h respect to a reference point will yield the lon- gitude difference. As the Earth rotates 360° in 1 day, each hour is equal to 15°, and therefore the difference in degrees between one's position and the reference point, Greenwich Village near London, can be calculated from the time difference. This accurate time measurement revolu- tionised navigation and gave Britain significant technological and military advantage. The emergence of satellites in the 1960s, and the US military's need for accurate posi- tioning measurement started a project in 1973 that led to the development and launch of GPS in 1993. In essence, GPS satellites are very accurate clocks in the sky, which broadcast this time and their exact position to receivers on the ground. The time is maintained according to an atomic clock with an accuracy loss of 1 S every 2.7 million years. The receiver can then calculate its distance relative to four of these satellites and triangulate its position based on these data as well account for inaccuracies of the local clock. Positioning satellites may be positioned at any orbit around the Earth. The only requirement is that they broadcast their present position and time. GPS operates at a MEO of 20 200 km using a constellation of 24 satellites orbiting at 20 200 km. These satellites orbit in 6 orbital planes each with 4 satellites as shown in Figure 11.14. The system is designed SO that at any time a receiver on the ground can measure its distance from four of these satellites. Position measurement is accomplished by determining the time it takes for a satellite signal to be received at a GPS device by measuring the signal propagation time from four or more visible satellites, as well as the satellites' physical x-y-z location in space as broadcast. This information is continuously updated and transmitted from each GPS satellite, and can be decoded to within a specified accuracy by a GPS receiver. The propagation delay and the satellites' physical posi- tions enable the receiving GSP device to calculate its po