SpacecraftState contains an orbit, an attitude and a mass, plus additional states if you want to add some. Both orbit and attitude reference a frame, and this frame must be the same when they are combined inside a SpacecraftState. In addtion, this frame must be inertial (this constraint comes from the orbit in fact).

So if want to use an Earth frame, you need to perform the conversion on objects that are lower level than spacecraft state and orbit. You should probably use TimeStampedPVCoordinates for this. If you define the pv coordinates in an Earth frame, than you can convert them to inertial frame and build your orbit and spacecraft state from here.

‘Orbit’ is an abstract class in Java so it cannot be instantiated.
You need to choose an orbit type. In your case, a ‘CartesianOrbit’ seems appropriate.

@bcazabonne,
would you be so kind as to confirm that this implementation of conversion from WGS84 to EME2000 is correct from the theoretical point of view?
Thank you very much.
Kind Regards,
Alfredo