/* OK, now put all of the above together to give the observatory coordinates
* wrt SSBary in ICRS/AU.
*/
void
earth_ssbary(double jd,
int obscode,
double *xtopo, double *ytopo, double *ztopo)
{
double xgeo[3], xobs,yobs,zobs;
/* Return zero if we are "observing" from ssbary */
if (obscode==OBSCODE_SSBARY) {
*xtopo = *ytopo = *ztopo = 0.;
return;
}
/* Use Horizons to get the coordinates of geocenter */
geocenter_ssbary(jd,xgeo);
/* fprintf(stderr,"geocenter ICRS: %g %g %g\n",xgeo[0],xgeo[1],xgeo[2]); */
/* Now get the geocenter->observatory vector */
observatory_geocenter(jd, obscode, &xobs, &yobs, &zobs);
/* fprintf(stderr,"observ ICRS: %g %g %g\n", xobs, yobs, zobs); */
/* Add in to give observatory coords in ICRS */
*xtopo = xgeo[0] + xobs;
*ytopo = xgeo[1] + yobs;
*ztopo = xgeo[2] + zobs;
return;
}
/* Return the angle btwn zenith (anti-earth) direction and target*/
double
zenith_angle(OBSERVATION *obs)
{
double xobs, yobs, zobs, r;
double xec, yec, zec, cosb;
/* Get the anti-Earth vector (in ICRS) for this observation */
observatory_geocenter(obs->obstime/DAY+jd0, obs->obscode,
&xobs, &yobs, &zobs);
r = sqrt(xobs*xobs+yobs*yobs+zobs*zobs);
if (r<=0.) {
fprintf(stderr,"Non-positive geocentric radius in zenith_angle()\n");
exit(1);
}
xobs /=r; yobs/=r; zobs/=r;
/* Rotate this ICRS vector into ecliptic, then projected coords */
xyz_eq_to_ec(xobs, yobs, zobs, &xec, &yec, &zec,NULL);
xyz_ec_to_proj(xec, yec, zec, &xobs, &yobs, &zobs, lat0, lon0, NULL);
/* dot the observation direction into the (Earth->bary) vector*/
cosb = obs->thetax*xobs + obs->thetay*yobs + zobs;
cosb /= sqrt(1+obs->thetax*obs->thetax+obs->thetay*obs->thetay);
return acos(cosb);
}
