are encountered. The surfaces exposed to the sun are heated by solar radiation and will rise to very high temperatures, whereas the other side that is not heated will be exceedingly cold. Only conduction will give any heating effect under these circumstances. There are a number of other effects that must be considered. Solar radiation itself has an effect on some materials, causing them to degrade. Notice must also be taken of meteorites. Very small ones cause the surfaces to be eroded slightly, but larger ones may penetrate the body the satellite causing significant damage. To overcome this satellites are protected by specially designed outer layers. These consist of sheets of metal which are slightly separated giving a cushioning effect when any meteorites impact on the satellite. Cosmic particles also degrade the performance of satellites. Particularly during solar flares the increase in solar particle flow can degrade solar cells, reducing their efficiency. The antennas used on satellites are particularly important. For geostationary satellites directional antennas are generally used. These are used because power use has to be optimized and antennas that give gain enable the best use to be made of the available transmitted power. Additionally they enable the signals from the earth to be received with the best signal to noise ratio. In view of the long path lengths required for geostationary satellites, there is a considerable path loss and the antenna gain is used to improve the received signal strength. It also helps reduce the reception of solar and cosmic noise that would further degrade the received signal. In a geostationary orbit the earth subtends only 18 degrees of arc. Any power not falling into this area is wasted. As a result, parabolic reflector or 'dish' antennas are widely used. Horn antennas are also popular and in some cases phased arrays may be employed, especially where coverage of a specific area of the globe is required. However, the use of directional antennas does mean that the orientati