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Geocaching NSW: How GPS works

Finding your location

Even though a grid system had been developed, it was still difficult to determine the exact latitude or longitude of your location. Some people used the position of stars, while others matched the position of the Sun with accurate timepieces.

Today, the satellite-based GPS allows anyone with a receiver to find their longitude and latitude quickly and easily.

The receiver uses signals from the US NAVSTAR satellites to triangulate its location.

Each satellite generates and transmits a signal called a 'pseudo-random code'. The GPS receiver also generates the same signal, but does not transmit.

Signals received by the GPS receiver from the satellites will have travelled several thousand kilometres. When it reaches the receiver it will be a fraction of a second behind the 'code' in the receiver.

Because the 'pseudo-random code' varies during the day (i.e. it is time dependant), the difference in time between the receiver's signal and the satellite's signal can be used to determine the distance between both. 

Radio signals travel at the speed of light, 300 000 kilometres per second, therefore, the distance can be calculated by multiplying the speed by the time difference. If it takes 0.05 seconds for the satellite signal to reach the receiver, the distance to the satellite is equal to 300 000 x 0.05 = 15 000 kilometres.

As the satellite sends out its signal in all directions, the GPS receiver could be anywhere on the surface of a sphere stretching 15 000 kilometres from the satellite.

Locking in your position

To find your exact position you need at least three satellites. Here is a two-dimensional example.

Imagine you are lost in the middle of a city, but you don't know which one. You ask someone where you are and they reply "490 kilometres from Melbourne".  If you draw a circle on a map of radius 490 kilometres centred on Melbourne, you realise you could be in any number of towns. 

Then you ask another person where you are, and they reply "240 kilometres from Sydney". When you draw a circle centred on Sydney, you now see that you could be in one of two locations. Finally, a third person tells you that you are 980 kilometres from Adelaide, so you draw a circle and realise that you are where the three circles intersect – in Canberra.

The GPS system works the same way, but because it is three-dimensional, it uses spheres, and requires four reference points to determine your location. 

Occasionally, the receiver can work with just three satellites, as this provides two possible locations, one located in outer space. Generally, the more satellites the receiver can 'see', the more accurately it can determine its position.

Text courtesy of CSIRO Education