GPS was developed by the US Department of Defense as a worldwide all weather navigation and positioning and timing resource, primarily for military use. It is based on a constellation of 24 satellites orbiting the earth. By measuring the travel time of radio signals transmitted from at least four satellites, a receiver can derive its distance from each satellite and combine these to compute its latitude, longitude, and altitude. GPS positions are accurate 95% of the time to ± 25 metres or better depending on the receiver used and ionespheric conditions. Position updates can be received every second. Higher levels of availability and accuracy are required for some navigation and positioning purposes and the integrity of the system needs to be monitored for safety-critical applications