on of the antenna is crucial, and any perturbation of the alignment of the satellite can have a major effect on its operation, both in reception and transmission. 228 Newnes Guide to Radio and Communications Technology For low earth orbit satellites the situation is somewhat different. The fact that they move across the sky and may need to be received by several users at any time means that they cannot use directive antennas. Additionally the earth subtends around half the celestial sphere and as a result users may be separated by angles ranging from zero to almost 180 degrees. Fortunately the satellites are much closer to the earth and path losses are very significantly less, reducing the need to high gain antennas. Ground stations also need an effective antenna system. For commu- nication with satellites in geostationary orbit the antenna remains fixed, except if there is a need to change to another satellite. Accordingly parabolic reflectors are often used. This can be seen from the number of satellite TV antennas that are in use. These are a form of parabolic reflector. However, it is possible to use other types such as arrays of Yagi antennas. Here they are stacked (one on top of the other) and bayed (side by side) to give additional gain. For some low earth orbit satellites the ground station antenna systems are designed to be able to track the satellite in azimuth and elevation. This is typically achieved by automatically tracking the satellite as it moves across the sky. This is normally achieved by taking a signal level output from the receiver. By ensuring that it is maintained at its peak level the satellite will be tracked. Many low earth orbit satellites are required for systems such as positioning or even telephone style communications. Here directional antennas are not used as the user will not want to reorientate the antenna all the time. Instead almost non-directional antennas are used and the transmitter powers and receiver sensitivities designed to give a sufficient level of signal to noise rat