About WatchSat

WatchSat is a real-time 3D satellite tracker that propagates roughly 16,000 active satellites live from public orbital data and renders them on a physically-motivated globe.

What WatchSat does

WatchSat shows where satellites actually are, right now. Open the live globe and every point is a real spacecraft whose position is being computed in your browser for the current instant — the International Space Station, thousands of Starlink and OneWeb broadband satellites, the GPS, GLONASS, Galileo and BeiDou navigation fleets, weather and Earth-observation satellites, and more. You can speed up time, follow a single object around its orbit, and see its ground footprint.

How it works

Every satellite’s orbit is described by a Two-Line Element set (TLE) — a standardized snapshot of its orbit published by the US Space Force and distributed through CelesTrak. WatchSat fetches the active-satellite catalog through an edge-cached proxy and feeds each TLE into the SGP4/SDP4 orbital model (via the open-source satellite.js library) to compute a live position and velocity. See the FAQ for what TLEs and SGP4 are in more depth.

Rendering thousands of moving objects smoothly is done on the GPU: the app sends two exact orbital samples per update interval and a vertex shader interpolates between them, so orbits stay correctly curved even at high time-warp with almost no per-frame CPU cost.

Built on real imagery and physics

Realism is a first-class goal, not decoration. The Earth uses real NASA imagery; the atmosphere is shaded with a physically-motivated altitude profile rather than a flat glow; and the scene includes a real celestial layer — the Sun, Moon, and all five naked-eye planets (Mercury, Venus, Mars, Jupiter and Saturn) are placed using astronomical ephemerides for the current time, against NASA’s Deep Star Maps all-sky background (Scientific Visualization Studio). Every planet renders with real mission-derived imagery — MESSENGER, Cassini, New Horizons — at its true sky position, with brightness driven by live apparent magnitude. That is why the lighting on Earth and the day/night terminator line up with reality.

Data sources & accuracy

Orbital data comes from CelesTrak (redistributing US Space Force tracking data); Earth and sky imagery comes from NASA, ESA and USGS. Full credits are on the credits page. Positions are typically accurate to within a few kilometres for recently-updated low-Earth-orbit objects and degrade as element sets age — WatchSat displays the age of its data so you can judge it. It is an educational and visualization tool, not a source for collision avoidance or antenna pointing.

Who it’s for

WatchSat is free and made for anyone curious about what is overhead — students, educators, astronomers, space enthusiasts, and people who just want to watch the sky fill up in real time. It is an independent project and is not affiliated with NASA, ESA, SpaceX, or any satellite operator.

Questions, corrections, or feedback? Get in touch.

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