SGP4.java
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package org.orekit.propagation.analytical.tle;
import org.hipparchus.util.FastMath;
import org.hipparchus.util.SinCos;
import org.orekit.annotation.DefaultDataContext;
import org.orekit.attitudes.AttitudeProvider;
import org.orekit.data.DataContext;
import org.orekit.frames.Frame;
/** This class contains methods to compute propagated coordinates with the SGP4 model.
* <p>
* The user should not bother in this class since it is handled internaly by the
* {@link TLEPropagator}.
* </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)
*/
public class SGP4 extends TLEPropagator {
/** If perige is less than 220 km, some calculus are avoided. */
private boolean lessThan220;
/** (1 + eta * cos(M0))³. */
private double delM0;
// CHECKSTYLE: stop JavadocVariable check
private double d2;
private double d3;
private double d4;
private double t3cof;
private double t4cof;
private double t5cof;
private double sinM0;
private double omgcof;
private double xmcof;
private double c5;
// CHECKSTYLE: resume JavadocVariable check
/** Constructor for a unique initial TLE.
*
* <p>This constructor uses the {@link DataContext#getDefault() default data context}.
*
* @param initialTLE the TLE to propagate.
* @param attitudeProvider provider for attitude computation
* @param mass spacecraft mass (kg)
* @see #SGP4(TLE, AttitudeProvider, double, Frame)
*/
@DefaultDataContext
public SGP4(final TLE initialTLE, final AttitudeProvider attitudeProvider,
final double mass) {
this(initialTLE, attitudeProvider, mass,
DataContext.getDefault().getFrames().getTEME());
}
/** Constructor for a unique initial TLE.
* @param initialTLE the TLE to propagate.
* @param attitudeProvider provider for attitude computation
* @param mass spacecraft mass (kg)
* @param teme the TEME frame to use for propagation.
* @since 10.1
*/
public SGP4(final TLE initialTLE,
final AttitudeProvider attitudeProvider,
final double mass,
final Frame teme) {
super(initialTLE, attitudeProvider, mass, teme);
}
/** Initialization proper to each propagator (SGP or SDP).
*/
protected void sxpInitialize() {
// For perigee less than 220 kilometers, the equations are truncated to
// linear variation in sqrt a and quadratic variation in mean anomaly.
// Also, the c3 term, the delta omega term, and the delta m term are dropped.
lessThan220 = perige < 220;
if (!lessThan220) {
final SinCos scM0 = FastMath.sinCos(tle.getMeanAnomaly());
final double c1sq = c1 * c1;
delM0 = 1.0 + eta * scM0.cos();
delM0 *= delM0 * delM0;
d2 = 4 * a0dp * tsi * c1sq;
final double temp = d2 * tsi * c1 / 3.0;
d3 = (17 * a0dp + s4) * temp;
d4 = 0.5 * temp * a0dp * tsi * (221 * a0dp + 31 * s4) * c1;
t3cof = d2 + 2 * c1sq;
t4cof = 0.25 * (3 * d3 + c1 * (12 * d2 + 10 * c1sq));
t5cof = 0.2 * (3 * d4 + 12 * c1 * d3 + 6 * d2 * d2 + 15 * c1sq * (2 * d2 + c1sq));
sinM0 = scM0.sin();
if (tle.getE() < 1e-4) {
omgcof = 0.;
xmcof = 0.;
} else {
final double c3 = coef * tsi * TLEConstants.A3OVK2 * xn0dp *
TLEConstants.NORMALIZED_EQUATORIAL_RADIUS * sini0 / tle.getE();
xmcof = -TLEConstants.TWO_THIRD * coef * tle.getBStar() *
TLEConstants.NORMALIZED_EQUATORIAL_RADIUS / eeta;
omgcof = tle.getBStar() * c3 * FastMath.cos(tle.getPerigeeArgument());
}
}
c5 = 2 * coef1 * a0dp * beta02 * (1 + 2.75 * (etasq + eeta) + eeta * etasq);
// initialized
}
/** Propagation proper to each propagator (SGP or SDP).
* @param tSince the offset from initial epoch (min)
*/
protected void sxpPropagate(final double tSince) {
// Update for secular gravity and atmospheric drag.
final double xmdf = tle.getMeanAnomaly() + xmdot * tSince;
final double omgadf = tle.getPerigeeArgument() + omgdot * tSince;
final double xn0ddf = tle.getRaan() + xnodot * tSince;
omega = omgadf;
double xmp = xmdf;
final double tsq = tSince * tSince;
xnode = xn0ddf + xnodcf * tsq;
double tempa = 1 - c1 * tSince;
double tempe = tle.getBStar() * c4 * tSince;
double templ = t2cof * tsq;
if (!lessThan220) {
final double delomg = omgcof * tSince;
double delm = 1. + eta * FastMath.cos(xmdf);
delm = xmcof * (delm * delm * delm - delM0);
final double temp = delomg + delm;
xmp = xmdf + temp;
omega = omgadf - temp;
final double tcube = tsq * tSince;
final double tfour = tSince * tcube;
tempa = tempa - d2 * tsq - d3 * tcube - d4 * tfour;
tempe = tempe + tle.getBStar() * c5 * (FastMath.sin(xmp) - sinM0);
templ = templ + t3cof * tcube + tfour * (t4cof + tSince * t5cof);
}
a = a0dp * tempa * tempa;
e = tle.getE() - tempe;
// A highly arbitrary lower limit on e, of 1e-6:
if (e < 1e-6) {
e = 1e-6;
}
xl = xmp + omega + xnode + xn0dp * templ;
i = tle.getI();
}
}