Re: [Orekit Developers] Ephemeris Object Extrapolation

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

Hank Grabowski <hank@applieddefense.com> a écrit :

> I have often run into a problem, especially during some propagation, where
> the Ephemeris propagator doesn't have quite enough data to allow the
> underlying event detector sampler to get to where it needs to go.  It
> doesn't happen often, but it's happened enough that I've considered looking
> at letting users of the library construct an Ephemeris object that has the
> option to allow extrapolation at the boundaries.  My thought is that the
> default behavior would be to keep it exactly as it is now.  I was going to
> add two additional constructors, one that simply has the flag that enables
> extrapolation and then another than allows the user to provide bounds on
> how much extrapolation on either side that is allowed.  This way a user
> can't accidentally sample way outside of the ephemeris range but they can
> avoid an exception for a near in sampling.
>
> Before confirming the add I was going to create test cases for LEO, HEO and
> GEO orbits and was going to see how fast the extrapolation degrades in
> accuracy for these standard orbits.  Assuming that:
>
> 1. The default behavior stays the same
> 2. There is a rigorous unit test for the additional cases
> 3. User options are as defined above
>
> Is this something that the community would like to have committed back to
> the main distribution?

Definitely.

I wonder if we could also set up some hook where the user could  
provide a function to extend the ephemeris if needed? This means the  
ephemeris data is not immutable anymore (which is bad) but may be an  
improvement. Some similar feature exists for example in the JPL  
celestial bodies classes, as each body loads only 50 or 100 days chunk  
at a time, and reloads a new chunk as needed. IT was done to avoid  
loading 300 years of ephemerides when only a few days were needed, but  
could perhaps be generalized. It is also completely hidden in the  
implementation, and not based on a user hook.

Even if we don't go all this way, your patch with the flag is already  
a good thing to commit.

By the way, did you solve the mismatch between Orekit and HORIZON?

best regards,
Luc

> Hank



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