Data & methodology

How WatchSat actually computes what you see — the data pipeline, the orbital math, and the honest limits of both.

Data pipeline

WatchSat’s catalog is the CelesTrak “active” GP catalog — roughly 16,000 objects. An edge proxy (/api/tle) fetches it and caches the response for about 30 minutes; the live 3D view re-fetches through that proxy roughly every 2 hours while its tab stays open. If the live fetch fails or has not run yet, the app falls back to a bundled snapshot and shows that snapshot’s catalog date honestly rather than pretending it is current. The snapshot itself is refreshed automatically twice a week (Mondays and Thursdays) by a scheduled job, independent of visitor traffic.

Position math

Every satellite’s position comes from SGP4 (or its deep-space companion SDP4 for higher orbits), run in your browser via the open-source satellite.js library. Nothing is prerecorded or fetched from a server-side position table: a background worker computes exact SGP4 position and velocity samples for every tracked satellite on a scheduled interval, and a vertex shader interpolates between those samples so thousands of orbits stay smoothly curved even at high time-warp. The satellite you select is additionally propagated directly at the exact displayed time.

Pass predictions

“When can I see it?” combines four checks computed on your device: SGP4 look angles (is the satellite above your horizon, and where), a cylindrical-shadow test for whether the satellite itself is sunlit, your local Sun altitude from astronomy-engine’s topocentric ephemeris (is your sky actually dark), and a standard-magnitude brightness model — combining range and sunlit phase — to filter out passes too faint for the naked eye. As with every SGP4-based prediction, accuracy degrades as the underlying element set ages and for satellites that manoeuvre; WatchSat shows the age of the data so you can judge it, and it is intended for visualization and education, not for collision avoidance or targeting an antenna.

Sky & ephemerides

The Sun, Moon, and planets are placed using Astronomy Engine, an open-source ephemeris library. In the sky view, topocentric body directions are checked against an independent astronomy-engine oracle across multiple sites and dates, and stay within 0.12° of the reference above 15° altitude — tight enough that the lighting on Earth’s day/night terminator and the position of every body in the sky are physically placed, not decorative.

Imagery provenance

Every texture and live imagery source — Earth, Sun, Moon, planets, the star field, and the ground tiles in sky mode — is documented on the Credits & data sources page, including which sources are official space-agency products and which are community redistributions of mission data.

Corrections

Found something wrong — a mismatched position, a stale claim on this page, a satellite that looks off? Tell us. We fix root causes, not symptoms.