[Orekit Developers] orbit determination in DSST

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

Hi all,

As mentioned during the Orekit day a few months ago, we have started
work on adding orbit determination capabilities in the Orekit implementation
of DSST.

I have an intern (Bryan Cazabonne) working on this since last week. Work will
be performed in a dedicated Orekit lab sub-project:
<https://www.orekit.org/forge/projects/orekit-dsst-orbit-determination>,
with its own git repository separated from the official Orekit repository, so
Bryan can have write access without touching the official tree. If people want
to look at what is done there, the project is public.

The first step is to set up the DSST specific force models so they provide
the Jacobians of the mean rates. This step is expected to take some
time to complete. We will use an approach to what is already done in
NumericalPropagator, i.e. using RealFieldElements/DerivativeStructure to
compute the derivatives by automated differentiation.

The second step will be to apply variational equations on the Jacobians to get
the state transition matrices during propagation in addition to the  
mean states
themselves. This step is expected to be easy because it is basically the same
that is already done in PartialDerivativesEquations for NumericalPropagator,
and DSSTPropagator already can accept additional equations  
(addAdditionalEquations
is provided by the parent class of both DSSTPropagator and  
NumericalPropagator).

The third step will be to link everything with the orbit determination engine.

With this setup, we will have a preliminary OD capability that will compute
the derivatives using only the mean elements, not the short periodic
elements. Therefore, additional steps will be required to improve the
derivatives, mainly by adding at least the J2 contribution to the derivatives.
However, I don't know if Bryan will have enough time for this, lets do
it step by step.

I think even a preliminary OD capability with not perfect derivatives
would be an interesting feature to add to Orekit, so we intend to merge
this project to the main tree as soon as possible.

We may ask for help or documentation to some of you to complete this project.

best regards,
Luc