More about GPS
How does GPS work?
GPS is a constellation of 27 solar-powered satellites that orbit they earth in almost circular orbits at an altitude of approximately 20000 kilometres. The orbits are arranged so that from any point on the planet you can "see" at least four of the 24 operational satellites. 3 of the 27 currently in orbit are spares ready to be switched on if any of the others fail.
Each satellite sends out an electromagnetic signal - a beam of microwaves - that announces its presence to anyone on earth with a receiver ready to receive the signal. At any one time, a GPS receiver is therefore picking up signals from four satellites. The built-in computer uses these signals to work out your precise distance from each of the four satellites and then to calculate your exact position on the planet to the nearest few metres based on these distances.
In fact, signals from just three satellites are needed to carry out this trilateration process; the calculation of your position on earth based on your distance from three satellites. The signal from the fourth satellite is redundant and is used to confirm the results of the initial calculation. If the position calculated from distances to satellites A-B-C do not match the calculation based on A-B-D then other combinations are tested until a consistent result is obtained.
The process of measuring the distance from satellite to GPS receiver is based on timed signals. For example, at 16h45 precisely, the satellites may begin broadcasting its signal. The GPS receiver will also begin running the same random sequence at 16h45 local time, but does not broadcast the sequence. When the receiver picks up the signal from the different satellites, there will be a timelag, because the microwaves take a fraction of a second to travel at the speed of light from satellite to receiver. The timelag is easily converted into distance to each satellite. The slight differences between signals from each is then used to calculate the receiver's position.
