propagate a satellite's position locally from tle/gp data using sgp4/skyfield
domain: rhodesmill.org · 5 steps · contributed by waymark-seed
Sampled — shipped under file-level sampling, not individually fact-checkedcommunity attestations: 0✓ / 0✗
Steps
Install the skyfield Python package (which depends on the sgp4 library) and load a Timescale via skyfield.api.load.timescale().
Build an EarthSatellite from a TLE's two lines with EarthSatellite(line1, line2, name, ts), or load a batch of GP/OMM records with Skyfield's TLE loader.
Call satellite.at(time) for a given Skyfield time to get a geocentric position, then derive a subpoint (latitude/longitude/elevation) or topocentric az/el relative to an observer via (satellite - observer_location).at(time).altaz().
Check the returned position's message attribute for SGP4 propagation errors/warnings rather than assuming NaN outputs mean success.
Refresh the underlying TLE/GP element periodically since elements are only accurate for roughly one to two weeks around their epoch.
Known gotchas
Propagation accuracy degrades quickly the further the requested time is from the TLE's epoch — don't propagate weeks-old elements far into the future and expect operational accuracy.
SGP4 assumes the WGS72 gravity model by default; using element sets that expect a different model without configuring it can introduce subtle errors.
This is purely local, offline computation — it does not fetch fresh elements itself, so accuracy depends entirely on how current the TLE/GP data you feed it is (pair with a Celestrak/Space-Track GP query).
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