Implementing Satellite J2Perturbation in STK Using orekit

1

Hello everyone, I would like to ask if using orekit can achieve j2p forecasting. I am using the BrouwerLyddanePropagatorTest class, but the calculated result seems to be inconsistent with stk

2

Hi @dreamdongx

Welcome to the Orekit forum.

That’s normal, Brouwer-Lyddane model implements the Zonal harmonics from J2 to J5. So you have more than just the J2 perturbation.

To implement a J2 only propagation model you can:

  • Initialize a NumericalPropagator
  • Create a HolmesFeatherstoneAttractionModel with only J2.

ForceModel gravityField = new HolmesFeatherstoneAttractionModel(FramesFactory.getITRF(IERSConventions.IERS_2010, true), GravityFieldFactory.getNormalizedProvider(2, 0));

  • Add the force model to your numerical propagator and propagate

To help you, you can look at the numerical propagation tutorial

Best regards,
Bryan

3

Hi Bryan,

Thank you so much for your response and the helpful solution you provided. Your explanation of the Brouwer-Lyddane model and the steps to implement a J2-only propagation model in Orekit was very clear and comprehensive.

I appreciate your time and efforts in assisting me with this issue. Your guidance has provided me with a better understanding of the model and how to apply it in my work. I will follow the steps you have recommended and implement the propagation model accordingly.

If I encounter any further difficulties or have further questions, I will definitely reach out to you for further assistance. Thank you once again for your invaluable support.

Best regards,
dreamdongx

4

Hi Bryan,
I followed the approach you provided to predict the orbit of the satellite, but it still seems different from STK. Did I make a mistake? Here is my code .Thank you in advance

public void testJ2P(){

        UTCScale utc = TimeScalesFactory.getUTC();
        AbsoluteDate initDate = new AbsoluteDate(2023, 9, 12, 04, 00, 00.000, utc);

        double a = 6675900;
        double e = 0.01256;
        double i = FastMath.toRadians(28.482);
        double omega = FastMath.toRadians(0);
        double rann = FastMath.toRadians(0);
        double mean = FastMath.toRadians(0);

        Orbit initOrbit = new KeplerianOrbit(a,e,i,omega,rann, mean, PositionAngle.MEAN,
                FramesFactory.getEME2000(), initDate,Constants.IERS2010_EARTH_MU);

        double dp = 1.0E-6;
        double minStep = 0.00001;
        double maxStep = 10000.0;
        ODEIntegrator integrator = new DormandPrince853IntegratorBuilder(minStep, maxStep, dp)
                .buildIntegrator(initOrbit, OrbitType.KEPLERIAN);

        // Initialize a NumericalPropagator
        NumericalPropagator propagator = new NumericalPropagator(integrator);
        propagator.setMu(Constants.EGM96_EARTH_MU);
        SpacecraftState initState = new SpacecraftState(initOrbit, 1000);
        propagator.setInitialState(initState);
        propagator.setPositionAngleType(PositionAngle.MEAN);

        // Create a HolmesFeatherstoneAttractionModel
        NormalizedSphericalHarmonicsProvider nshp = GravityFieldFactory.getNormalizedProvider(2, 0);
        HolmesFeatherstoneAttractionModel gravityField = new HolmesFeatherstoneAttractionModel(
                CelestialBodyFactory.getEarth().getBodyOrientedFrame(), nshp);

        // Add the force model
        propagator.addForceModel(gravityField);

        AbsoluteDate start = new AbsoluteDate(2023, 9, 12, 04, 00, 00.000, utc);
        AbsoluteDate end = new AbsoluteDate(2023, 9, 12, 05, 00, 00.000, utc);

        // Start forecasting
        while (start.isBefore(end)){
            TimeStampedPVCoordinates pvCoordinates = propagator.getPVCoordinates(start, FramesFactory.getEME2000());
            System.out.println(pvCoordinates);
            start = start.shiftedBy(60);
        }
    }
// myJ2P :{2023-09-12T04:01:00.000, P(6575521.924382813, 412315.4616164101, 223700.6558825176),
// stkJ2P :12 Sep 2023 04:01:00.000     6575.482849      412.975133      224.382459

Best regards,
dreamdongx

5

Your Orekit code is good.

But I maybe understood why you have differences. Please find below the description of STK’s J2Perturbation:

STK’s J2Perturbation and J4Perturbation propagators model only the secular drift in the elements (the drift in mean anomaly can best be seen as a change to the period of motion of the satellite)

In other words, Orekit’s numerical propagator computes osculating elements while STK’s J2-Perturbation computes secular elements. That’s why results are differents.
Unfortunately, we don’t have secular propagation model in Orekit. We have a semi-analytical theory (i.e., DSST) that can compute mean elements (i.e., the sum of secular and long terms variation).

To summarize, they are different ways to model orbital elements: osculating, mean and secular. And here it looks that we’re comparing two differents and that’s why results are differents. But both are corrects :smiley:

Regards,
Bryan

6

Thank you very much for your reply and detailed explanation. I understand why my code and STK produce different results.
Regards,
dreamdongx