 to receive the signals. This means that very large arrays of solar cells are required along with large batteries to support the broadcasting in periods of darkness. They also have a number of antenna systems accurately directing the transmitted power to the required areas. Different antennas on the same satellite may have totally different footprints. Satellites have also been used for cellular style communications. They have not been nearly as successful as initially anticipated because of the enormously rapid growth of terrestrial cellular telecommunications, and its spread into far more countries and areas than predicted when the ideas for satellite personal communications was originally envisaged. The systems that were set up used low earth orbiting satellites, typically with a constellation of around 66 satellites. Hand-held phones then communi- cated directly with the satellites which would then process and relay the signals as required. The main advantage of the satellite system is that it is truly global and communications can be made from ships, in remote locations where there Satellites 235 would be no possibility of there being a communications network. However, against this the network is expensive to run because of the cost of building and maintaining the satellite network, as well as the more sophisticated and higher power handsets required to operate with the satellite. As a result these networks have not been financially successful. Navigational satellites One of the most recent developments in satellite applications is in the Global Positioning System or GPS. Although initially conceived as a military system, it is now very widely used for commercial applications. Part of the appeal of the system is that it gives worldwide coverage with accuracies of just a few metres. In recent years the system has come into widespread use. Receivers for domestic use are available at low cost enabling them to be used in domestic vehicles, and in many other everyday applications as well as for very many professi