8.9 Operation of the Global Positioning System (GPS) these stations the satellites can be tracked and monitored for 92 per cent of the time. This results in two 1.5 hour periods each day when the satellite is out of contact with the ground stations. Using these stations the performance of the satellites is monitored very closely. The information that is received at the remote stations is passed to the main operational centre at Colorado Springs and the received information is assessed. Parameters such as the orbit and clock performance are monitored and actions taken to reposition the satellite if it is drifting even very slightly out of its orbit, or if any adjustment to the clock is necessary. This information is passed to three uplink stations co-located with the downlink monitoring stations at Ascension Island, Diego Garcia and Kwejalein Weather and observation satellites Satellites are used for a variety of other purposes. They can be used for a variety of observation tasks. One of the more familiar of these applications is in weather forecasting where they can take a bird's eye view of the weather formations from high above the earth. This gives weather forecasters a much better view of how the weather formations are developing and moving. There are two main types of weather satellite. Geostationary ones are used for short-range warning. Polar orbiting satellites are used for longer- term forecasting. However, both types of satellite are needed to provide a complete picture. Satellites 237 The geostationary satellites are sufficiently high to gain a view of a large section of the earth's surface although their view of regions towards the poles is limited because of the low angle at which the satellites see these regions. As they remain over a certain point on the earth's surface they can provide a constant view of the prevailing conditions below them. They are usually used for monitoring trigger conditions for severe weather conditions such as hurricanes, tornadoes, thunderstorms and other forms of severe w