Re: [Orekit Users] CelestialBody Position extrapolation

Hello Orekit users,

I find this post interesting because Simon tried to extrapolate the position and velocity of celestial bodies in the solar system. We don't need to do it as JPL already did it for us for instance, but I tried to propagate the orbit of Venus (for instance) during 30 days and I have a difference in the position, compare to the translated JPL ephemerides, of more than 35,000 km !

I was interested by this when I was comparing results of propagation of spacecrafts far from Earth to their "actual" positions, given by the Horizon JPL website. I then decided to do the same comparison on a simple body, like Venus. You will find enclosed a main class in which I am just using keplerian attractions from the Sun and the 8 other planets of the Solar System (assuming Pluto is a planet...). I know that I should add the solar radiation pressure to the propagator to be more realistic, but this doesn't work (or gives very strange results, such as Venus at a distance of 100AU from the Sun, using a sphere model with the Venus radius). Even if I do not take into account the solar radiation pressure, I do not expect to have more than 35,000km of difference after 30 days !

I tried to do the same thing propagating around the Sun, but it almost gives the same results (not exactly, as the propagation around the Sun and the Earth are not equivalent for the moment, see the "PropagationFromDifferentPointOfViewPb" post).

Does anyone have an idea of what I did wrong or what is not working ? This may also help the discussion about the problem of propagation from different points of view.

Thank you very much !

Quentin

2013-03-29 19:48 GMT+01:00 Spörri Simon (s) <simon.spoerri@students.fhnw.ch>:

Hi Luc,

thanks a lot for your help on even the most amateurish questions!

it works charmingly now.

cheers,

simon

From: MAISONOBE Luc <luc.maisonobe@c-s.fr>
Reply-To: <orekit-users@orekit.org>
Date: Fri, 29 Mar 2013 14:19:07 +0100
To: <orekit-users@orekit.org>
Subject: Re: [Orekit Users] CelestialBody Position extrapolation

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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