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//
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// cEci.cpp
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//
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// Copyright (c) 2002-2003 Michael F. Henry
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//
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#include "stdafx.h"
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#include "cEci.h"
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#include "globals.h"
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//////////////////////////////////////////////////////////////////////
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// cEci Class
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//////////////////////////////////////////////////////////////////////
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cEci::cEci(const cVector &pos,
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const cVector &vel,
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const cJulian &date,
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bool IsAeUnits /* = true */)
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{
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m_pos = pos;
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m_vel = vel;
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m_date = date;
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m_VecUnits = (IsAeUnits ? UNITS_AE : UNITS_NONE);
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}
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//////////////////////////////////////////////////////////////////////
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// cEci(cCoordGeo&, cJulian&)
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// Calculate the ECI coordinates of the location "geo" at time "date".
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// Assumes geo coordinates are km-based.
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// Assumes the earth is an oblate spheroid as defined in WGS '72.
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// Reference: The 1992 Astronomical Almanac, page K11
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// Reference: www.celestrak.com (Dr. TS Kelso)
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cEci::cEci(const cCoordGeo &geo, const cJulian &date)
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{
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m_VecUnits = UNITS_KM;
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double mfactor = TWOPI * (OMEGA_E / SEC_PER_DAY);
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double lat = geo.m_Lat;
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double lon = geo.m_Lon;
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double alt = geo.m_Alt;
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// Calculate Local Mean Sidereal Time (theta)
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double theta = date.toLMST(lon);
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double c = 1.0 / sqrt(1.0 + F * (F - 2.0) * sqr(sin(lat)));
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double s = sqr(1.0 - F) * c;
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double achcp = (XKMPER_WGS72 * c + alt) * cos(lat);
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m_date = date;
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m_pos.m_x = achcp * cos(theta); // km
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m_pos.m_y = achcp * sin(theta); // km
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m_pos.m_z = (XKMPER_WGS72 * s + alt) * sin(lat); // km
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m_pos.m_w = sqrt(sqr(m_pos.m_x) +
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sqr(m_pos.m_y) +
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sqr(m_pos.m_z)); // range, km
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m_vel.m_x = -mfactor * m_pos.m_y; // km / sec
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m_vel.m_y = mfactor * m_pos.m_x;
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m_vel.m_z = 0.0;
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m_vel.m_w = sqrt(sqr(m_vel.m_x) + // range rate km/sec^2
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sqr(m_vel.m_y));
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}
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//////////////////////////////////////////////////////////////////////////////
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// toGeo()
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// Return the corresponding geodetic position (based on the current ECI
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// coordinates/Julian date).
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// Assumes the earth is an oblate spheroid as defined in WGS '72.
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// Side effects: Converts the position and velocity vectors to km-based units.
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// Reference: The 1992 Astronomical Almanac, page K12.
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// Reference: www.celestrak.com (Dr. TS Kelso)
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cCoordGeo cEci::toGeo()
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{
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ae2km(); // Vectors must be in kilometer-based units
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double theta = AcTan(m_pos.m_y, m_pos.m_x);
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double lon = fmod(theta - m_date.toGMST(), TWOPI);
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if (lon < 0.0)
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lon += TWOPI; // "wrap" negative modulo
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double r = sqrt(sqr(m_pos.m_x) + sqr(m_pos.m_y));
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double e2 = F * (2.0 - F);
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double lat = AcTan(m_pos.m_z, r);
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const double delta = 1.0e-07;
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double phi;
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double c;
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do
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{
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phi = lat;
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c = 1.0 / sqrt(1.0 - e2 * sqr(sin(phi)));
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lat = AcTan(m_pos.m_z + XKMPER_WGS72 * c * e2 * sin(phi), r);
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}
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while (fabs(lat - phi) > delta);
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double alt = r / cos(lat) - XKMPER_WGS72 * c;
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return cCoordGeo(lat, lon, alt); // radians, radians, kilometers
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}
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//////////////////////////////////////////////////////////////////////////////
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// ae2km()
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// Convert the position and velocity vector units from AE-based units
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// to kilometer based units.
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void cEci::ae2km()
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{
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if (UnitsAreAe())
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{
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MulPos(XKMPER_WGS72 / AE); // km
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MulVel((XKMPER_WGS72 / AE) * (MIN_PER_DAY / 86400)); // km/sec
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m_VecUnits = UNITS_KM;
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}
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}
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