Pick a satellite and a location, and this tracker lists the upcoming passes, draws the selected one on a live sky plot and shows the ground track on a 3D globe. Pass prediction runs on my own server, with no third-party API key.
At a glance
Part
Choice
Orbital data
CelesTrak orbital elements (TLE/GP), cached on the server
Visible pass: the satellite is sunlit (outside Earth's cylindrical shadow) while the sun is at least 6° below the observer's horizon, the limit of civil twilight. Both conditions are checked at the peak.
Next passes across satellites
The "Next passes" list covers many satellites at once, so the candidate set is cut down first:
Candidates come from CelesTrak's group of bright satellites.
Orbits are filtered by comparing their inclination with the observer's latitude.
At most 80 candidates are searched, with up to 6 passes each.
In the browser
Sky plot: polar, north up and east on the left, as when looking up at the sky. The browser recomputes the selected pass at a few seconds' resolution and moves a live marker along it.
Globe: the ground track for the next 120 min on a three.js globe, refreshed in the background, with a day/night terminator from the computed sub-solar point.
Location: browser geolocation or manual entry.
Sharing: the satellite and location are kept in the URL, so any view can be sent as a link.
Timeline: passes are laid out in local time.
Caching
Data
Scope
Kept for
Orbital elements, group
Per CelesTrak group
6 h
Orbital elements, single
Per satellite
24 h
Next passes list
Per observer
10 min
If a CelesTrak refresh fails, the server serves the stale copy instead of an error.
Limits
No refraction: elevations are geometric, so positions near the horizon are slightly off.
No terrain mask: the horizon is assumed flat and clear.
No penumbra: the shadow is a cylinder, so the switch from sunlit to shadow is a hard edge.
Visibility at the peak only: a pass that enters or leaves shadow partway through is classified by its peak.
No magnitude: the tracker says whether a pass is visible, not how bright it is.
No Doppler: no prediction of the frequency shift needed to work a satellite by radio.
