@Test
public void testSpin() throws OrekitException {
AbsoluteDate date =
new AbsoluteDate(
new DateComponents(1970, 01, 01),
new TimeComponents(3, 25, 45.6789),
TimeScalesFactory.getUTC());
KeplerianOrbit orbit =
new KeplerianOrbit(
7178000.0,
1.e-4,
FastMath.toRadians(50.),
FastMath.toRadians(10.),
FastMath.toRadians(20.),
FastMath.toRadians(30.),
PositionAngle.MEAN,
FramesFactory.getEME2000(),
date,
3.986004415e14);
final AttitudeProvider law =
new LofOffsetPointing(
earthSpheric,
new LofOffset(orbit.getFrame(), LOFType.VVLH, RotationOrder.XYX, 0.1, 0.2, 0.3),
Vector3D.PLUS_K);
Propagator propagator = new KeplerianPropagator(orbit, law);
double h = 0.01;
SpacecraftState sMinus = propagator.propagate(date.shiftedBy(-h));
SpacecraftState s0 = propagator.propagate(date);
SpacecraftState sPlus = propagator.propagate(date.shiftedBy(h));
// check spin is consistent with attitude evolution
double errorAngleMinus =
Rotation.distance(
sMinus.shiftedBy(h).getAttitude().getRotation(), s0.getAttitude().getRotation());
double evolutionAngleMinus =
Rotation.distance(sMinus.getAttitude().getRotation(), s0.getAttitude().getRotation());
Assert.assertEquals(0.0, errorAngleMinus, 1.0e-6 * evolutionAngleMinus);
double errorAnglePlus =
Rotation.distance(
s0.getAttitude().getRotation(), sPlus.shiftedBy(-h).getAttitude().getRotation());
double evolutionAnglePlus =
Rotation.distance(s0.getAttitude().getRotation(), sPlus.getAttitude().getRotation());
Assert.assertEquals(0.0, errorAnglePlus, 1.0e-6 * evolutionAnglePlus);
Vector3D spin0 = s0.getAttitude().getSpin();
Vector3D reference =
AngularCoordinates.estimateRate(
sMinus.getAttitude().getRotation(), sPlus.getAttitude().getRotation(), 2 * h);
Assert.assertTrue(spin0.getNorm() > 1.0e-3);
Assert.assertEquals(0.0, spin0.subtract(reference).getNorm(), 1.0e-10);
}
/**
* Conversion from osculating to mean, orbit.
*
* <p>Compute osculating state <b>in a DSST sense</b>, corresponding to the mean SpacecraftState
* in input, and according to the Force models taken into account.
*
* <p>Since the osculating state is obtained with the computation of short-periodic variation of
* each force model, the resulting output will depend on the force models parametrized in input.
*
* <p>The computing is done through a fixed-point iteration process.
*
* @param osculating Osculating state to convert
* @param forces Forces to take into account
* @return mean state in a DSST sense
* @throws OrekitException if computation of short periodics fails or iteration algorithm does not
* converge
*/
public static SpacecraftState computeMeanState(
final SpacecraftState osculating, final Collection<DSSTForceModel> forces)
throws OrekitException {
// Creation of a DSSTPropagator instance
final AbstractIntegrator integrator = new ClassicalRungeKuttaIntegrator(43200.);
final DSSTPropagator dsst = new DSSTPropagator(integrator, false);
// Create the auxiliary object
final AuxiliaryElements aux = new AuxiliaryElements(osculating.getOrbit(), I);
// Set the force models
for (final DSSTForceModel force : forces) {
force.initialize(aux, false);
dsst.addForceModel(force);
}
dsst.setInitialState(osculating, true);
final Orbit meanOrbit = dsst.mapper.computeMeanOrbit(osculating);
return new SpacecraftState(
meanOrbit,
osculating.getAttitude(),
osculating.getMass(),
osculating.getAdditionalStates());
}
/** {@inheritDoc} */
public SpacecraftState apply(final SpacecraftState state1) {
if (state1.getDate().compareTo(referenceDate) <= 0 && !applyBefore) {
// the orbit change has not occurred yet, don't change anything
return state1;
}
return new SpacecraftState(
updateOrbit(state1.getOrbit()), state1.getAttitude(), state1.getMass());
}
/**
* Compute mean state from osculating state.
*
* <p>Compute in a DSST sense the mean state corresponding to the input osculating state.
*
* <p>The computing is done through a fixed-point iteration process.
*
* @param osculating initial osculating state
* @return mean state
* @throws OrekitException if the underlying computation of short periodic variation fails
*/
private Orbit computeMeanOrbit(final SpacecraftState osculating) throws OrekitException {
// rough initialization of the mean parameters
EquinoctialOrbit meanOrbit = new EquinoctialOrbit(osculating.getOrbit());
// threshold for each parameter
final double epsilon = 1.0e-13;
final double thresholdA = epsilon * (1 + FastMath.abs(meanOrbit.getA()));
final double thresholdE = epsilon * (1 + meanOrbit.getE());
final double thresholdAngles = epsilon * FastMath.PI;
int i = 0;
while (i++ < 200) {
final SpacecraftState meanState =
new SpacecraftState(meanOrbit, osculating.getAttitude(), osculating.getMass());
// recompute the osculating parameters from the current mean parameters
final EquinoctialOrbit rebuilt = (EquinoctialOrbit) computeOsculatingOrbit(meanState);
// adapted parameters residuals
final double deltaA = osculating.getA() - rebuilt.getA();
final double deltaEx = osculating.getEquinoctialEx() - rebuilt.getEquinoctialEx();
final double deltaEy = osculating.getEquinoctialEy() - rebuilt.getEquinoctialEy();
final double deltaHx = osculating.getHx() - rebuilt.getHx();
final double deltaHy = osculating.getHy() - rebuilt.getHy();
final double deltaLm = MathUtils.normalizeAngle(osculating.getLM() - rebuilt.getLM(), 0.0);
// check convergence
if ((FastMath.abs(deltaA) < thresholdA)
&& (FastMath.abs(deltaEx) < thresholdE)
&& (FastMath.abs(deltaEy) < thresholdE)
&& (FastMath.abs(deltaLm) < thresholdAngles)) {
return meanOrbit;
}
// update mean parameters
meanOrbit =
new EquinoctialOrbit(
meanOrbit.getA() + deltaA,
meanOrbit.getEquinoctialEx() + deltaEx,
meanOrbit.getEquinoctialEy() + deltaEy,
meanOrbit.getHx() + deltaHx,
meanOrbit.getHy() + deltaHy,
meanOrbit.getLM() + deltaLm,
PositionAngle.MEAN,
meanOrbit.getFrame(),
meanOrbit.getDate(),
meanOrbit.getMu());
}
throw new PropagationException(OrekitMessages.UNABLE_TO_COMPUTE_DSST_MEAN_PARAMETERS, i);
}
/** {@inheritDoc} */
public FieldVector3D<DerivativeStructure> accelerationDerivatives(
final SpacecraftState s, final String paramName) throws OrekitException {
complainIfNotSupported(paramName);
final AbsoluteDate date = s.getDate();
final Frame frame = s.getFrame();
final Vector3D position = s.getPVCoordinates().getPosition();
final Vector3D sunSatVector =
position.subtract(sun.getPVCoordinates(date, frame).getPosition());
final double r2 = sunSatVector.getNormSq();
// compute flux
final double rawP = kRef * getLightningRatio(position, frame, date) / r2;
final Vector3D flux = new Vector3D(rawP / FastMath.sqrt(r2), sunSatVector);
return spacecraft.radiationPressureAcceleration(
date, frame, position, s.getAttitude().getRotation(), s.getMass(), flux, paramName);
}
private SpacecraftState shiftState(
SpacecraftState state,
OrbitType orbitType,
PositionAngle angleType,
double delta,
int column) {
double[] array = stateToArray(state, orbitType, angleType, true);
array[column] += delta;
return arrayToState(
array,
orbitType,
angleType,
state.getFrame(),
state.getDate(),
state.getMu(),
state.getAttitude());
}
/** {@inheritDoc} */
public void addContribution(final SpacecraftState s, final TimeDerivativesEquations adder)
throws OrekitException {
final AbsoluteDate date = s.getDate();
final Frame frame = s.getFrame();
final Vector3D position = s.getPVCoordinates().getPosition();
final Vector3D sunSatVector =
position.subtract(sun.getPVCoordinates(date, frame).getPosition());
final double r2 = sunSatVector.getNormSq();
// compute flux
final double rawP = kRef * getLightningRatio(position, frame, date) / r2;
final Vector3D flux = new Vector3D(rawP / FastMath.sqrt(r2), sunSatVector);
final Vector3D acceleration =
spacecraft.radiationPressureAcceleration(
date, frame, position, s.getAttitude().getRotation(), s.getMass(), flux);
// provide the perturbing acceleration to the derivatives adder
adder.addAcceleration(acceleration, s.getFrame());
}
