TLEPropagator.java
/* Copyright 2002-2019 CS Systèmes d'Information
* Licensed to CS Systèmes d'Information (CS) under one or more
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package org.orekit.propagation.analytical.tle;
import org.hipparchus.geometry.euclidean.threed.Vector3D;
import org.hipparchus.util.FastMath;
import org.hipparchus.util.MathUtils;
import org.orekit.attitudes.Attitude;
import org.orekit.attitudes.AttitudeProvider;
import org.orekit.errors.OrekitException;
import org.orekit.errors.OrekitMessages;
import org.orekit.frames.Frame;
import org.orekit.frames.FramesFactory;
import org.orekit.orbits.CartesianOrbit;
import org.orekit.orbits.Orbit;
import org.orekit.propagation.SpacecraftState;
import org.orekit.propagation.analytical.AbstractAnalyticalPropagator;
import org.orekit.time.AbsoluteDate;
import org.orekit.utils.PVCoordinates;
/** This class provides elements to propagate TLE's.
* <p>
* The models used are SGP4 and SDP4, initially proposed by NORAD as the unique convenient
* propagator for TLE's. Inputs and outputs of this propagator are only suited for
* NORAD two lines elements sets, since it uses estimations and mean values appropriate
* for TLE's only.
* </p>
* <p>
* Deep- or near- space propagator is selected internally according to NORAD recommendations
* so that the user has not to worry about the used computation methods. One instance is created
* for each TLE (this instance can only be get using {@link #selectExtrapolator(TLE)} method,
* and can compute {@link PVCoordinates position and velocity coordinates} at any
* time. Maximum accuracy is guaranteed in a 24h range period before and after the provided
* TLE epoch (of course this accuracy is not really measurable nor predictable: according to
* <a href="https://www.celestrak.com/">CelesTrak</a>, the precision is close to one kilometer
* and error won't probably rise above 2 km).
* </p>
* <p>This implementation is largely inspired from the paper and source code <a
* href="https://www.celestrak.com/publications/AIAA/2006-6753/">Revisiting Spacetrack
* Report #3</a> and is fully compliant with its results and tests cases.</p>
* @author Felix R. Hoots, Ronald L. Roehrich, December 1980 (original fortran)
* @author David A. Vallado, Paul Crawford, Richard Hujsak, T.S. Kelso (C++ translation and improvements)
* @author Fabien Maussion (java translation)
* @see TLE
*/
public abstract class TLEPropagator extends AbstractAnalyticalPropagator {
// CHECKSTYLE: stop VisibilityModifier check
/** Initial state. */
protected final TLE tle;
/** final RAAN. */
protected double xnode;
/** final semi major axis. */
protected double a;
/** final eccentricity. */
protected double e;
/** final inclination. */
protected double i;
/** final perigee argument. */
protected double omega;
/** L from SPTRCK #3. */
protected double xl;
/** original recovered semi major axis. */
protected double a0dp;
/** original recovered mean motion. */
protected double xn0dp;
/** cosinus original inclination. */
protected double cosi0;
/** cos io squared. */
protected double theta2;
/** sinus original inclination. */
protected double sini0;
/** common parameter for mean anomaly (M) computation. */
protected double xmdot;
/** common parameter for perigee argument (omega) computation. */
protected double omgdot;
/** common parameter for raan (OMEGA) computation. */
protected double xnodot;
/** original eccentricity squared. */
protected double e0sq;
/** 1 - e2. */
protected double beta02;
/** sqrt (1 - e2). */
protected double beta0;
/** perigee, expressed in KM and ALTITUDE. */
protected double perige;
/** eta squared. */
protected double etasq;
/** original eccentricity * eta. */
protected double eeta;
/** s* new value for the contant s. */
protected double s4;
/** tsi from SPTRCK #3. */
protected double tsi;
/** eta from SPTRCK #3. */
protected double eta;
/** coef for SGP C3 computation. */
protected double coef;
/** coef for SGP C5 computation. */
protected double coef1;
/** C1 from SPTRCK #3. */
protected double c1;
/** C2 from SPTRCK #3. */
protected double c2;
/** C4 from SPTRCK #3. */
protected double c4;
/** common parameter for raan (OMEGA) computation. */
protected double xnodcf;
/** 3/2 * C1. */
protected double t2cof;
// CHECKSTYLE: resume VisibilityModifier check
/** TLE frame. */
private final Frame teme;
/** Spacecraft mass (kg). */
private final double mass;
/** Protected constructor for derived classes.
* @param initialTLE the unique TLE to propagate
* @param attitudeProvider provider for attitude computation
* @param mass spacecraft mass (kg)
*/
protected TLEPropagator(final TLE initialTLE, final AttitudeProvider attitudeProvider,
final double mass) {
super(attitudeProvider);
setStartDate(initialTLE.getDate());
this.tle = initialTLE;
this.teme = FramesFactory.getTEME();
this.mass = mass;
initializeCommons();
sxpInitialize();
// set the initial state
final Orbit orbit = propagateOrbit(initialTLE.getDate());
final Attitude attitude = attitudeProvider.getAttitude(orbit, orbit.getDate(), orbit.getFrame());
super.resetInitialState(new SpacecraftState(orbit, attitude, mass));
}
/** Selects the extrapolator to use with the selected TLE.
* @param tle the TLE to propagate.
* @return the correct propagator.
*/
public static TLEPropagator selectExtrapolator(final TLE tle) {
return selectExtrapolator(tle, DEFAULT_LAW, DEFAULT_MASS);
}
/** Selects the extrapolator to use with the selected TLE.
* @param tle the TLE to propagate.
* @param attitudeProvider provider for attitude computation
* @param mass spacecraft mass (kg)
* @return the correct propagator.
*/
public static TLEPropagator selectExtrapolator(final TLE tle, final AttitudeProvider attitudeProvider,
final double mass) {
final double a1 = FastMath.pow( TLEConstants.XKE / (tle.getMeanMotion() * 60.0), TLEConstants.TWO_THIRD);
final double cosi0 = FastMath.cos(tle.getI());
final double temp = TLEConstants.CK2 * 1.5 * (3 * cosi0 * cosi0 - 1.0) *
FastMath.pow(1.0 - tle.getE() * tle.getE(), -1.5);
final double delta1 = temp / (a1 * a1);
final double a0 = a1 * (1.0 - delta1 * (TLEConstants.ONE_THIRD + delta1 * (delta1 * 134.0 / 81.0 + 1.0)));
final double delta0 = temp / (a0 * a0);
// recover original mean motion :
final double xn0dp = tle.getMeanMotion() * 60.0 / (delta0 + 1.0);
// Period >= 225 minutes is deep space
if (MathUtils.TWO_PI / (xn0dp * TLEConstants.MINUTES_PER_DAY) >= (1.0 / 6.4)) {
return new DeepSDP4(tle, attitudeProvider, mass);
} else {
return new SGP4(tle, attitudeProvider, mass);
}
}
/** Get the Earth gravity coefficient used for TLE propagation.
* @return the Earth gravity coefficient.
*/
public static double getMU() {
return TLEConstants.MU;
}
/** Get the extrapolated position and velocity from an initial TLE.
* @param date the final date
* @return the final PVCoordinates
*/
public PVCoordinates getPVCoordinates(final AbsoluteDate date) {
sxpPropagate(date.durationFrom(tle.getDate()) / 60.0);
// Compute PV with previous calculated parameters
return computePVCoordinates();
}
/** Computation of the first commons parameters.
*/
private void initializeCommons() {
final double a1 = FastMath.pow(TLEConstants.XKE / (tle.getMeanMotion() * 60.0), TLEConstants.TWO_THIRD);
cosi0 = FastMath.cos(tle.getI());
theta2 = cosi0 * cosi0;
final double x3thm1 = 3.0 * theta2 - 1.0;
e0sq = tle.getE() * tle.getE();
beta02 = 1.0 - e0sq;
beta0 = FastMath.sqrt(beta02);
final double tval = TLEConstants.CK2 * 1.5 * x3thm1 / (beta0 * beta02);
final double delta1 = tval / (a1 * a1);
final double a0 = a1 * (1.0 - delta1 * (TLEConstants.ONE_THIRD + delta1 * (1.0 + 134.0 / 81.0 * delta1)));
final double delta0 = tval / (a0 * a0);
// recover original mean motion and semi-major axis :
xn0dp = tle.getMeanMotion() * 60.0 / (delta0 + 1.0);
a0dp = a0 / (1.0 - delta0);
// Values of s and qms2t :
s4 = TLEConstants.S; // unmodified value for s
double q0ms24 = TLEConstants.QOMS2T; // unmodified value for q0ms2T
perige = (a0dp * (1 - tle.getE()) - TLEConstants.NORMALIZED_EQUATORIAL_RADIUS) * TLEConstants.EARTH_RADIUS; // perige
// For perigee below 156 km, the values of s and qoms2t are changed :
if (perige < 156.0) {
if (perige <= 98.0) {
s4 = 20.0;
} else {
s4 = perige - 78.0;
}
final double temp_val = (120.0 - s4) * TLEConstants.NORMALIZED_EQUATORIAL_RADIUS / TLEConstants.EARTH_RADIUS;
final double temp_val_squared = temp_val * temp_val;
q0ms24 = temp_val_squared * temp_val_squared;
s4 = s4 / TLEConstants.EARTH_RADIUS + TLEConstants.NORMALIZED_EQUATORIAL_RADIUS; // new value for q0ms2T and s
}
final double pinv = 1.0 / (a0dp * beta02);
final double pinvsq = pinv * pinv;
tsi = 1.0 / (a0dp - s4);
eta = a0dp * tle.getE() * tsi;
etasq = eta * eta;
eeta = tle.getE() * eta;
final double psisq = FastMath.abs(1.0 - etasq); // abs because pow 3.5 needs positive value
final double tsi_squared = tsi * tsi;
coef = q0ms24 * tsi_squared * tsi_squared;
coef1 = coef / FastMath.pow(psisq, 3.5);
// C2 and C1 coefficients computation :
c2 = coef1 * xn0dp * (a0dp * (1.0 + 1.5 * etasq + eeta * (4.0 + etasq)) +
0.75 * TLEConstants.CK2 * tsi / psisq * x3thm1 * (8.0 + 3.0 * etasq * (8.0 + etasq)));
c1 = tle.getBStar() * c2;
sini0 = FastMath.sin(tle.getI());
final double x1mth2 = 1.0 - theta2;
// C4 coefficient computation :
c4 = 2.0 * xn0dp * coef1 * a0dp * beta02 * (eta * (2.0 + 0.5 * etasq) +
tle.getE() * (0.5 + 2.0 * etasq) -
2 * TLEConstants.CK2 * tsi / (a0dp * psisq) *
(-3.0 * x3thm1 * (1.0 - 2.0 * eeta + etasq * (1.5 - 0.5 * eeta)) +
0.75 * x1mth2 * (2.0 * etasq - eeta * (1.0 + etasq)) * FastMath.cos(2.0 * tle.getPerigeeArgument())));
final double theta4 = theta2 * theta2;
final double temp1 = 3 * TLEConstants.CK2 * pinvsq * xn0dp;
final double temp2 = temp1 * TLEConstants.CK2 * pinvsq;
final double temp3 = 1.25 * TLEConstants.CK4 * pinvsq * pinvsq * xn0dp;
// atmospheric and gravitation coefs :(Mdf and OMEGAdf)
xmdot = xn0dp +
0.5 * temp1 * beta0 * x3thm1 +
0.0625 * temp2 * beta0 * (13.0 - 78.0 * theta2 + 137.0 * theta4);
final double x1m5th = 1.0 - 5.0 * theta2;
omgdot = -0.5 * temp1 * x1m5th +
0.0625 * temp2 * (7.0 - 114.0 * theta2 + 395.0 * theta4) +
temp3 * (3.0 - 36.0 * theta2 + 49.0 * theta4);
final double xhdot1 = -temp1 * cosi0;
xnodot = xhdot1 + (0.5 * temp2 * (4.0 - 19.0 * theta2) + 2.0 * temp3 * (3.0 - 7.0 * theta2)) * cosi0;
xnodcf = 3.5 * beta02 * xhdot1 * c1;
t2cof = 1.5 * c1;
}
/** Retrieves the position and velocity.
* @return the computed PVCoordinates.
*/
private PVCoordinates computePVCoordinates() {
// Long period periodics
final double axn = e * FastMath.cos(omega);
double temp = 1.0 / (a * (1.0 - e * e));
final double xlcof = 0.125 * TLEConstants.A3OVK2 * sini0 * (3.0 + 5.0 * cosi0) / (1.0 + cosi0);
final double aycof = 0.25 * TLEConstants.A3OVK2 * sini0;
final double xll = temp * xlcof * axn;
final double aynl = temp * aycof;
final double xlt = xl + xll;
final double ayn = e * FastMath.sin(omega) + aynl;
final double elsq = axn * axn + ayn * ayn;
final double capu = MathUtils.normalizeAngle(xlt - xnode, FastMath.PI);
double epw = capu;
double ecosE = 0;
double esinE = 0;
double sinEPW = 0;
double cosEPW = 0;
// Dundee changes: items dependent on cosio get recomputed:
final double cosi0Sq = cosi0 * cosi0;
final double x3thm1 = 3.0 * cosi0Sq - 1.0;
final double x1mth2 = 1.0 - cosi0Sq;
final double x7thm1 = 7.0 * cosi0Sq - 1.0;
if (e > (1 - 1e-6)) {
throw new OrekitException(OrekitMessages.TOO_LARGE_ECCENTRICITY_FOR_PROPAGATION_MODEL, e);
}
// Solve Kepler's' Equation.
final double newtonRaphsonEpsilon = 1e-12;
for (int j = 0; j < 10; j++) {
boolean doSecondOrderNewtonRaphson = true;
sinEPW = FastMath.sin( epw);
cosEPW = FastMath.cos( epw);
ecosE = axn * cosEPW + ayn * sinEPW;
esinE = axn * sinEPW - ayn * cosEPW;
final double f = capu - epw + esinE;
if (FastMath.abs(f) < newtonRaphsonEpsilon) {
break;
}
final double fdot = 1.0 - ecosE;
double delta_epw = f / fdot;
if (j == 0) {
final double maxNewtonRaphson = 1.25 * FastMath.abs(e);
doSecondOrderNewtonRaphson = false;
if (delta_epw > maxNewtonRaphson) {
delta_epw = maxNewtonRaphson;
} else if (delta_epw < -maxNewtonRaphson) {
delta_epw = -maxNewtonRaphson;
} else {
doSecondOrderNewtonRaphson = true;
}
}
if (doSecondOrderNewtonRaphson) {
delta_epw = f / (fdot + 0.5 * esinE * delta_epw);
}
epw += delta_epw;
}
// Short period preliminary quantities
temp = 1.0 - elsq;
final double pl = a * temp;
final double r = a * (1.0 - ecosE);
double temp2 = a / r;
final double betal = FastMath.sqrt(temp);
temp = esinE / (1.0 + betal);
final double cosu = temp2 * (cosEPW - axn + ayn * temp);
final double sinu = temp2 * (sinEPW - ayn - axn * temp);
final double u = FastMath.atan2(sinu, cosu);
final double sin2u = 2.0 * sinu * cosu;
final double cos2u = 2.0 * cosu * cosu - 1.0;
final double temp1 = TLEConstants.CK2 / pl;
temp2 = temp1 / pl;
// Update for short periodics
final double rk = r * (1.0 - 1.5 * temp2 * betal * x3thm1) + 0.5 * temp1 * x1mth2 * cos2u;
final double uk = u - 0.25 * temp2 * x7thm1 * sin2u;
final double xnodek = xnode + 1.5 * temp2 * cosi0 * sin2u;
final double xinck = i + 1.5 * temp2 * cosi0 * sini0 * cos2u;
// Orientation vectors
final double sinuk = FastMath.sin(uk);
final double cosuk = FastMath.cos(uk);
final double sinik = FastMath.sin(xinck);
final double cosik = FastMath.cos(xinck);
final double sinnok = FastMath.sin(xnodek);
final double cosnok = FastMath.cos(xnodek);
final double xmx = -sinnok * cosik;
final double xmy = cosnok * cosik;
final double ux = xmx * sinuk + cosnok * cosuk;
final double uy = xmy * sinuk + sinnok * cosuk;
final double uz = sinik * sinuk;
// Position and velocity
final double cr = 1000 * rk * TLEConstants.EARTH_RADIUS;
final Vector3D pos = new Vector3D(cr * ux, cr * uy, cr * uz);
final double rdot = TLEConstants.XKE * FastMath.sqrt(a) * esinE / r;
final double rfdot = TLEConstants.XKE * FastMath.sqrt(pl) / r;
final double xn = TLEConstants.XKE / (a * FastMath.sqrt(a));
final double rdotk = rdot - xn * temp1 * x1mth2 * sin2u;
final double rfdotk = rfdot + xn * temp1 * (x1mth2 * cos2u + 1.5 * x3thm1);
final double vx = xmx * cosuk - cosnok * sinuk;
final double vy = xmy * cosuk - sinnok * sinuk;
final double vz = sinik * cosuk;
final double cv = 1000.0 * TLEConstants.EARTH_RADIUS / 60.0;
final Vector3D vel = new Vector3D(cv * (rdotk * ux + rfdotk * vx),
cv * (rdotk * uy + rfdotk * vy),
cv * (rdotk * uz + rfdotk * vz));
return new PVCoordinates(pos, vel);
}
/** Initialization proper to each propagator (SGP or SDP).
*/
protected abstract void sxpInitialize();
/** Propagation proper to each propagator (SGP or SDP).
* @param t the offset from initial epoch (min)
*/
protected abstract void sxpPropagate(double t);
/** {@inheritDoc} */
public void resetInitialState(final SpacecraftState state) {
throw new OrekitException(OrekitMessages.NON_RESETABLE_STATE);
}
/** {@inheritDoc} */
protected void resetIntermediateState(final SpacecraftState state, final boolean forward) {
throw new OrekitException(OrekitMessages.NON_RESETABLE_STATE);
}
/** {@inheritDoc} */
protected double getMass(final AbsoluteDate date) {
return mass;
}
/** {@inheritDoc} */
protected Orbit propagateOrbit(final AbsoluteDate date) {
return new CartesianOrbit(getPVCoordinates(date), teme, date, TLEConstants.MU);
}
/** Get the underlying TLE.
* @return underlying TLE
*/
public TLE getTLE() {
return tle;
}
/** {@inheritDoc} */
public Frame getFrame() {
return teme;
}
}