A real-time 3D tracker for the Earth-observation fleet. Satellite positions are computed in your browser by propagating each spacecraft's NORAD Two-Line Element set (TLE) with the SGP4 analytical model — the same near-Earth orbital model used by mission operators.
Orbital elements are fetched live from the TLE API (sourced daily from CelesTrak / U.S. Space Force 18th SDS), with an embedded snapshot as offline fallback. Positions are converted ECI → geodetic, propagated via satellite.js (an SGP4/SDP4 port of Vallado et al.).
The day/night terminator is rendered from the true solar sub-point (low-precision solar ephemeris), so the lit hemisphere matches reality at the simulated time.
"Imaging revisit" is the instrument's coverage frequency (e.g. MODIS/VIIRS near-daily, OLCI ≤1.9 days), which differs from the orbital ground-track repeat cycle shown separately. "Next acquisitions" uses the operator's official plan via the Spectator Earth API (Copernicus/USGS) when an API key is provided for Sentinel-1/2 and Landsat-8/9; otherwise it falls back to a geometric model that times when the imaging swath actually covers your location.
SGP4 on a fresh TLE is good to roughly 1–3 km near epoch, degrading over days/weeks. This is excellent for visualization and planning but is not a substitute for operational ephemerides.
Hoots & Roehrich (1980) Spacetrack Report No. 3; Vallado, Crawford, Hujsak & Kelso (2006) Revisiting Spacetrack Report #3, AIAA 2006-6753; CelesTrak GP data.