the rotation has no effect. Tropospheric bending can also affect signals. This occurs particularly when the satellite is low in the sky and the path length through the troposphere is longer. When this occurs the path losses may increase. Again the effect is less pronounced at higher frequencies. The Doppler effect can also be noticed under some circumstances. Naturally this is not noticed in the case of geostationary satellites because they are stationary above a given location. However, it is often noticed when using satellites such as those in low earth orbits or elliptical orbits that move relative to a point on the earth's surface. Relative velocities may be quite high, and frequencies are often as high as several GHz resulting in some noticeable frequency shifts. As a result some satellite systems have to be able to accommodate frequency shifts arising from this. Communications satellites Satellites fulfil a number of roles. They can be used for communications, direct broadcasting, weather monitoring, navigation and a number of other uses. Of these the most widespread is possibly their use in communications. Every day many thousands of international telephone 232 Newnes Guide to Radio and Communications Technology calls are made. These are often carried via satellites. In the early days of telephones, calls were routed via undersea cables. These were expensive to lay, and were difficult to maintain. They also had a relatively narrow bandwidth and this meant the number of calls they could carry was limited. For use in communications like this the satellite acts as a repeater. Its height above the earth means that signals can be transmitted over distances that are very much greater than the line of sight as shown in Figure 8.6. An earth station transmits the signal, normally consisting of Satellite Uplink Downlink (6 GHz) (4 GHz) Transmitting Receiving earth station earth station Figure 8.6 Using a satellite for long distance communications many telephone calls up to the satellite. This is called the uplink an