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Re: [Orekit Users] 0.1 deg Inclination bias?


"Hammel, Gregory (ASC/CSA)" <gregory.hammel@canada.ca> a écrit :


Hello Luc,
Thanks for this. This was actually going to be my next question - how to use LS to convert from osculating to DSST - was digging around the estimation project and was starting to run into a brick wall at the PropagatorBuilder. I didn't realize that my two problems may be actually only one.
If this is the source of the problem, then what purpose does the DSSTPropagator.computeMeanState function serve? Is is intended to be a quick way to get mean states when precision isn't required? 

No, it is really intended to do what you want, but using only
DSST models, not converting models at the same time, i.e.
without a "first propagation" from another model beforehand.

Some other things you could check are the difference between mean
and osculating, i.e. the amplitude of the short periodic
terms and the evolution rate of the mean elements, and compare
this to the 0.1° you see. It may help understand what happens.

best regards,
Luc



Will give this a go. Thanks,

Greg
________________________________________
From: orekit-users-request@orekit.org <orekit-users-request@orekit.org> on behalf of MAISONOBE Luc <luc.maisonobe@c-s.fr> 
Sent: Thursday, July 19, 2018 2:08 PM
To: orekit-users@orekit.org
Subject: Re: [Orekit Users] 0.1 deg Inclination bias?

"Hammel, Gregory (ASC/CSA)" <gregory.hammel@canada.ca> a écrit :


Hello,


Hi Greg,



This is my first post to this list, so I would just like to start by
thanking everyone who contributes to this project. Its an incredible
resource.


I am trying to use Orekit for mean state generation for
stationkeeping using DSST. To that end I want to take in a vector
from an external source (in this case, STK) read it in, propagate
it, including manoeuvres, and then use DSST to get pre and
post-maneouvre mean states. If I take a J2000 vector and provide it
to DSST to get a mean state, I get an inclination of 97.75 deg.
However, if I first propagate that same vector and then pass it
through DSST I get an inclination of 97.65 (in this case, without
any manoeuvres).


I'm using the same methods as outlined in the "MasterMode.java"
tutorial for my "pre-mean" propagation, but I've modified it by
including solar and lunar grav and DTM2000 drag.


Can anyone explain why there is a 0.1 deg shift in the inclination?
I can provide the code if it helps.


If you convert starting from a single state vector, and change from
one propagator to another one, you can have this sort of problems.

I'll suggest to look at the propagation.conversion package and
at the associated test cases to do the conversion from a full
time range (say one or two periods) instead of a single state
vector. What this package does is a fitting, trying to ensure
the trajectories stay close to each other, in a least squares
sense.

Hope this helps,
Luc




Thanks,

Greg Hammel

___________________________________________________
Gregory Hammel
Analyste de la dynamique de vol
Contractuel SED a division of Calian Ltd. Utilisation de l'espace
Agence spatiale canadienne / Gouvernement du Canada
gregory.hammel@canada.ca / Tél.: 450-926-5138

Flight Dynamics Analyst
Consultant SED a division of Calian Ltd. Space Utilization
Canadian Space Agency / Government of Canada
gregory.hammel@canada.ca / Tel.: 450-926-5138