[Orekit Users] Please help me understanding normalizedParameters

["Maria Sousa" <estellamarissousa@gmx.com>]

Dear Orekit group,
 

I'm been using Orekit mainly for Orbit Determination with considerable success (only with simulated data). Thank you for your effort and generosity.

 

However, I'm still confused about the normalized parameters and the impact on the propagation.

 

I am using the NumericalPropagatorBuilder class to build a numerical propagator. It is used for propagation and for orbit determination. When used for propagation, I build a NumericalPropagator from it providing using buildPropgator, with a normalizedParameters argument.
 

However, I don't quite understand the effect of these normalizedParameters for propagation. I usually provide an array of zeros, but I've noticed that if I provide other numbers, the propagation results are different.
 

In addition, I've noticed that after performing an orbital determination, if I use the orbit returned by the estimator to build a new PropagatorBuilder and use zeroes to get a NumericalPropagator, the propagation is slightly different than the propagator returned by the estimation. In fact, I can get the same propagation if I extract the NormalizedParameters from the estimated propagator and use them as normalizedParameters for the new propagator.

I'm a bit confused by this, as it seems that the orbit returned by the estimator is not all the information I need to create a different propagator and propagate the same orbit. It seems that I also need the normalized parameters, which makes no sense for me as I not understand the underlying process.
 

When reading the documentation, I understand that those normalized parameters are defined as (reference value - current value) / scale. So my first idea is that it should not affect the propagation, as the current value is the same in the estimator and in the newly created propagator. However, it differs.

 

I would need a way to extract an orbit from the estimator that I could store and retrieve later to replicate the propagation, without additional normalized parameters. In fact I may use that orbit with a completely different propagator (not Orekit) where those parameters have no use.
 

Could you help me to understand this behaviour?
 

Thank you for your help.