Despite the frequent stories of drivers being advised to take last minute turnings and the government's zealous enthusiasm for all drivers to install sat-nav so they are easier to track, GPS devices are becoming more and more popular on dashboards around the country. The list of pros for installing a GPS system is always increasing too, if your vehicle gets stolen it is easier to track, if you breakdown on an isolated back-lane in the middle of nowhere you can work out where you are, and it may even make your insurance premiums cheaper. So how does GPS work?

Currently circling the world there are between 24 and 32 global positioning system satellites at a medium earth orbit. At any one time above your head there are several of these satellites in the sky, meaning that if you had a GPS system with you right now it would be receiving the messages they are sending. In order to calculate where you are, there usually needs to be four satellites, one for each of the three dimensions, and a fourth to make up for any possible error (which might result in the deduction of a wrong position).

Your GPS system will then, by use of geometric trilateration, be able to work out your position. In simple terms, each satellite will be covering a circular portion of the earth at any one time, the GPS receiver calculates the position of, for example, Sat A, and then does the same for Sat B by establishing the differences between times of each. By calculating the distances of A and B your GPS device can deduce a line of where you could be by acknowledging the points at which A and B's portions cross. Add to that Sat C and you get a very specific point of where you are, and then the backup Sat D can clarify the estimation.

Time is a very important factor. Each GPS satellite is set to exactly the right time and is repeatedly broadcasting a message 50 bits in size. The message includes the health of the satellite as well as the position and time. The satellite takes just thirty seconds to send 1500 bits.

With GPS, there is room for error outside of the possibility of calculations being affected by the speed of light. For example, Einstein's theory of relativity highlights that clocks on satellites will run faster due to speed (special relativity) and gravity (general relativity) at about a rate of 45.9 microseconds a day. This is accounted for giving each clock onboard a GPS satellite a rate offset, in order to make it run slightly slower than it would on earth.

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