geostationary

Geostationary refers to a specific type of Earth-centered orbit in which a satellite moves in a circular path around the equator with an orbital period that matches the Earth's rotation. In a true geostationary orbit, the satellite remains fixed over a single longitude on the equator, so its appearance in the sky does not change and it provides continuous coverage to the area directly beneath.

The geostationary orbit lies at an altitude of about 35,786 kilometers (22,236 miles) above the equator. The orbital period is approximately 23 hours, 56 minutes, and 4 seconds, the length of a sidereal day. To maintain a fixed ground position, the orbit must be nearly perfectly equatorial; even small inclinations cause the ground track to oscillate north and south of the equator, requiring periodic station-keeping maneuvers. Perturbations from the Earth's equatorial bulge, the gravity of the Moon and Sun, and solar radiation pressure can gradually alter the orbit, which is corrected by onboard propulsion.

Geostationary orbits are especially valued for communications and weather satellites, because a single GEO satellite can continuously cover a broad region along its fixed longitude. However, coverage is best near the equator; high-latitude regions experience limited visibility, and to achieve near-global coverage, networks deploy multiple satellites at different longitudes. The high altitude also introduces latency in signal transmission.

The concept was proposed by Arthur C. Clarke in 1945, and the first successful geostationary satellite, Syncom 3, achieved the condition in 1964. Since then, geostationary satellites have become a foundational element of modern telecommunications and meteorology.