// ///////////////////////////////////////////////////////////////////////////
// getLookAngle()
// Return the topocentric (azimuth, elevation, etc.) coordinates for a target
// object described by the input ECI coordinates.
public CoordTopo getLookAngle(Eci eci)
{
// Calculate the ECI coordinates for this Site object at the time
// of interest.
Julian date = eci.getDate();
Eci eciSite = new Eci(m_geo, date);
// The Site ECI units are km-based; ensure target ECI units are same
if (!eci.UnitsAreKm())
throw new Exception("ECI units must be kilometer-based");
Vector vecRgRate = new Vector(eci.getVel().X - eciSite.getVel().X,
eci.getVel().Y - eciSite.getVel().Y,
eci.getVel().Z - eciSite.getVel().Z);
double x = eci.getPos().X - eciSite.getPos().X;
double y = eci.getPos().Y - eciSite.getPos().Y;
double z = eci.getPos().Z - eciSite.getPos().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(Lon);
double sin_lat = Math.Sin(Lat);
double cos_lat = Math.Cos(Lat);
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;
CoordTopo topo = new CoordTopo(az, // azimuth, radians
el, // elevation, radians
vecRange.W, // range, km
rate); // rate, km / sec
#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.m_El += Globals.Deg2Rad((1.02 /
Math.Tan(Globals.Deg2Rad(Globals.Rad2Deg(el) + 10.3 /
(Globals.Rad2Deg(el) + 5.11)))) / 60.0);
if (topo.m_El < 0.0)
topo.m_El = el; // Reset to true elevation
if (topo.m_El > (Globals.PI / 2))
topo.m_El = (Globals.PI / 2);
#endif
return topo;
}
private bool InitFromTime(DateTime utc)
{
utc = Globals.InsureUTC(utc);
TimeSpan ts = utc - this.EpochUTC;
double minutes = ts.TotalMinutes;
Eci position = new Eci();
try
{
position = this._orbit.getPosition(minutes);
}
catch (Exception)
{
return false;
}
this._lat = new Angle(position.toGeo().LatDeg, AngleUnits.DEGREES);
this._lon = new Angle(position.toGeo().LonDeg, AngleUnits.DEGREES);
this._alt = position.toGeo().Altitude;
Vector velVec = position.getVel();
this._velX = velVec.X;
this._velY = velVec.Y;
this._velZ = velVec.Z;
this._vel = Math.Sqrt(Math.Pow(this._velX, 2.0) + Math.Pow(this._velY, 2.0) + Math.Pow(this._velZ, 2.0));
CoordTopo ct = this.Site.getLookAngle(position);
this._az = new Angle(ct.Azimuth);
this._el = new Angle(ct.Elevation);
this._rg = new Distance(ct.Range, DistanceUnits.KILOMETERS);
this._rr = ct.RangeRate;
this.MakeFootprint(utc);
return true;
}