How does GPS work?

It’s the system that changed the world – and how we find our way around it. Jake Port explains.

An artist's impression of the satellites that make up the global positioning system.

It is only 16 short years since the power of the global positioning system (GPS) was released by the US military and made available to the civilian public, no matter how much we might take it for granted now. This system of at least 24 satellites, flying around the Earth 20,000 kilometres above our head, usually provides a reliable and accurate way of getting from point A to point B.

The whole network relies on an atomic clock and radio waves.

The speed of light (radio, light and electromagnetic waves) is set at just shy of 300,000 kilometres per second and is a reliable way of measuring, for instance, the time it takes a signal to travel from one place to another.

When you switch on your GPS-enabled device, it will connect to at least three of the 24 GPS satellites orbiting earth in order to calculate a 2D position, or four satellites to calculate altitude as well. The GPS device, known as the receiver, will measure, with extreme precision, the time it takes for the signal to travel from the satellite to your receiver.

Because the slightest change in timing can alter the precision of this measurement, GPS satellites use what is known as an atomic clock, the most precise way of measuring time known today.

These clocks in the satellites actually tick a few microseconds faster than the clocks on Earth, as time slows by a few millionths of a second as you move away from the planet. A slightly faster tick rate corrects time dilation so clocks in space match those on Earth.

A GPS receiver connected to the three or more satellites will measure the radio signal travel time, known as Time Of Transmission and calculate the relative position. This allows it to determine its location to within a few metres.

GPS satellites actually broadcast two signals, one that is very precise for military use and one that is slightly offset by the US Department of Defense for civilian applications.

This then allows you to know where you are, and in combination with information such as a road map, the route you need to take to reach your destination.

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