Geosynchronous Satellite Orbits

Communications Satellites Orbit above Same Location on Earth

Mar 22, 2008

Communications satellites often use geosynchronous orbits because they remain stationary above the same location on Earth.

History of Geosynchronous Orbits

Arthur C. Clarke, who achieved his greatest fame as a science fiction writer, first suggested the idea of communication satellites in geosynchronous orbits in 1945. At that time, prior to the dawning of the space age, Clarke's brilliant yet elegantly simple idea seemed too far fetched to be anything but science fiction.

With the launching of the first Sputnik satellite in late 1957, the idea of satellites in geosynchronous orbits became a real possibility. During the 1960s, the possibility became a reality. NASA launched the first geosynchronous satellites, Syncom I and II, in 1963. On of the first big communications job was broadcasting the 1964 Tokyo Olympics live to audiences in North America and Europe.

Today satellites in geosynchronous orbits have found widespread use in satellite communications.

What Is a Geosynchronous Orbit?

A geosynchronous orbit is simply an orbit that allows the satellite to remain apparently stationary above the same location on Earth.

In order for a satellite's orbit to remain stationary above the same location on Earth, the satellite's orbital period (the time required for one complete orbit) needs to equal the Earth's rotational period - one day. Because the satellite orbits at the same rate the Earth rotates, the satellite remains stationary above the same longitude on Earth.

In addition for the satellite orbit to be truly geostationary, the orbit must not be inclined to the equator. Otherwise the satellite will oscillate in a north south direction above the same longitude.

Applications of Geosynchronous Orbits

Anyone with a satellite TV receiver knows that the receiving antenna remains pointed toward the satellite at the same position in the sky. The receiver does not need to move to track the satellite because these satellites are in geosynchronous orbits. The fact that it is not necessary for a receiver or transmitter to move to track a satellite that is in a geosynchronous orbit makes these orbits particularly useful for communications satellites.

Distance for Geosynchronous Orbit

Kepler's third law allows us to calculate the distance from the center of the Earth for a geosynchronous orbit. Kepler's third law is a mathematical equation relating the orbital period, orbital radius, and the mass of the Earth or other object being orbited. See the illustration for the mathematical details. Using Earth's rotational period in seconds and Earth's mass in kilograms gives a value of 42,200 kilometers (or 26,200 miles) for the distance a satellite has to be from the center of the Earth to orbit geosynchronously. Subtracting Earth's radius gives a distance from Earth's surface of 35,900 kilometers or 22,300 miles.

Earth's Rotational Period

One might think Earth's rotational period is 24 hours, but it is actually 3 minutes and 56 seconds faster. A solar day, or 24 hours is Earth's apparent rotational period using the Sun as a reference. However the true rotational period of the Earth must be measured using the stars, rather than the Sun, as a reference. The day measured using stars as a reference is called a sidereal day. The length of the sidereal and solar days differ by nearly 4 minutes because the Earth travels almost a degree in its orbit around the Sun each day.