Covariance Interpolation/Blending from OEM

Orekit usage
1

Dear Orekit community,

I am working on parsing and manipulating data from an OEM. In particular, I would like to read the covariance evolution and interpolate it at any epoch.
I am using Orekit v12.2.

After reading the OEM and getting the propagator:

OemParser oemParser = new ParserBuilder().buildOemParser();
Oem oem = oemParser.parse(new DataSource(new File(oemStringPath)));
OemSegment oemSegment = oem.getSegments().get(0);
prop =  oem.getPropagator(); // Gives an EphemerisSegmentPropagator

I tried, at first, to interpolate the covariance matrix with a StateCovarianceBlender using an OrbitBlender. Here I create an instance of StateCovarianceBlender and obtain the interpolationSamples:

// 1) Create Blender 
// Smoothing function
SmoothStepFunction sf = SmoothStepFactory.getQuintic();

// Build orbit blender
OrbitBlender ob = new OrbitBlender(sf, prop, FramesFactory.getEME2000());
	
// Assign state covariance blender to instance variable
stateCovInterpolator = new StateCovarianceBlender(sf, ob, LOFType.TNW);

// 2) Create samples
List<TimeStampedPair<Orbit, StateCovariance>> samples = new ArrayList<>();

for (CartesianCovariance cov : oem.getCovarianceMatrices()) {
	AbsoluteDate date = cov.getEpoch();
	Orbit orbit = prop.propagate(date).getOrbit(); // orbit at covariance epoch
	StateCovariance stateCov = new StateCovariance(
		cov.getCovarianceMatrix(), date, cov.getReferenceFrame().asFrame(),
		OrbitType.CARTESIAN, null);
	
	samples.add(new TimeStampedPair<>(orbit, stateCov));
}

// Assign samples to instance variable
intepolationSamples = samples;

However, on the line:

StateCovariance cov = stateCovInterpolator.interpolate(s.getDate(), intepolationSamples).getSecond()

I get the following error:

Caused by: org.orekit.errors.OrekitException: reset state not allowed
        at org.orekit.files.general.EphemerisSegmentPropagator.resetInitialState(EphemerisSegmentPropagator.java:143)
        at org.orekit.orbits.OrbitBlender.propagateOrbit(OrbitBlender.java:131)
        at org.orekit.orbits.OrbitBlender.interpolate(OrbitBlender.java:104)
        at org.orekit.orbits.OrbitBlender.interpolate(OrbitBlender.java:59)
        at org.orekit.time.AbstractTimeInterpolator.interpolate(AbstractTimeInterpolator.java:94)
        at org.orekit.orbits.AbstractOrbitInterpolator.interpolate(AbstractOrbitInterpolator.java:81)
        at org.orekit.orbits.AbstractOrbitInterpolator.interpolate(AbstractOrbitInterpolator.java:34)
        at org.orekit.propagation.AbstractStateCovarianceInterpolator.interpolateOrbit(AbstractStateCovarianceInterpolator.java:189)
        at org.orekit.propagation.AbstractStateCovarianceInterpolator.interpolate(AbstractStateCovarianceInterpolator.java:129)
        at org.orekit.propagation.AbstractStateCovarianceInterpolator.interpolate(AbstractStateCovarianceInterpolator.java:41)
        at org.orekit.time.AbstractTimeInterpolator.interpolate(AbstractTimeInterpolator.java:94)

After some research, it seems that the EphemerisSegmentPropagator (as other AnalyticalPropagators) does not support the resetInitialState method (“Try (and fail) to reset the initial state.”).

I also tried to implement it with the StateCovarianceKeplerianHermiteInterpolator with an OrbitHermiteInterpolator, with a very similar approach (I will not share my code for now, otherwise this post will become a TLDR), but I get this error:

Caused by: java.lang.NullPointerException
        at org.orekit.orbits.Orbit.getJacobianWrtParameters(Orbit.java:606)
        at org.orekit.propagation.StateCovariance.changeTypeAndCreate(StateCovariance.java:458)
        at org.orekit.propagation.StateCovariance.changeCovarianceType(StateCovariance.java:257)
        at org.orekit.propagation.StateCovarianceKeplerianHermiteInterpolator.computeAndCombineCovarianceValueAndDerivatives(StateCovarianceKeplerianHermiteInterpolator.java:311)
        at org.orekit.propagation.StateCovarianceKeplerianHermiteInterpolator.computeInterpolatedCovarianceInOrbitFrame(StateCovarianceKeplerianHermiteInterpolator.java:275)
        at org.orekit.propagation.AbstractStateCovarianceInterpolator.interpolate(AbstractStateCovarianceInterpolator.java:134)
        at org.orekit.propagation.AbstractStateCovarianceInterpolator.interpolate(AbstractStateCovarianceInterpolator.java:41)
        at org.orekit.time.AbstractTimeInterpolator.interpolate(AbstractTimeInterpolator.java:94)

I suppose it is because, in the interpolated states I am providing, the Jacobian is missing.

What am I missing? What is the common way to obtain covariance interpolation from an OEM? Is there any simpler way than this?
I guess that creating a NumericalPropagator and re-propagate might work but I would avoid re-propagation of OEM data.

I am very likely missing something here, so I apologize in advance.
Thank you for your answer and for your work!

Cheers,
Giacomo

2

Hello @gnisi and welcome to the Orekit forum :slight_smile: !

In short, you probably want to use a StateCovarianceKeplerianHermiteInterpolator with an OrbitHermiteInterpolator. I believe this thread is identical to yours : Covariance Matrix Interpolation from OEM

Feel free to ask any questions you may have :+1: .

Cheers,
Vincent

3

Hello @Vincent and thanks for your answer!

I had not noticed there was such a similar thread already. It solved the problem.

Cheers,
Giacomo

1 Like
4

Glad it helped :+1: !

Cheers,
Vincent