Question 1

Is the satellite tracking real-time?

Accepted Answer

Yes. Every satellite is propagated live in your browser from its most recent orbital elements, so the position you see is computed for the current moment (or the moment you fast-forward to). Nothing is pre-recorded — the globe advances in real time until you pause or warp the clock.

Question 2

How accurate are the positions?

Accepted Answer

Positions come from the same SGP4/SDP4 model and public TLE data that professional trackers use, so they are typically accurate to within a few kilometres for recently updated low-Earth-orbit objects. Accuracy degrades as an element set ages and for satellites that manoeuvre; WatchSat shows the age of the data so you can judge it. It is intended for visualization and education, not for collision avoidance or targeting an antenna.

Question 3

What is a TLE?

Accepted Answer

A Two-Line Element set (TLE) is a compact, standardized description of a satellite’s orbit at a specific moment (the "epoch"): its inclination, eccentricity, mean motion, and other Keplerian-style parameters packed into two 69-character lines. TLEs are published by the US Space Force and distributed through CelesTrak, which is where WatchSat gets them.

Question 4

What is SGP4?

Accepted Answer

SGP4 (Simplified General Perturbations 4) is the standard mathematical model that turns a TLE into a position and velocity at any given time. It accounts for the most important orbital perturbations — Earth’s oblateness and atmospheric drag among them. WatchSat runs SGP4 (via the open-source satellite.js library) for thousands of objects continuously.

Question 5

How often does the orbital data update?

Accepted Answer

WatchSat’s freshness has three layers: the edge-cached proxy in front of CelesTrak caches responses for about 30 minutes; the live 3D view itself re-fetches through that proxy roughly every 2 hours while its tab stays open; and CelesTrak typically publishes new element sets for active objects one to several times a day. Prerendered pages — satellite, constellation, and directory pages — show the catalog date of the snapshot they were built from rather than claiming to be live; the tracker also ships with that snapshot so it loads instantly and keeps working if the live feed is briefly unavailable.

Question 6

Why can’t I find a particular satellite?

Accepted Answer

WatchSat tracks the active-satellite catalog (~16,000 objects), not the full ~30,000-object catalog that includes spent rocket bodies and debris. If a satellite has decayed, was recently launched and not yet catalogued, or is classified, it may not appear. You can jump straight to any object by NORAD id via the URL, e.g. /sat/25544 for the ISS.

Question 7

What do the colors and groups mean?

Accepted Answer

Satellites are grouped by operator and function: space stations, the SpaceX Starlink constellation, OneWeb, global navigation systems (GPS, GLONASS, Galileo, BeiDou), and all other active satellites. Each group has its own color and can be toggled on or off using the layer controls, so you can isolate, say, just the navigation satellites or just Starlink.

Question 8

Can I see the ISS or other satellites from where I live?

Accepted Answer

Yes — select a satellite (for example the ISS) and use “When can I see it?” in its panel. Share your location (or type coordinates) and WatchSat computes the next naked-eye passes from where you stand: rise time, direction, maximum elevation, and duration. Passes cluster around dawn and dusk, when your sky is dark but the satellite is still catching sunlight. Your location is rounded to about a kilometre, stays on your device, and is never transmitted.

Question 9

What are the Sun, Moon, and planets in the view?

Accepted Answer

WatchSat renders 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. This is why the lighting on Earth and the day/night terminator match reality — the scene is physically positioned, not decorative.

Question 10

Where does the data come from, and can I reuse it?

Accepted Answer

Orbital data comes from CelesTrak (which redistributes US Space Force tracking data); Earth and celestial imagery comes from NASA, ESA, and USGS. All are credited on the attribution page. TLE data is public. If you want to build on the underlying data, go to CelesTrak directly rather than scraping WatchSat, so you get fresh element sets and respect their terms.

Question 11

Does WatchSat work on mobile and without a fast GPU?

Accepted Answer

Yes. The 3D globe runs on any device with WebGL, including modern phones. If WebGL is unavailable, WatchSat falls back to a static table of live satellite positions computed from the same data, so the core information is still accessible.

Frequently asked questions