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Geostationary (GEO) and Geosynchronous orbits (GSO)

Artistic impression of European Data Relay Satellite (EDRS) system (not to scale). These satellites work in GEO and relay data to and from non-GEO satellites, spacecraft, and stations that aren’t otherwise capable of constant communications.
Artistic impression of European Data Relay Satellite (EDRS) system (not to scale). These satellites work in GEO and relay data to and from non-GEO satellites, spacecraft, and stations that aren’t otherwise capable of constant communications. Image: ESA – P. Carril

Satellites in LEO zip across the sky, but that’s not the case for objects in geostationary orbit (GEO). Objects in GEO work directly above Earth’s equator, and their speed matches the rotational speed of Earth. The consequence of this is that satellites in geostationary orbit stay in a fixed position relative to the surface (fun fact: the orbital location of GEO satellites is known as the Clarke Belt, in honor of scifi author Arthur C. Clarke, who conceived of the idea). Satellites in GEO feature an orbital speed of 1.86 miles per second (3 km/s) and an orbit that’s roughly 6-times larger than LEO. As “few as three equally-spaced satellites can provide near global coverage,” the European Space Agency says.

Satellites in geosynchronous orbits (GSOs) aren’t positioned over the equator, so they appear to move across the sky. “A spacecraft in geosynchronous orbit appears to remain above Earth at a constant longitude, although it may seem to wander north and south,” according to NASA. “The spacecraft returns to the same point in the sky at the same time each day.” Objects in GSO complete a single rotation once every 23 hours, 56 minutes, and 4 seconds (i.e. the sidereal day). Geostationary orbits are a special type of geosynchronous orbit, in that objects in this orbit appear stationary from a person’s perspective on Earth.

A drawback of GEO is that satellites in this orbit work 22,236 miles (35,786 km) above the surface, which is reasonably far away. That’s one-tenth the distance to the Moon, which amounts to a round-trip delay of 250 milliseconds when transmitting data. At the same time, however, an antenna on Earth’s surface can stay pointed at a satellite in space, which is hugely advantageous.

Indeed, GEO is ideal for weather and telecommunications satellites, both of which need to stay above a specific point on the surface. Around 565 operational satellites currently work in GEO, examples being NASA and NOAA’s Geostationary Operational Environmental Satellite-R (GOES-R), an advanced weather satellite, ESA’s European Data Relay System (EDRS), a sophisticated laser communication network, and the John Deere StarFire navigation system, used for agriculture management.

Correction: A previous version of this slide incorrectly conflated geostationary and geosynchronous orbits.

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