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Re: [Orekit Developers] STM/Jacobians w.r.t. Keplerian Parameters

Hi Christophe,

On Thu, 2017-08-10 at 17:59 +0200, Christophe LE BRIS wrote:

With PartialDerivativeEquations, STM are given wrt cartesian parameters

If you want to have it wrt Keplerian parameters, you need to compute the
Jacobian "dKep/Cart" with the method computeJacobianMeanWrtCartesian in
the class KeplerianOrbit (on the final state) and then use it to change
of basis of your STM (the STM is a linear transformation).

Thanks for the reply. I tried your suggestion and it worked well, once I made the tolerance a little tighter. 

What is PDE actually computing then when orbitType is not Cartesian? I thought the mapper returned by PartialDerivativeEquations.getMapper() took care of converting orbital element representations since it calls Orbit.getJacobianWrtCaresian() both when setting the initial STM and when retrieving the STM. [1-2] Should I add some documentation warning the user to never use the PDE class with non-Cartesian elements?

[1] https://www.orekit.org/forge/projects/orekit/repository/revisions/master/entry/src/main/java/org/orekit/propagation/numerical/JacobiansMapper.java#L140
[2] https://www.orekit.org/forge/projects/orekit/repository/revisions/master/entry/src/main/java/org/orekit/propagation/numerical/JacobiansMapper.java#L182

Best Regards,


Le 10/08/2017 à 17:24, Ward, Evan a écrit :
Hi, I'm trying to compute the State Transition Matrix (STM) for Keplerian orbit w.r.t. Keplerian parameters. I'm not sure if I'm doing it right so I thought I'd post my question here. So I'm trying to compute dx(t)/dx(0) where x = {a, e, i, arg. of perigee, RAAN, M (mean anomaly)}. According to Montenbruck & Gill (section 7.1.1 of Satellite Orbits) the result should be the identity matrix except for the dM(t)/da(0). term. In Orekit I've set the orbitType to Keplerian and the positionAngleType to Mean, but the result looks very different from what I would expect. I've attached my code and included the output below. The code is in groovy and it should be able to run if you want to try it. Am I doing something wrong? Is this a bug? Initial State Keplerian parameters: {a: 6678137.0; e: 0.1; i: 50.0; pa: 0.0; raan: 0.0; v: 0.0;} Final state Keplerian parameters: {a: 6678137.0; e: 0.1; i: 50.0; pa: 0.0; raan: 0.0; v: 359.9999999999999;} STM 1.084033316 1048746.583 -1.881271601e-07 338443.1765 217547.0804 689070.0095 8.986475977e-08 -0.4460115259 5.972999872e-14 -0.2344750310 -0.1507176447 -0.5273227631 -5.987960454e-21 1.482147738e-14 1.008352980 -2.359223927e-15 0.003748479827 5.162537065e-15 -1.049794901e-06 19.17181914 -0.009257359011 1.103560184 0.05508206389 -1.305994282 3.388131789e-21 9.503509091e-14 0.01440189399 9.742207041e-15 1.017867705 2.688127498e-14 7.973387020e-09 -23.67118846 1.041684976e-12 -0.8494761263 -0.5460327287 0.9198999327 Expected dM(t)/da(t0) -1.411288501683835E-6 Best Regards, Evan