Geosynchronous Orbits + Geostationary Orbits
Webster's Dictionary defines a Geostationary orbit as of, relating to, or being a satellite that travels above Earth's equator from west to east at an altitude of approximately 35,900 kilometers (22,300 miles) and at a speed matching that of Earth's rotation, thus remaining stationary in relation to Earth. 2. Of, relating to, or being the orbit of such a satellite. In plain English, a satellite matches the earth's rotation making it seemingly hover over one spot of the globe enabling coverage of half the earth's surface.

Three such satellites, appropriately spaced longitudinally, have worldwide coverage except for relatively small areas over the poles. Three main classes are typically placed into a GSO: Communications, missile early warning, and navigational satellites. The uses are unlimited ranging from commercial use to weather forecasts!

The GSO originated in the mid-1970's. The U.S. Air Force designed a two-stage interim upper stage (later renamed inertial upper stage, or IUS) to carry satellites weighing as much as 5,000 pounds (2,300 kg) from the shuttle to Geostationary orbit, and a three-stage version for boosting NASA' s space probes from the shuttle into interplanetary trajectories. IUS development problems, however, prompted NASA in the early 1980' s to design a widebody version of the Centaur upper stage to replace the three-stage IUS.

In its first use (1983) aboard the shuttle, the IUS's second-stage nozzle burned through and left the first Tracking and Data Relay Satellite (TDRS-1) in a useless orbit. Ground controllers were able to use the satellite's onboard thrusters to put it in the proper Geostationary orbit over a period of weeks, but the IUS was grounded until the nozzle problem was resolved.

Because the IUS was too large and expensive for most satellites going to Geostationary orbit, McDonnell Douglas developed the payload assist module, a special cradle with a turntable to spin and then release satellites. A small rocket motor and the satellite's own rockets then boost it into Geostationary orbit.