s or 860 km. Geostationary orbits are very popular because once the earth station is set onto the satellite it can remain in the same position, and no tracking is normally necessary. This considerably simplifies the design and construction of the antenna. For direct broadcast satellites it means that people with dishes outside the home do not need to adjust them once they have been directed towards the satellite. The path length to any geostationary satellite is a minimum of 22 300 miles. This gives a small but significant delay of 0.24 seconds. For a communications satellite this must be doubled to account for the uplink and downlink times giving virtually half a second. This delay can make telephone conversations rather difficult when satellite links are used. It can also be seen when news reporters are using satellite links. When asked a question from the broadcasters studio, the reporter appears to take some time to answer. This delay is the reason why many long- distance links use cables rather than satellites as the delays incurred are far less. Satellites The requirements for satellites are very stringent, both in terms of their design and construction. They must be capable of operating in extreme conditions while still maintaining the highest standards of reliability because they cannot be retrieved for maintenance or repair. Satellites also contain a number of systems used for what is called station keeping. These systems are used to maintain the satellite in the correct orbit and position. The satellite will tend to drift away from its correct position over a period of time. To correct this small thrusters are used. Often they consist of canisters of a gas which when released with a catalyst gives a form of rocket propulsion to move the satellite back on station. Often the 226 Newnes Guide to Radio and Communications Technology service life of a satellite is determined by the amount of fuel for repositioning the satellite rather than the reliability of the electronics. The other problem with a satellite