How it works: GPS

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Satellites, science and a bit of maths make getting lost a little harder. Gavin Treadgold explains the workings of GPS.

When I bought my first Global Positioning System receiver (GPSr) in 2001, it was rare to see someone else with them. These days, you can find them just about everywhere – particularly suckered to car windscreens. But how does the GPS work?

It's quite appropriate that in-car GPS receivers are usually located close to the radio, as the GPS is similar in concept to a radio. Instead of ground-based aerials to transmit radio waves, the GPS uses about 30 GPS satellites orbiting 20,200km above the Earth's surface to transmit signals that GPS receivers listen to, and use these signals to calculate their position. Each satellite accurately knows the time and its own position in space based on its orbit and onboard atomic clocks. This information is then broadcast from space for any receiver to hear. The GPS receiver uses the precise time and position information broadcast from each satellite to trilaterate and determine its location based on how far it is from known reference points – the satellites.

Whereas triangulation uses angles to calculate location, trilateration uses distance. Listening to one satellite, the receiver could be located anywhere in a circle around that satellite. If the receiver knows how far it is from a second satellite, then the number of possible locations of the receiver is reduced to two.

The known location of a third satellite would identify a unique location, and reduce the calculation error to an acceptable level.

A GPS receiver is capable of listening to 12-20 satellites at the same time. Generally, the more satellites the GPS can listen to at the same time, the more accurate it is at calculating your latitude, longitude and, to a lesser extent, altitude.

The receiver needs to be listening to at least three satellites to calculate a location. In good conditions the location is accurate to 3-5 metres.

The default model of the Earth used for calculation is the World Geodetic System 1984 (WGS-84). This is used on all new GPS receivers. Most receivers allow alternative display systems to be selected – in New Zealand the combination of Geodetic Datum '49 and New Zealand Map Grid should be selected when using a receiver with the current 260 series topographic maps.

GPS receivers aren't always able to report accurate positions – this is usually due to the receiver not being able to "hear" the satellite signals correctly. When the receiver has a good clear view of the sky – for example standing on top of the Port Hills – there is nothing stopping the signals reaching the receiver.

Problems arise when you place the receiver under thick bush canopy or in buildings where the signals have trouble penetrating. Similar problems can be found in gullies and valleys, or urban canyons in high-rise cities. In these situations, the receiver can only produce approximate locations and often will fail to even produce a location.

During the past two to three years, the processing technology within receivers has reached the point where they can produce accurate locations even in demanding environments. With a mixture of low and high sensitivity receivers on the market, this can be an important consideration if you are going to be using the receiver in challenging locations.

Change is also afoot in space, with Japan, China, India, Russia and Europe announcing new or updated navigation satellite systems.

The EU's Galileo system recently announced an agreement that will make their system compatible with the US GPS. More satellites and updated receivers will make it easier to get a signal and more accurate locations.

Gavin Treadgold is president of the New Zealand Recreational GPS Society, www.gps.org.nz

- © Fairfax NZ News