No "ToGeodetic" Method Available (for calculating latitude/longitude from ephemeris)?

[gbmhunter]

The sgp4 C++ reference library has a very useful Eci::ToGeodetic() method (see https://github.com/dnwrnr/sgp4/blob/0cc181ee50c707d1a433e97be98de2b925dd78a4/libsgp4/Eci.cpp).

This is very useful for converting a propagated ephemeris into latitude/longitude values. As far as I can see, this functionality does not exist in this python version. Is there any plan to, or was there a specific reason why this was not incorporated?

The Javascript version also has this functionality (https://github.com/shashwatak/satellite-js).

[brandon-rhodes]

My intention was for sgp4 to bring the bare algorithm from the paper into Python, and then to layer more sophisticated routines on top of this library by incorporating it into Skyfield. Which, incidentally, I am adding a to-geodetic routine to this week (yay, finally!). Here's the issue tracking my work:

https://github.com/skyfielders/python-skyfield/pull/42

Is there any chance you could try out Skyfield and see how it works for you? If not, maybe you could snag the routine I wrote for the geodetic work, and pull it into your own project.

[gbmhunter]

Ah o.k., is it the GCRS_to_Topos(xyz, jd) method in skyfield/positionlib.py?

[brandon-rhodes]

Alas, January 2018! I didn’t answer immediately because (if I recall) I was at that moment working on Skyfield’s subpoint() routine but maybe wasn’t done yet. In any case, Skyfield does now support computing the longitude and latitude beneath any sky object:

https://rhodesmill.org/skyfield/api-position.html#skyfield.positionlib.Geocentric.subpoint

Enjoy!

[gbmhunter]

Thank you!