Ground tracks

Applies to Orbit Visualizer v1.1 · Updated July 5, 2026

The Ground Track tab draws each selected satellite’s sub-satellite path on a 2D world map using the same propagator as the 3D scene, with a live position marker at the current scenario time and cross symbols for any ground sites you have added.

Opening the Ground Track tab

1. Add at least one object to the scenario (e.g. from Load from Catalog).
2. Press A or click the ▲ toggle at the bottom of the 3D scene to open the Analysis panel.
3. The Ground Track tab is active by default. If the Access or Conjunction tab is showing, click Ground Track.

▶ Open the Orbit Visualizer with the ISS pre-loaded

Reading the map

The map is an equirectangular projection: longitude is the horizontal axis and latitude is the vertical, with the antimeridian (180°) at both edges. The track is split at the antimeridian so crossing longitudes never draw a horizontal streak across the whole map.

For a low-Earth orbit like the ISS, the sinusoidal shape reflects the orbit’s inclination: the track swings between +inclination and −inclination in latitude as the spacecraft circles the Earth. Successive ascending-node crossings (where the track crosses the equator heading north) march westward by roughly 23° per revolution for a ~90-minute orbit — the result of Earth rotating eastward beneath an inertial orbit.

A bright dot on the track marks the sub-satellite point at the current scenario time. Advance the scenario time to watch the dot move along the track.

Why the propagator matters for ground tracks

Tracks use each object’s selected propagator, and the choice matters more than it might appear:

• SGP4 accounts for atmospheric drag and Earth’s oblateness. For a real satellite with a fresh TLE, the track accurately reflects where the satellite actually passes over the Earth.
• J2 secular adds the long-term nodal regression (the westward orbital-plane drift) but no drag. For circular-orbit design work this is the right choice.
• Two-body produces a fixed ellipse with no perturbations. The sub-satellite track still forms a sinusoid, but the ascending-node longitude stays constant — the real westward regression that SGP4 or J2 shows is absent. The disclaimer “Idealized two-body propagation — educational geometry only; real orbits diverge within hours” applies here too.

See Propagation models for the underlying theory.

Controls

• Object checkboxes in the Ground Track panel’s controls column toggle individual tracks on and off without removing objects from the scenario.
• Ground site markers appear automatically for every site defined in the Ground Sites sidebar section. The marker is a cross labeled with the site name.
• Save Ground Track PNG — exports the current map canvas as a PNG file named exoatlas-groundtrack-<timestamp>.png.

Accuracy & model notes

Longitudes are computed by rotating SGP4’s TEME positions (or J2000 positions for other propagators) to ECEF using GMST — the standard approximation for TLE work, without polar motion. This is documented in the app’s ground-track disclaimer: “Sub-satellite tracks use each object’s selected propagator. Longitudes are Earth-fixed (GMST); no polar motion.” For SGP4 with a fresh TLE the resulting sub-satellite positions are operationally useful. See the validation page for the accuracy figures behind SGP4.

Related

• Ground sites & access — predict pass times over the ground sites shown on this map.
• Propagation models — how SGP4, J2, and two-body differ for ground tracks.
• Interface tour: Analysis panel — how to open and resize the panel.