/// <summary>
/// Returns the topo-centric (azimuth, elevation, etc.) coordinates for
/// a target object described by the given ECI coordinates.
/// </summary>
/// <param name="eci">The ECI coordinates of the target object.</param>
/// <returns>The look angle to the target object.</returns>
public TopoTime GetLookAngle(EciTime eci)
{
// Calculate the ECI coordinates for this Site object at the time
// of interest.
Julian date = eci.Date;
EciTime eciSite = PositionEci(date);
Vector vecRgRate = new Vector(eci.Velocity.X - eciSite.Velocity.X,
eci.Velocity.Y - eciSite.Velocity.Y,
eci.Velocity.Z - eciSite.Velocity.Z);
double x = eci.Position.X - eciSite.Position.X;
double y = eci.Position.Y - eciSite.Position.Y;
double z = eci.Position.Z - eciSite.Position.Z;
double w = Math.Sqrt(Globals.Sqr(x) + Globals.Sqr(y) + Globals.Sqr(z));
Vector vecRange = new Vector(x, y, z, w);
// The site's Local Mean Sidereal Time at the time of interest.
double theta = date.ToLmst(LongitudeRad);
double sin_lat = Math.Sin(LatitudeRad);
double cos_lat = Math.Cos(LatitudeRad);
double sin_theta = Math.Sin(theta);
double cos_theta = Math.Cos(theta);
double top_s = sin_lat * cos_theta * vecRange.X +
sin_lat * sin_theta * vecRange.Y -
cos_lat * vecRange.Z;
double top_e = -sin_theta * vecRange.X +
cos_theta * vecRange.Y;
double top_z = cos_lat * cos_theta * vecRange.X +
cos_lat * sin_theta * vecRange.Y +
sin_lat * vecRange.Z;
double az = Math.Atan(-top_e / top_s);
if (top_s > 0.0)
{
az += Globals.Pi;
}
if (az < 0.0)
{
az += 2.0 * Globals.Pi;
}
double el = Math.Asin(top_z / vecRange.W);
double rate = (vecRange.X * vecRgRate.X +
vecRange.Y * vecRgRate.Y +
vecRange.Z * vecRgRate.Z) / vecRange.W;
TopoTime topo = new TopoTime(az, // azimuth, radians
el, // elevation, radians
vecRange.W, // range, km
rate, // rate, km / sec
eci.Date);
#if WANT_ATMOSPHERIC_CORRECTION
// Elevation correction for atmospheric refraction.
// Reference: Astronomical Algorithms by Jean Meeus, pp. 101-104
// Note: Correction is meaningless when apparent elevation is below horizon
topo.ElevationRad += Globals.ToRadians((1.02 /
Math.Tan(Globals.ToRadians(Globals.ToDegrees(el) + 10.3 /
(Globals.ToDegrees(el) + 5.11)))) / 60.0);
if (topo.ElevationRad < 0.0)
{
topo.ElevationRad = el; // Reset to true elevation
}
if (topo.ElevationRad > (Math.PI / 2.0))
{
topo.ElevationRad = (Math.PI / 2.0);
}
#endif
return(topo);
}