Expected Rotation different from manually computed rotation

Hipparchus usage
1

Hello everyone,

So I’ve been working on the issue #964 and to improve numerical precision, i added a direct conversion method from LOFType to LOFType in the LOFType class without having to resort to an inertial “pivot” frame.

For example, in the case where we want to convert a covariance from NTW to RTN, instead of doing :
NTW -> Inertial frame -> RTN.
We could directly do :
NTW -> RTN.

The problem here is that when I build this rotation using the Rotation class from hipparchus, my test is failing whereas if i build the rotation matrix myself, the test is passing.

You’ll find attached the unit test I’m talking about. In this test, I want to express a covariance matrix, initially expressed in the NTW coordinate system, in the RTN(=RSW=QSW) coordinate system. This test is using tests cases from this Vallado’s paper. In this particular case, a very small rotation around the orbital momentum is needed as the NTW and the RTN are almost identical.

I don’t know why the rotation obtained from the Rotation’s constructor is different from what i manually compute and I’m currently out of ideas. That is why I’m now requesting your help !

Cheers,
Vincent

RotationProblemTest.java (6.9 KB)

2

Hi Vincent,

looks to me as if in one case you define the rotation matrix in the RTN frame while in the other you define it in the inertial frame.

Best,
Romain.

3

Hello Romain,

Now that you mention it, I’m going to take a look at it with your idea in mind.

Thank you,
Vincent

4

Hi Romain,

This was indeed the solution ! I’m now composing rotation to and from the ECI in which the given PVCoordinates are defined so now it works as expected.

Thank you for your insight :+1: