Re: [Orekit Users] CelestialBody Position extrapolation

[MAISONOBE Luc <luc.maisonobe@c-s.fr>]

simon.spoerri@students.fhnw.ch a écrit :

> Dear all,

Hi Simon,

> i'd like to create a discrete sampling of celestial body positions for a
> complete orbit of all bodies. I tried both getting positions from the
> celestialBody object directly and using a Propagator to get positions of the
> orbits. However i run into ephemerides constraints and cannot obtain
> coordinates past around 2030 (Error: out of range date for EARTH_MOON
> ephemerides: 2030-05-20T14:36:30.712).

I'm not sure about your needs. Orekit uses JPL ephemerides (or INPOP  
ephemerides) to compute position/velocity of celestial bodies. It does  
not do any propagation by itself for these bodies.

The limitation you get is simply the limitation of the JPL ephemerides  
you use. I guess you simply relied on the orekit-data.zip file  
provided for convenience in the download section of the forge and did  
not change the default set of data contained in this zip archive. If  
you want to go past 2030, you can look inside the archive and you will  
see it contains a file named unxp1962.406. This is a reduce JPL  
ephemeris file. Remove this file and replace it by the original files  
from JPL that you will find here:  
<ftp://ssd.jpl.nasa.gov/pub/eph/planets/SunOS/de406/>. Each file  
contains ephemerides for 300 years. If you retrieve all 20 files, you  
will be able to propagate from year -3000 to year +3000. You can look  
at the wiki page  
<https://www.orekit.org/forge/projects/orekit/wiki/Configuration> for  
further information on how Orekit does load such data.

Please note that JPL ephemerides are already sampled. The correspond  
to large sets of Chebyshev polynomials. Adding another sampling layer  
on top of that is probably not worth the trouble. Just using the  
celestial body itself is probably sufficient for most purposes. The  
current version of celestial bodies that use JPL ephemerides is also  
thread safe (it was not thread safe in the 5.x series of the Orekit  
library), and uses efficient cache mechanisms even when several  
threads uses dates very wide apart from each other. This may be useful  
in a web application as you cannot know beforehand if two different  
clients will request positions around the same dates.

best regards,
Luc

> I try to obtain the positions in the SolarSystemBarycenter Frame.
> My question is if it is possible at all in Orekit to extrapolate  
> body positions
> around 150 years into the future/past? I know that this is not the  
> main purpose
> of Orekit and I do not require very accurate results, nevertheless i  
> wanted to
> use Orekit for that for convenience...
> And if it is possible, how should i do it?
>
> thanks a lot in advance,
> Simon



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