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I was wondering how Gpredict calculates the ground tracks, Lat, Long and elevation based on a ground station.

If I understand correctly, Gpredict takes the values from a TLE (Two-Line Element) set, then propagates these values using the SGP4 model to obtain geocentric coordinates. It subsequently converts these coordinates to topocentric coordinates based on the ground station's latitude and longitude. Is my understanding correct, or have I misunderstood the process? How does it compute the elevation and azimuth? Are these calculated from the topocentric coordinates?

In Gpredict, it displays latitude and longitude within the black rectangles. Are these values derived from topocentric or ECEF (Earth-Centered, Earth-Fixed) coordinates?

I'm genuinely only interested in understanding the structural aspects of the Gpredict algorithm. EDIT: I added some links. Main page: http://gpredict.oz9aec.net/ From source, open source: https://github.com/csete/gpredict

I added this image based on a ground station in Bremen: enter image description here

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    $\begingroup$ @uhoh I added two links to my post. $\endgroup$
    – ed190
    Commented Oct 21, 2023 at 20:10
  • $\begingroup$ Great, thanks! $\endgroup$
    – uhoh
    Commented Oct 21, 2023 at 22:23

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