Satellite orbits, passes and eclipses
Track a satellite from its TLE: its live orbit on a globe, predicted ground station passes, and eclipse and beta angle along the orbit.
Satellite orbits on a globe
Add a satellite by its NORAD catalog number, and the Earth panel shows its live position and orbit, computed from its TLE.
Why
Where is the spacecraft right now, and where is it going next? That is the first thing anyone asks before a pass, and it should not need a separate tool open in another window.
How it works
When you add a satellite, Plexus looks up its current two-line element set (TLE) from a public satellite catalog by NORAD number and stores it on the device.
The Earth panel propagates that TLE with the standard orbit model in a background worker in your browser. It draws the satellite's position and its orbit, and moves them in real time. Day and night shading shows which side of Earth is lit.
Using it
- Go to Devices, choose Add device, and type the satellite's NORAD number. Plexus finds the TLE as you type.
- Give it a name, for example sat-alpha, and save.
- On a dashboard, add an Earth panel and select the device. Add more satellites to the same panel to see them together.
Limits
A TLE describes the orbit at one moment. Predictions get worse the further you are from that moment, so an old TLE means an approximate position. Past 30 satellites the panel switches to simpler point markers to stay fast, and it draws orbit paths for up to 100.
Predicted ground station passes
The Gantt panel can predict ground-station passes from a satellite's TLE and your station's location, so the pass schedule is there before the first packet arrives.
How it works
For each satellite and station pair, Plexus propagates the TLE with the standard orbit model and finds when the satellite is above the station's horizon mask. Each pass is a bar from acquisition of signal to loss of signal, and is marked planned, in progress or completed.
Nothing is read from a telemetry source, so it works for a spacecraft that has never sent a point.
Using it
Add a Gantt panel and open Ground station contacts in its settings.
- Pick the satellites, for example sat-alpha. They come from your devices, so the TLE is the one stored on the device.
- Add a station with a name, latitude and longitude. Height is optional.
- Set the horizon mask. The default is 5 degrees.
- Set how far ahead to look. The default is 24 hours when the dashboard is on a live time range. Otherwise the panel predicts over the dashboard's window.
Limits
A pass is geometric. The horizon mask stands in for terrain, atmosphere and the antenna's pattern, so pick a mask that matches your site. Pass times are only as good as the TLE, and they drift as it ages.
Eclipse, beta angle and orbit events on the globe
Turn on flight dynamics in an Earth panel and each orbit shows where it passes through Earth's shadow, along with the beta angle, the next events and a simulation clock. Every number has an (i) that says how it was computed.
How it works
Positions come from the spacecraft's TLE, propagated with the standard orbit model. The Sun's position comes from a standard solar model, and Earth's shadow is a cone on the WGS84 ellipsoid.
- Eclipse shading: each orbit ring is one revolution, sampled 240 times. Full color in sunlight, dimmed in penumbra, dark in umbra.
- Beta angle: the Sun's angle above the orbit plane. It sets how long eclipses last.
- Next events: ascending and descending node, penumbra and umbra entry and exit, and perigee and apogee for noticeably elliptical orbits. Each is found to 0.1 s and marked on the orbit where it will happen. The console looks 24 hours ahead by default.
- Sim clock: Live follows the wall clock. Play runs a simulation at 1x, 10x, 60x or 100x. Hover or pin a time on any chart on the dashboard and the globe jumps to that moment.
Every value explains itself
Each value has an (i) that says how it was computed and how good it is. For example, shadow boundaries were checked against an independent reference to within 0.25 s, and the beta angle to within 0.01 degrees. No atmosphere is modelled, so real shadow entry and exit can differ by a few seconds.
TLE age is shown next to everything. It turns amber after 3 days and red after 7. When the stored element set is more than 7 days old, the panel fetches a fresh one from a public satellite catalog.
Using it
Add an Earth panel with a satellite device, for example sat-alpha. In the panel settings, turn on Flight dynamics. You can turn eclipse shading and event markers on or off, and change how far ahead the console looks.
Then put a battery voltage chart on the same dashboard and hover over it. The globe follows your cursor.
Limits
Everything inherits the accuracy of the TLE. Element sets for small or manoeuvring satellites are usually worse. The penumbra percentage uses a flat-disc approximation, which is fine for a readout but not for a power budget.