Geosynchronous Orbit Calculator

A satellite at GEO appears stationary to ground observers, eliminating the need for tracking antennas. One GEO satellite can cover about 40% of Earth's surface (excluding polar regions). Three GEO satellites at 120-degree spacing provide nearly global coverage (except poles). This makes GEO ideal for broadcast TV, weather satellites (GOES), and broadband communications (ANIK, INTELSAT).

GEO satellites hover above the equator. From high latitudes (above about 75-80 degrees), the satellite is below the horizon or at very low elevation angles, making the signal path through a thick atmosphere (high airmass) unusable. Polar regions use elliptical high-inclination orbits (Molniya, Tundra) for communications instead.

Retired GEO satellites are boosted to a graveyard orbit about 300 km above GEO (at roughly 36,100 km altitude). This preserves the valuable GEO orbital slots for operational satellites. International regulations require satellite operators to remove satellites from GEO slots within a few years of end-of-life using residual propellant for the graveyard boost maneuver.

A satellite slightly above GEO has a longer orbital period than the Earth's rotation — it drifts slowly westward relative to the ground. One slightly below GEO drifts eastward. Operators use these drifting maneuvers to reposition GEO satellites to new longitude slots using minimal propellant.

As of 2024, there are over 500 active satellites in geostationary orbit, plus hundreds of defunct ones. The equatorial GEO arc is administered by the ITU (International Telecommunication Union), which assigns orbital slots (by longitude) and radio frequencies. Popular longitudes over high-population regions are heavily contested.

A sidereal day is Earth's rotation period relative to distant stars: 23h 56m 4s. A solar day (24 hours) is relative to the Sun and is longer because Earth also orbits the Sun. For geosynchronous orbit, we use the sidereal rotation period because the orbit must repeat relative to inertial space (the stars), not relative to the Sun.

The Moon rotates once per month (its rotation is tidally locked to its orbital period around Earth). A lunar geosynchronous orbit would have an altitude of about 88,400 km, far outside the Moon's sphere of influence (about 66,000 km from the Moon). Lunar geosynchronous orbits are not stable in practice due to Earth and Sun perturbations.

A semi-synchronous orbit has a period of exactly half the planet's rotation period, so the satellite passes over the same ground points twice per day (at 12-hour intervals). GPS satellites are in semi-synchronous orbits at 20,200 km altitude (period 11.97 hours, approximately half Earth's sidereal day of 23.93 hours). This repeating ground track simplifies GPS receiver predictions.

All GEO satellites are at the same altitude (35,786 km) but at different longitudes around the equator. Station keeping — small thruster burns every 1-2 weeks — corrects for perturbations from the Moon, Sun, and Earth's non-uniform gravity that would otherwise cause longitude drift and inclination change. Each maneuver uses a small amount of propellant; the satellite's operational lifetime is limited by its propellant budget for station keeping.