What is Satellite Coverage?
Satellite coverage refers to the geographical area on the Earth's surface within which a satellite is visible at or above a specific minimum elevation angle. This area is often called the satellite's "footprint." Understanding this footprint is crucial for telecommunications, weather monitoring, and GPS services, as it determines how many ground stations or users a single satellite can serve simultaneously.
How the Calculation Works
The coverage area of a satellite depends on two primary factors: the orbital altitude and the minimum elevation angle required for a reliable signal. Because the Earth is curved, a satellite at a higher altitude "sees" more of the surface. However, signals near the horizon often suffer from atmospheric interference or physical obstructions (like buildings and mountains). Therefore, engineers use a "Minimum Elevation Angle" (typically between 5° and 20°) to define the usable coverage zone.
The calculation uses spherical geometry. By calculating the Earth Central Angle (ψ), we can determine the arc length along the Earth's surface (the radius of the footprint) and the total surface area covered using the formula for a spherical cap.
LEO vs. MEO vs. GEO Coverage
Satellites are generally categorized by their orbital height:
- Low Earth Orbit (LEO): Altitudes between 160km and 2,000km. These provide low latency but have small footprints, requiring large constellations (like Starlink) for global coverage.
- Medium Earth Orbit (MEO): Altitudes between 2,000km and 35,786km. This is the home of GPS satellites.
- Geostationary Orbit (GEO): Fixed at 35,786km. A single GEO satellite can cover nearly one-third of the Earth's surface.
Frequently Asked Questions
Q: Why does the coverage decrease as the elevation angle increases?
A: A higher elevation angle means the ground observer must look more "straight up." This restricts the usable area to the region more directly beneath the satellite, effectively shrinking the footprint.
Q: How many satellites are needed for global coverage?
A: For Geostationary satellites, three are theoretically enough to cover most of the Earth (excluding poles). For LEO satellites, hundreds or thousands are needed due to their small individual footprints.