Failing EKF without orbit estimation

Orekit usage
1

Hi all,

I found an issue with KalmanEstimator if no orbital coordinate is estimated but a propagation model parameter is. It basically crashes (whilst it works fine with UKF as far as I am aware).
Code sample to reproduce:

    @Test
    void testEstimationStepWithBStarOnly() {
        // GIVEN
        TLEEstimationTestUtils.eccentricContext("regular-data:potential:tides");
        String line1 = "1 07276U 74026A   00055.48318287  .00000000  00000-0  22970+3 0  9994";
        String line2 = "2 07276  71.6273  78.7838 1248323  14.0598   3.8405  4.72707036231812";
        final TLE tle = new TLE(line1, line2);
        final TLEPropagatorBuilder propagatorBuilder = new TLEPropagatorBuilder(tle,
                PositionAngleType.TRUE, 1., new FixedPointTleGenerationAlgorithm());
        for (final ParameterDriver driver: propagatorBuilder.getOrbitalParametersDrivers().getDrivers()) {
            driver.setSelected(false);
        }
        propagatorBuilder.getPropagationParametersDrivers().getDrivers().get(0).setSelected(true);
        final KalmanEstimatorBuilder builder = new KalmanEstimatorBuilder();
        builder.addPropagationConfiguration(propagatorBuilder,
                new ConstantProcessNoise(MatrixUtils.createRealMatrix(1, 1)));
        final KalmanEstimator estimator = builder.build();
        final AbsoluteDate measurementDate = tle.getDate().shiftedBy(1.0);
        final TLEPropagator propagator = TLEPropagator.selectExtrapolator(tle);
        final Position positionMeasurement = new Position(measurementDate, propagator.getPosition(measurementDate,
                propagator.getFrame()), 1., 1., new ObservableSatellite(0));
        // WHEN & THEN
        Assertions.assertDoesNotThrow(() -> estimator.estimationStep(positionMeasurement));
    }

I naively tried to replace the “6” by “nbOrbParams” line 157 of KalmanModel, but it’s not enough.

Cheers,
Romain.

1 Like
2

Thanks @Serrof. That’s a good indication we don’t have tests for the case where there are fewer than 6 estimated orbital parameters.

There’s a lot of complexity in keeping track of which propagator orbital/propagation parameters map to which entry in the covariance matrix (and state transition matrix) - the covarianceIndirection indicies. At some point there’s been a mixup in how that mapping is used. I haven’t looked very closely at those before, because they work fine with 6 orbital parameters!

If you’d like to make your specific case work, then you can change like 151 in KalmanModel from

if (nbParams > 0) {

to

if (nbOrbParams > 0 && nbParams > 0) {

That’s not a general fix, but it’ll do the trick when there aren’t any orbital parameters being estimated.

3

Hi Mark,

Thanks for your answer!
Just to double check, whilst your trick would allow the code not to crash, then the covariance matrix would not be computed correctly right?

Cheers,
Romain.

4

The covariance should be fine in your case. The transition matrix is just the identity matrix when there aren’t any orbital parameters being estimated.

5

I see, thanks. In that case I will create another issue for when there is between 1 and 5 orbital parameters estimated and at least one propagation model parameter. This way we can already have a fix for the original problem soon.

Cheers,
Romain.

1 Like