Tracking a satellite in space is a major part of any mission. Tracking involves determining the position of a satellite optically or by radio, either continuously or at regular intervals, and is necessary for maintaining radio communications with the satellite.
The tracking of satellites that no longer transmit radio signals can be used (regardless of the satellites' original purpose) to study the earth's gravity and the upper atmosphere. Variations in the strength of gravity from place to place and variations in the density of the atmosphere at very high altitudes cause changes in the orbits of satellites. When such orbital changes are observed, useful data can be acquired about the conditions that produced them.
During lunar or planetary flights, tracking determines the exact path of a spacecraft and the need for any corrections in flight.
The United States has several tracking networks, each for a different purpose. The networks include the Tracking and Data Relay Satellite System (TDRSS), the Deep Space Network, and the Space Surveillance Network (SSN). The first two networks are operated by the National Aeronautics and Space Administration (NASA); the third, by the U.S. Space Command, a joint military command.
The Tracking and Data Relay Satellite System
permits continuous tracking of and communication with spacecraft in relatively low orbits virtually anywhere around the earth. The system uses geostationary satellites called Tracking and Data Relay Satellites to provide a radio link between the spacecraft and a ground facility in New Mexico.
The Deep Space Network
is a radio link with spacecraft that travel beyond earth orbit. There are three main stations, set up around the earth so that contact can be maintained as the earth rotates. The three stations are located about 120 degrees of longitude apart at Goldstone, California; Madrid, Spain; and Canberra, Australia. These stations have large movable antennas that can receive information from the spacecraft and transmit instructions to start or stop experiments and make course corrections at great distances.
The Space Surveillance Network
is designed for air defense. It consists of a worldwide network of stations to track, identify, and catalogue all objects orbiting the earth. The information is relayed to the Space Surveillance Center of the U.S. Space Command. The center supplies the information to the North American Aerospace Defense Command (NORAD). Satellites in low orbits are usually tracked by radar. Satellites beyond radar range are tracked by specially designed telescopes that record images photographically on film or electronically by means of light-sensitive electronic devices.