load OEM file

[mikezang]

Can you provide a new API to load OEM file? So that we can get what we get from TLE file.

[brandon-rhodes]

Could you provide an example of an OEM file, that illustrates the format? Or a link to a site providing OEM files?

[mikezang]

Here you are!
CSS_OEM_20241009001030_0001.txt

[brandon-rhodes]

Thanks for the example! Having looked over the file, I can see documentation for it (among other places) here:

https://public.ccsds.org/Pubs/502x0b3e1.pdf

I am not, alas, aware of any Skyfield contributors who are currently planning to sit down and implement OEM support in the near term. Everyone seems to be busy using OMM instead.

[mikezang]

Is it difficult to use OEM file instead of TLE data?

[brandon-rhodes]

You can look over chapter "5 ORBIT EPHEMERIS MESSAGE" of the PDF to see the details. It looks like it would take a few days of work, perhaps, for someone to implement? Parsing the file would, happily, be fairly simple, as the format is easy to read and understand. The work involved would be: implementing any new reference frames that OEM files might use, like the EME2000 reference frame used in your example file CSS_OEM_20241009001030_0001.txt; implementing the interpolation methods that OEM files can request through the INTERPOLATION and INTERPOLATION_DEGREE keywords; and, finally, figuring out what to do with files like this one, that don't specify an INTERPOLATION. Would Skyfield have a default option? Or, maybe much better, the person loading the file without an INTERPOLATION would have to select an interpolation method themselves.

In any case, my guess is that implementing the reference frame and at least one initial interpolation method would be the biggest tasks.

[gfireman]

I second this request. We use OEM files for planning PACE instrument operations. TLEs are not updated until after a delta-V, but predicted ephemerides have the expected new orbital parameters built in.

Alas, I have neither the time nor the expertise to implement.