// /////////////////////////////////////////////////////////////////// // 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); }