_llh[0]);
s16 lon_deg = R2D * (soln->pos_llh[1]);
double lon_min = MINUTES(soln->pos_llh[1]);
lat_deg = abs(lat_deg);
lon_deg = abs(lon_deg);
char lat_dir = soln->pos_llh[0] < 0 ? 'S' : 'N';
char lon_dir = soln->pos_llh[1] < 0 ? 'W' : 'E';
float velocity;
float x,y,z;
x = soln->vel_ned[0];
y = soln->vel_ned[1];
z = soln->vel_ned[2];
float course = atan2(y,x);
/* Conversion to magnitue knots */
velocity = MS2KNOTTS(x,y,z);
double az, el;
wgsecef2azel(nav_meas[0].sat_pos, soln->pos_ecef, &az, &el);
char buf[100];
u8 n = sprintf(buf,
"$GPRMC,%02d%02d%06.3f,A," /* Command, Time (UTC), Valid */
"%02d%010.7f,%c,%03d%010.7f,%c," /* Lat/Lon */
"%06.2f,%05.1f," /* Speed, Course */
"%02d%02d%02d," /* Date Stamp */
",", /* Variation */
t.tm_hour, t.tm_min, t.tm_sec + frac_s,
lat_deg, lat_min, lat_dir, lon_deg, lon_min, lon_dir,
velocity, course * R2D,
t.tm_mday, t.tm_mon, t.tm_year-100);
u8 sum = nmea_checksum(buf);
sprintf(buf + n, "*%02X\r\n", sum);
nmea_output(buf);
}
/** Using available almanac and ephemeris information, determine
* whether a satellite is in view and the range of doppler frequencies
* in which we expect to find it.
*
* \param prn 0-indexed PRN
* \param t Time at which to evaluate ephemeris and almanac (typically system's
* estimate of current time)
* \param dopp_hint_low, dopp_hint_high Pointers to store doppler search range
* from ephemeris or almanac, if available and elevation > mask
* \return Score (higher is better)
*/
static u16 manage_warm_start(u8 prn, gps_time_t t,
float *dopp_hint_low, float *dopp_hint_high)
{
/* Do we have any idea where/when we are? If not, no score. */
/* TODO: Stricter requirement on time and position uncertainty?
We ought to keep track of a quantitative uncertainty estimate. */
if (time_quality < TIME_GUESS &&
position_quality < POSITION_GUESS)
return SCORE_COLDSTART;
float el = 0;
double el_d, _, dopp_hint = 0, dopp_uncertainty = DOPP_UNCERT_ALMANAC;
/* Do we have a suitable ephemeris for this sat? If so, use
that in preference to the almanac. */
if (ephemeris_good(&es[prn], t)) {
double sat_pos[3], sat_vel[3], el_d;
calc_sat_state(&es[prn], t, sat_pos, sat_vel, &_, &_);
wgsecef2azel(sat_pos, position_solution.pos_ecef, &_, &el_d);
el = (float)(el_d) * R2D;
if (el < elevation_mask)
return SCORE_BELOWMASK;
vector_subtract(3, sat_pos, position_solution.pos_ecef, sat_pos);
vector_normalize(3, sat_pos);
/* sat_pos now holds unit vector from us to satellite */
vector_subtract(3, sat_vel, position_solution.vel_ecef, sat_vel);
/* sat_vel now holds velocity of sat relative to us */
dopp_hint = -GPS_L1_HZ * (vector_dot(3, sat_pos, sat_vel) / GPS_C
+ position_solution.clock_bias);
/* TODO: Check sign of receiver frequency offset correction */
if (time_quality >= TIME_FINE)
dopp_uncertainty = DOPP_UNCERT_EPHEM;
} else if (almanac[prn].valid) {
calc_sat_az_el_almanac(&almanac[prn], t.tow, t.wn-1024,
position_solution.pos_ecef, &_, &el_d);
el = (float)(el_d) * R2D;
if (el < elevation_mask)
return SCORE_BELOWMASK;
dopp_hint = -calc_sat_doppler_almanac(&almanac[prn], t.tow, t.wn,
position_solution.pos_ecef);
} else {
return SCORE_COLDSTART; /* Couldn't determine satellite state. */
}
/* Return the doppler hints and a score proportional to elevation */
*dopp_hint_low = dopp_hint - dopp_uncertainty;
*dopp_hint_high = dopp_hint + dopp_uncertainty;
return SCORE_COLDSTART + SCORE_WARMSTART * el / 90.f;
}
/** Assemble a NMEA GPGSV message and send it out NMEA USARTs.
* NMEA GPGSV message contains GPS satellites in view.
*
* \param n_used Number of satellites currently being tracked.
* \param nav_meas Pointer to navigation_measurement struct.
* \param soln Pointer to gnss_solution struct.
*/
void nmea_gpgsv(u8 n_used, navigation_measurement_t *nav_meas,
gnss_solution *soln)
{
if (n_used == 0)
return;
u8 n_mess = (n_used + 3) / 4;
char buf[80];
char *buf0 = buf + sprintf(buf, "$GPGSV,%d,", n_mess);
char *bufp = buf0;
u8 n = 0;
double az, el;
for (u8 i = 0; i < n_mess; i++) {
bufp = buf0;
bufp += sprintf(bufp, "%d,%d", i + 1, n_used);
for (u8 j = 0; j < 4; j++) {
if (n < n_used) {
wgsecef2azel(nav_meas[n].sat_pos, soln->pos_ecef, &az, &el);
bufp += sprintf(
bufp, ",%02d,%02d,%03d,%02d",
nav_meas[n].prn + 1,
(u8)round(el * 180.0 / M_PI),
(u16)round(az * 180.0 / M_PI),
(u8)(10.0 * nav_meas[n].snr)
);
} else {
bufp += sprintf(bufp, ",,,,");
}
n++;
}
sprintf(bufp, "*%02X\r\n", nmea_checksum(buf));
nmea_output(buf);
}
}
/** Assemble an NMEA GPVTG message and send it out NMEA USARTs.
* NMEA VTG contains course and speed
*
* \param nav_meas Pointer to navigation_measurement struct.
* \param soln Pointer to gnss_solution struct
*/
void nmea_gpvtg(const navigation_measurement_t *nav_meas,
const gnss_solution *soln)
{
/* NMEA Parameters for GPVTG
* Ex.
* $GPVTG,054.7,T,034.4,M,005.5,N,010.2,K
* | | | | | | | | |
* Command | 'T' | 'M' | 'N' | 'K'
* True Course | Speed (K) |
* Mag. course Speed (km/hr)
*/
double az, el;
wgsecef2azel(nav_meas[0].sat_pos, soln->pos_ecef, &az, &el);
float vknots, vkmhr;
float x,y,z;
x = soln->vel_ned[0];
y = soln->vel_ned[1];
z = soln->vel_ned[2];
float course = atan2(y,x);
/* Conversion to magnitue knots */
vknots = MS2KNOTTS(x,y,z);
/* Conversion to magnitue km/hr */
vkmhr = MS2KMHR(x,y,z);
char buf[80];
u8 n = sprintf(buf,
"$GPVTG,%05.1f,T," /* Command, course, */
",M," /* Magnetic Course (omitted) */
"%06.2f,N,%06.2f,K", /* Speed (knots, km/hr) */
course* R2D,
vknots, vkmhr);
u8 sum = nmea_checksum(buf);
sprintf(buf + n, "*%02X\r\n", sum);
nmea_output(buf);
}
/** Using available almanac and ephemeris information, determine
* whether a satellite is in view and the range of doppler frequencies
* in which we expect to find it.
*
* \param prn 0-indexed PRN
* \param t Time at which to evaluate ephemeris and almanac (typically system's
* estimate of current time)
* \param dopp_hint_low, dopp_hint_high Pointers to store doppler search range
* from ephemeris or almanac, if available and elevation > mask
* \return Score (higher is better)
*/
static u16 manage_warm_start(gnss_signal_t sid, const gps_time_t* t,
float *dopp_hint_low, float *dopp_hint_high)
{
/* Do we have any idea where/when we are? If not, no score. */
/* TODO: Stricter requirement on time and position uncertainty?
We ought to keep track of a quantitative uncertainty estimate. */
if (time_quality < TIME_GUESS &&
position_quality < POSITION_GUESS)
return SCORE_COLDSTART;
float el = 0;
double _, dopp_hint = 0, dopp_uncertainty = DOPP_UNCERT_ALMANAC;
bool ready = false;
/* Do we have a suitable ephemeris for this sat? If so, use
that in preference to the almanac. */
const ephemeris_t *e = ephemeris_get(sid);
u8 eph_valid;
s8 ss_ret;
double sat_pos[3], sat_vel[3], el_d;
ephemeris_lock();
eph_valid = ephemeris_valid(e, t);
if (eph_valid) {
ss_ret = calc_sat_state(e, t, sat_pos, sat_vel, &_, &_);
}
ephemeris_unlock();
if (eph_valid && (ss_ret == 0)) {
wgsecef2azel(sat_pos, position_solution.pos_ecef, &_, &el_d);
el = (float)(el_d) * R2D;
if (el < elevation_mask)
return SCORE_BELOWMASK;
vector_subtract(3, sat_pos, position_solution.pos_ecef, sat_pos);
vector_normalize(3, sat_pos);
/* sat_pos now holds unit vector from us to satellite */
vector_subtract(3, sat_vel, position_solution.vel_ecef, sat_vel);
/* sat_vel now holds velocity of sat relative to us */
dopp_hint = -GPS_L1_HZ * (vector_dot(3, sat_pos, sat_vel) / GPS_C
+ position_solution.clock_bias);
/* TODO: Check sign of receiver frequency offset correction */
if (time_quality >= TIME_FINE)
dopp_uncertainty = DOPP_UNCERT_EPHEM;
ready = true;
}
if(!ready) {
const almanac_t *a = &almanac[sid_to_global_index(sid)];
if (a->valid &&
calc_sat_az_el_almanac(a, t, position_solution.pos_ecef,
&_, &el_d) == 0) {
el = (float)(el_d) * R2D;
if (el < elevation_mask)
return SCORE_BELOWMASK;
if (calc_sat_doppler_almanac(a, t, position_solution.pos_ecef,
&dopp_hint) != 0) {
return SCORE_COLDSTART;
}
dopp_hint = -dopp_hint;
} else {
return SCORE_COLDSTART; /* Couldn't determine satellite state. */
}
}
/* Return the doppler hints and a score proportional to elevation */
*dopp_hint_low = dopp_hint - dopp_uncertainty;
*dopp_hint_high = dopp_hint + dopp_uncertainty;
return SCORE_COLDSTART + SCORE_WARMSTART * el / 90.f;
}