Understanding Satellite Ground Tracks & Visibility Passes
A satellite ground track is the path on the surface of the Earth directly beneath a satellite. Plotting ground tracks is a fundamental technique in space operations, astronomy, and telecommunications to predict when a satellite will pass over specific ground stations or be visible to observers on Earth.
Orbital Propagation with the SGP4 Model
Satellites in low Earth orbit (LEO) are subjected to various gravitational forces, atmospheric drag, and solar radiation pressure. To calculate a satellite's exact position over time, aerospace software uses the SGP4 (Simplified General Perturbations 4) model. By inputting standardized NORAD Two-Line Element (TLE) datasets, SGP4 calculates the satellite's inertial position (ECI) at any given second, which is then transformed into geodetic latitude and longitude coordinates.
Earth Rotation and Ground Track Drift
Because the Earth rotates on its axis, a satellite's ground track drifts westward with each successive orbit. For example, a typical LEO satellite completes one orbit in about 90 minutes. During this time, the Earth rotates 22.5 degrees to the east. As a result, the satellite's path over the ground shifts 22.5 degrees longitude westward, covering different geographic areas on every pass.
Orbital Inclination and Geographic Coverage
The latitude limits of a satellite's ground track are defined by its **orbital inclination** (the angle of its orbit relative to the equator). A satellite with an inclination of 53 degrees (common for Starlink satellites) will travel between 53° North and 53° South latitude, never passing directly over the polar regions. Polar-orbiting satellites with inclinations near 90 degrees pass over both poles, providing total global coverage.