Ground Track & Pass Predictor

Everything here starts from a Two-Line Element set (TLE) — the compact NORAD element format that encodes a satellite's mean orbital elements at a reference epoch. TLEs are mean elements fitted to the SGP4 (Simplified General Perturbations 4) analytical propagator, so they only mean something when propagated with SGP4 — feeding TLE elements into a generic Keplerian propagator gives kilometres of error immediately. This tool runs the genuine article: satellite.js, a faithful port of the reference SGP4/SDP4 implementation from "Revisiting Spacetrack Report #3" (Vallado et al. 2006).

SGP4 outputs position in the TEME (True Equator, Mean Equinox) inertial frame. To place the sub-satellite point on the map, the tool rotates TEME into the Earth-fixed ECEF (Earth-Centered, Earth-Fixed) frame using Greenwich Mean Sidereal Time (GMST) at each sample, then converts to geodetic latitude/longitude on the ellipsoid. Pass geometry uses the standard topocentric transform (Vallado §4): the station's look angles — azimuth and elevation — are computed from the ECEF vector between station and satellite. A pass is a contiguous interval with elevation at or above your elevation mask; AOS (Acquisition of Signal) and LOS (Loss of Signal) are the rising and falling mask crossings, interpolated between samples, and TCA (Time of Closest Approach) — the elevation peak — is refined numerically inside the pass.

What bounds the accuracy: TLE freshness, more than anything else. A fresh element set is good to roughly a kilometre; the error grows about 1–3 km per day away from epoch, so pass predictions from a week-old TLE can drift by tens of seconds, and a month-old one is geometry practice, not a schedule. The tool flags a stale epoch but can't fix it — paste a current TLE from CelesTrak or Space-Track before committing an antenna. Drag is modelled only as far as SGP4's own B* term goes (no density weather, no storms), maneuvers are invisible until they show up in the next published element set, and the bundled example TLEs are deliberately historical — fixed epoch, fixed window — so the examples are reproducible rather than current.

The map is deliberately a grid-only equirectangular plot — a ground track, a station marker, and the station's visibility circle (the locus where the satellite clears the mask, from standard coverage geometry). No basemap tiles: nothing on this page phones out, and a graticule is all a ground track needs to be read correctly. This is the same physics family as the orbit-propagator and ground-station-pass tools ENKI uses server-side.