CartesianOrbit: JacobianWrtParameters

Orekit development
1

I encountered an interesting issue while using the CartesianOrbit class. My goal is to approximate a covariance in cartesian elements into a covariance in Keplerian elements.

To my knowledge this should be done as follows:

CartesianOrbit initialGuess = new CartesianOrbit(.....)

// Orbital covariance matrix initialization
final double[][] dYdC = new double[6][6];
            initialGuess.getJacobianWrtParameters(getPositionAngle, dYdC);
final RealMatrix Jac = inv(MatrixUtils.createRealMatrix(dYdC));
final RealMatrix orbitalP = Jac.multiply(cartesianlP.multiply(Jac.transpose()));

However under the CartesianOrbit class getJacobianWrtParameters() is only defined in the parent Orbit.java class, which essentially takes the inverse of the jacobian w.r.t cartesian elements (but in this case that’s always the identity matrix.

Am I missing a way to get the Jacobian with respect to Parameters for a cartesian orbit?

2

Hi @jajamian

Welcome to the Orekit forum!

What you could try is to convert your initialGuess into a KeplerianOrbit using OrbitType.KEPLERIAN.convertType(initialGuess) and call the getJacobianWrtParameters() method

Regards,
Bryan

3

Please also see the following description for the Jacobian methods of Orekit:

getJacobianWrtParameters : Compute the Jacobian of the Cartesian parameters with respect to the orbital parameters

getJacobianWrtCartesian : Compute the Jacobian of the orbital parameters with respect to the Cartesian parameters

In the description, the “orbital parameters” are defined by the orbit type you use when calling the method. Therefore, using a CartesianOrbit you will have identity matrices. dC/dC and dC/dC
If you convert your orbit into a KeplerianOrbit, you will have :

  • getJacobianWrtParameters : dC/dK
  • getJacobianWrtCartesian dK/dC