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I want to compute the distance of a certain satellite, given the TLE, to a ground station in the earth. I have the latitude, longitude and altitude of the ground station. But I don't know if the TLEs are computed using the ECEF or ECI models.

Does this question even make sense? Or am I mixing things?

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What you see in the comments are all valid partial answer. Your question is not specific but almost make sense.

TLEs are not written in a cartesian frame, they are in the orbital element frame. That said, as TLE are special mean orbital elements, you need to use them with SGP4 propagator (see celestrak.com/software/vallado-sw.php suggested by @Greg Miller or the Astrodynamic Standard Package released on Space-Track.org). SGP4 does output spacecraft position and velocity at some requested time. So with respect to your question: TLEs inserted in a SGP4 propagator produce ephemeris (position and velocity) in an ECI (Earth Centered Inertial) frame.

There are however different kind of ECI frames, slightly different definitions depending on what is more "handy" and precise for the application. SGP4 outputs (given a TLE) are given in TEME and its precise definition is given by Seago and Vallado in Coordinate frames of the U.S. Space Object Catalogs (DOI: 10.2514/6.2000-4025).

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  • $\begingroup$ You really should not reference "see comments" in your answer. Comments are ephemeral and subject to deletion at any time. Please incorporate the information from the comments into your answer. You can also give props to the author of the comment, but it's not especially required. $\endgroup$
    – CGCampbell
    Mar 25, 2022 at 14:22
  • $\begingroup$ @CGCampbell Although comments are ephemeral, they're still copyright material covered by the CC-BY-SA license, and so copying from comments requires attribution. Please see meta.stackoverflow.com/q/416665/4014959 Although it's on meta.SO, it's a recent post on current network-wide policy. $\endgroup$
    – PM 2Ring
    Mar 25, 2022 at 15:49
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    $\begingroup$ @PM2Ring Aha, thank you, I stand corrected. $\endgroup$
    – CGCampbell
    Mar 25, 2022 at 15:59

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