Space-Based Measurements using AngularRaDec

1

Hello everyone, I kindly ask for your feedback on the following approach!

What I need

I would like to perform Batch Least Squares Orbit Determination of an object, given space-based observations.

What I know

  • Right Ascension / Declination measurements of a space object, from a space-based telescope
  • Measurement uncertainty expressed in ra / dec respectively
  • Position vector of the space-based telescope at each observation time, expressed in J2000.

What I would like to confirm

  1. Since the uncertainty is expressed in ra/dec terms, I feel it makes sense to use AngularRaDec to add the measurements (subject to your critical review).

  2. In my effort to add AngularRaDec measurements, I notice that the constructor requires a GroundStation location, but my telescope is not fixed to the ground!

Considering that:

  • GroundStation instanciates with a TopocentricFrame, and
  • TopocentricFrame implements PVCoordinatesProvider

Is there any problem with assuming that my space-based telescope is momentarily fixed in ITRF, and use the J2000-to-ITRF position of my observer to define a “fake” GroundStation (geodetic point) for every measurement, as the location of my space telescope?

As I understand, an alternative way would be to define a new Measurement type, very similar to AngularRaDec, but for space-based measurements, according to this thorough explanation by Luc.

Thank you deeply in advance!
Manny

2

Hi @Manny

We have already a feature request for this type of measurement here: Add support for Space-Based Visible (SBV) observations (#754) · Issues · Orekit / Orekit · GitLab.
So the best thing would be to implement the measurement and contribute it for closinf the feature request. This is a simple measurement, and looking at both the AngularRaDec and the InterSatelliteRange measurements would help to do this work, which I guess is a good and interesting first contribution.
You could also use the trick to use a fake GroundStation, but you would have a wrong relativistic effect (angular deviation due to telescope own velocity). It is probably not a big problem as I don’t think the accuracy of on-board telescopes will sufficient to notice it.

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