Geosynchronous orbit.

Geosynchronous and Geostationary Orbit

A geosynchronous orbit may be defined as one with an orbital period (the time needed to orbit once around the Earth) that matches the rotation rate of the Earth. This is a sidereal day, which is 23 hours 56 minutes and 4 seconds in length, and represents the time taken for the Earth to rotate once about its polar axis relative to a distant fixed point. This is about four minutes shorter than the common day length of 24 hours, which is relative to the sun.

A geostationary orbit is a special case of a geosynchronous orbit. A satellite is in a geostationary orbit when it appears stationary from the point of view of an observer on the Earth's surface. This can only occur when:

• The orbit is geosynchronous
• The orbit is a circle
• The orbit lies in the plane of the Earth's equator

Thus, a geosynchronous satellite will be geostationary only with the additional restrictions of it being in a circular orbit situated over the equator.

The following parameters are always true for any geostationary satellite: