A TLE (Two-Line Element set) is a compact, standardized encoding of a satellite's orbit at a moment in time, and SGP4 is the propagation model that turns a TLE into the satellite's position and velocity at any future time.
Pass prediction starts from the most recent TLE for each satellite (published by sources such as Space-Track and CelesTrak) and uses the SGP4 model to propagate the orbit forward. From the propagated positions it computes ground tracks, look angles, and the times a satellite can see an area.
TLE accuracy degrades over days as the real orbit drifts from the modelled one, so fresh TLEs matter: a stale element set produces inaccurate passes. PassPrediction refreshes its catalog on a strict schedule to keep predictions reliable.
Predictions are only as accurate as the underlying TLE. Pass GEOMETRY — off-nadir angle, sun elevation, look direction — is generally representative even when the element set is a few days old. Exact pass DATES and TIMES are more sensitive: the ground-track walk can drift from minutes up to several days when a satellite's public TLE is not current.
The effect is largest for RECENTLY LAUNCHED and ACTIVELY MANOEUVRING satellites (new constellations still raising or phasing their orbits), whose publicly available elements can lag the operator's own precise ephemeris. For these, PassPrediction may find the correct overpasses but place them on a shifted schedule.
Treat predictions as a planning aid. Before committing to an acquisition or tasking window, confirm timing against the operator's ephemeris or schedule.
A Two-Line Element set is a standardized two-line text encoding of a satellite's orbital elements at a specific epoch, used as the input to orbit-propagation models like SGP4.
The real orbit drifts from the modelled one due to drag and perturbations, so a TLE loses accuracy over days; fresh TLEs keep propagated positions and pass predictions accurate.
Pass geometry (off-nadir angle, sun elevation) is representative, but exact dates and times depend on TLE currency and can drift — from minutes up to several days — for satellites with stale element sets or that are recently launched or actively manoeuvring. Verify timing against the operator before any firm tasking.
Plan a real acquisition over your area on the interactive map, browse the satellite catalog, or read the tasking guides.