//------------------------------------------------------------------------------------
double GMST(DayTime t)
{
try {
// days' since epoch = +/-(integer+0.5)
double days = t.JD() - 2451545;
int d=int(days);
if(d < 0 && days==double(d)) d++;
days = d + (days<0.0 ? -0.5 : 0.5);
double Tp = days/36525.0;
// Compute GMST
double G;
//G = 24060.0 + 50.54841 + 8640184.812866*Tp; // seconds (24060s = 6h 41min)
//G /= 86400.0; // instead, divide the above equation by 86400.0 manually...
G = 0.27847222 + 0.00058505104167 + 100.0021390378009*Tp;
G += (0.093104 - 6.2e-6*Tp)*Tp*Tp/86400.0; // seconds/86400 = circles
double r=1.002737909350795 + (5.9006e-11 - 5.9e-15*Tp)*Tp;
G += r*t.secOfDay()/86400.0; // circles
G *= 360.0; // degrees
//G = fmod(G,360.0);
//if(G < -180.0) G += 360.0;
//if(G > 180.0) G -= 360.0;
return G;
}
catch(Exception& e) { GPSTK_RETHROW(e); }
catch(exception& e) { Exception E("std except: "+string(e.what())); GPSTK_THROW(E); }
catch(...) { Exception e("Unknown exception"); GPSTK_THROW(e); }
}
//------------------------------------------------------------------------------------
// Solar ephemeris, in ECEF coordinates.
// Accuracy is about 1 arcminute, when t is within 2 centuries of 2000.
// Ref. Astronomical Almanac pg C24, as presented on USNO web site.
// input
// t epoch of interest
// output
// lat,lon,R latitude, longitude and distance (deg,deg,m in ECEF) of sun at t.
// AR apparent angular radius of sun as seen at Earth (deg) at t.
void SolarPosition(DayTime t, double& lat, double& lon, double& R, double& AR)
{
try {
//const double mPerAU = 149598.0e6;
double D; // days since J2000
double g,q;
double L; // sun's geocentric apparent ecliptic longitude (deg)
//double b=0; // sun's geocentric apparent ecliptic latitude (deg)
double e; // mean obliquity of the ecliptic (deg)
//double R; // sun's distance from Earth (m)
double RA; // sun's right ascension (deg)
double DEC; // sun's declination (deg)
//double AR; // sun's apparent angular radius as seen at Earth (deg)
D = t.JD() - 2451545.0;
g = (357.529 + 0.98560028 * D) * DEG_TO_RAD;
q = 280.459 + 0.98564736 * D;
L = (q + 1.915 * ::sin(g) + 0.020 * ::sin(2*g)) * DEG_TO_RAD;
e = (23.439 - 0.00000036 * D) * DEG_TO_RAD;
RA = atan2(::cos(e)*::sin(L),::cos(L)) * RAD_TO_DEG;
DEC = ::asin(::sin(e)*::sin(L)) * RAD_TO_DEG;
//equation of time = apparent solar time minus mean solar time
//= [q-RA (deg)]/(15deg/hr)
// compute the hour angle of the vernal equinox = GMST and convert RA to lon
lon = fmod(RA-GMST(t),360.0);
if(lon < -180.0) lon += 360.0;
if(lon > 180.0) lon -= 360.0;
lat = DEC;
// ECEF unit vector in direction Earth to sun
//xhat = ::cos(lat*DEG_TO_RAD)*::cos(lon*DEG_TO_RAD);
//yhat = ::cos(lat*DEG_TO_RAD)*::sin(lon*DEG_TO_RAD);
//zhat = ::sin(lat*DEG_TO_RAD);
// R in AU
R = 1.00014 - 0.01671 * ::cos(g) - 0.00014 * ::cos(2*g);
// apparent angular radius in degrees
AR = 0.2666/R;
// convert to meters
R *= 149598.0e6;
}
catch(Exception& e) { GPSTK_RETHROW(e); }
catch(exception& e) { Exception E("std except: "+string(e.what())); GPSTK_THROW(E); }
catch(...) { Exception e("Unknown exception"); GPSTK_THROW(e); }
}
