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Re: [Orekit Users] Problem when doing propagation from different point of views



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