// ///////////////////////////////////////////////////////////////////
// Calculate the ECI coordinates of the location "geo" at time "date".
// Assumes geo coordinates are km-based.
// Assumes the earth is an oblate spheroid as defined in WGS '72.
// Reference: The 1992 Astronomical Almanac, page K11
// Reference: www.celestrak.com (Dr. TS Kelso)
public Eci(CoordGeo geo, Julian date)
{
m_VectorUnits = VectorUnits.Km;
double mfactor = Globals.TWOPI * (Globals.OMEGA_E / Globals.SEC_PER_DAY);
double lat = geo.Latitude;
double lon = geo.Longitude;
double alt = geo.Altitude;
// Calculate Local Mean Sidereal Time (theta)
double theta = date.toLMST(lon);
double c = 1.0 / Math.Sqrt(1.0 + Globals.F * (Globals.F - 2.0) * Globals.Sqr(Math.Sin(lat)));
double s = Globals.Sqr(1.0 - Globals.F) * c;
double achcp = (Globals.XKMPER * c + alt) * Math.Cos(lat);
m_Date = date;
m_Position = new Vector();
m_Position.X = achcp * Math.Cos(theta); // km
m_Position.Y = achcp * Math.Sin(theta); // km
m_Position.Z = (Globals.XKMPER * s + alt) * Math.Sin(lat); // km
m_Position.W = Math.Sqrt(Globals.Sqr(m_Position.X) +
Globals.Sqr(m_Position.Y) +
Globals.Sqr(m_Position.Z)); // range, km
m_Velocity = new Vector();
m_Velocity.X = -mfactor * m_Position.Y; // km / sec
m_Velocity.Y = mfactor * m_Position.X;
m_Velocity.Z = 0.0;
m_Velocity.W = Math.Sqrt(Globals.Sqr(m_Velocity.X) + // range rate km/sec^2
Globals.Sqr(m_Velocity.Y));
}
// ///////////////////////////////////////////////////////////////////////////
// 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.Date;
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.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(Longitude);
double sin_lat = Math.Sin(Latitude);
double cos_lat = Math.Cos(Latitude);
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);
}
