onal and military uses. The system uses a constellation of 24 satellites orbiting the earth at an altitude of 20 183 km. Not all the satellites are operational at any given time. Of the total quantity there is a minimum of 21 that are operational 98 per cent of the time to provide the required level of service. The satellites are arranged in six orbital planes of 55 degrees inclination (four satellites per plane) and they take 11 hours 58 minutes (12 sidereal hours) to complete each orbit. This arrangement enables each receiver to pick up at least four satellites under normal conditions. In fact satellites are visible above the horizon for five hours. Each satellite transmits a message containing three pieces of informa- tion: the satellite number, its position in space and the time at which the message was sent. Each satellite is held in a very accurate orbit, and on board it contains an atomic clock to give a highly accurate time source. This time information is transmitted with the signal from each satellite. From this it is possible for the receiver to calculate the different path lengths from the satellites. The time for the signal to travel from the satellite to the earth is proportional to the path length, and since the velocity of the signal is known it is possible to calculate the path length. This information is computed by the processor in the receiver to give the position on the earth. If three satellites are being received, then it is possible to compute the latitude and longitude. If four satellites are being received, then it is also possible to compute the altitude as well. The GPS satellites are controlled by a master station located at Falcon Air Force Base, Colorado Springs, USA. Other remote stations are located on Hawaii, Ascension Island, Diego Garcia and at Kwajalein. Using all 236 Newnes Guide to Radio and Communications Technology Time code leaves Shortest path will mean all the satellites at the time code from this the same time satellite reaches the receiver first Receiver Earth Figure 