Hello,Thanks for the suggestion. I think that you refer to the paper updated every three year by the IAU Working group on cartographic coordinates and rotational elements. In this paper, the IAU publishes with a high precision information such as shape, size, pole position and rotation axis and rates for the celestial bodies in the solar system.
I am still a beginner with Orekit and I do not know if all these parameters are taken into account in the definition of the celestial bodies of the CelestialBodyFactory. I am quite sure that these ones are taken into account for the Earth as the EOP (Earth Orientation Parameters) are implemented and may be provided by the IMCCE database for instance.
But in the way I see Orekit and in the way I am trying to use it, these parameters are not important. I am only working with newtonian attractions from the different celestial bodies. I therefore do the approximation that there is an homogeneous distribution of the mass in a spherical body and that the attraction of each body can be reduced to the one produced by an infinitisimal point with a mass. If the orientation parameters and the flattening of the different bodies are taken into account for the computation of the central body attraction and are not for the ThirdBodyAttraction, then a difference may appear due to the fact that you are on the "big side" or the "light side" of the planet. My mistake would therefore be to change from a central body attraction dealing with the orientation parameters and mass repartition corrections to a simple newtonian attraction. This may explain the results I have, even if I am surprised of the magnitude of the errors due to this effect...
I may be wrong but I think that Orekit allows for motion about any arbitrary central body. I understand that when you want define an orbit, this one should be defined in an inertial frame as the concepts of frames and referentials seem to be combined in Orekit. Then, the computation of the acceleration of the spacecraft is done using the inertial frame/referential of the central body. In Orekit, each celestial body has a function that returns an inertially oriented frame that is oriented like the Earth EME2000 frame (meaning with an inclination of about 23.4° with the ecliptic plane). I therefore think that Orekit is designed for what I want but, once again, I may be wrong.
Thank you very much again for your answer, I will create my own celestial bodies with only a spherical shape, a simple mass model and a position provider given by JPL ephemerides. I then hope to avoid any problem due to orientation and mass corrections.
I keep you informed very soon, Quentin Le 31/03/2014 21:23, paulcefo a écrit :
Hello. Is Orekit designed to allow for motion about an arbitrary cental body? If so, what characteristics can this central body have?I am thinking of the paper that Ken Seidelmann updates for the IAU every couple of years.Paul