I am using the SGP4 python package to get the coordinates of satellites at a given epoch. The algorithm computes the positions (X, Y, Z) in the True Equator Mean Equinox coordinate frame (TEME). From that, I would like to find the corresponding RA DEC coordinates at the given epoch. I thought this was just a change of frame from Cartesian to spherical coordinates, with RA = arctan(Y/X) and DEC = arcsin(Z/R). However, it does not seem to be the case at all. Looking at Stellarium, which give both TEME and RA/DEC coordinates for satellites, I have no idea how to get from one to the other. Is it complicated?

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    $\begingroup$ There will be a few rotations, and long algorithms to compute the angles. Routines to do it are at the link below, and have been ported to more languages than listed there. For Python, the "skfield" library is often used for this. celestrak.org/software/vallado-sw.php $\endgroup$ Dec 9, 2022 at 18:02
  • $\begingroup$ Thank you for your answer, the skfield library looks like exactly what I needed! I will look into it. I just struggle to understand the difference between the TEME frame and the RA/DEC frame of the current epoch, it seems like exactly the same equatorial coordinate system to me. $\endgroup$
    – laetitia
    Dec 9, 2022 at 20:24
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    $\begingroup$ There are many different RA/Dec systems. Usually for satellites, you want apparent topocentric coodinates, the coordinates that you get from SGP4 are geometric geocentric coordinates. $\endgroup$ Dec 9, 2022 at 23:38
  • $\begingroup$ This is why then! I did not realize Stellarium implicitly shifted the center of the frame for the RA/Dec, but this makes sense of course. I usually work with stars, for which this problem does not occur. $\endgroup$
    – laetitia
    Dec 10, 2022 at 19:09


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