The first GPS satellite was placed in orbit in 1978, with another 23 launched and positioned up to 1994. These satellites were originally for use by the US Department of Defence, and transmit a very low power radio signal. This allows anyone with a receiver to determine their location on Earth, whether they are on land, sea or in the air, with an accuracy of 5 to 10 metres under normal conditions.
The satellites are positioned in a geo-stationary orbit at about 12,000 miles above the Earth and are arranged so that at least 4 are visible to any GPS receiver at any time. They are mainly powered by solar energy but also have battery backup facilities.
The GPS system works by receiving a signal from multiple satellites, usually 4, and calculates the time it takes the signal to reach the receiver multiplied by the speed of light, which gives the distance from the satellite to Earth. It does this with each satellite, and from this the receiver is able to work out its position on Earth. As long as the receiver can see three satellites then a position can be calculated.
Most car GPS systems check their position every second and from this they are accurately able to calculate the speed of the vehicle. There are now however some GPS units which check their position every tenth of a second and offer a greater degree of accuracy.
Vehicle GPS systems which detect speed cameras or display information on static camera locations do so by checking their current location against a database of information stored within the device. GPS navigation systems work in the same way but compare their location against the route required.
GPS systems are now helping business users with things like their mileage which the GPS system will log and can then be downloaded to the users PC and imported into a spreadsheet. This helps keep track of mileage and expenses easily.
Performance GPS systems are now available which drivers and manufacturers use to evaluate performance and products. The race driver can now use a device called a Driftbox which keeps track of lap times, as well as variables such as acceleration, top speed and G-loading. These devices are accurate to a tenth of a kilometre an hour, and check the GPS signal every tenth of a second. Manufacturers also use these devices to evaluate things such as tyres.
With GPS devices now as small as wrist watches, anyone can now check their position - handy if you're out walking, cycling or on a boat.