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The Global Positioning System (1)

Many key processes in Earth Science, from plate tectonic motions to the tiny local adjustments of the Earth's crust after a large earthquake, can now be studied using repeated precise measurements of points on the Earth's surface; this is achieved using the Global Positioning System (GPS).

Using GPS for navigation

The GPS consists of a set of satellites and land-based control stations owned and operated by the US Department of Defense. Each satellite transmits a coded signal on two carrier frequencies that says, amongst other things, where it is in the sky. In its simplest form a GPS receiver receives the signal from the GPS satellite and uses the code to determine its distance from the satellite. If the distance to four satellites can be measured at once then the receiver can calculate its own position in real time. A position calculated in this way is accurate to better than 10 metres, easily sufficient for most navigational purposes, and can be achieved using a commonly-available hand-held receiver.

Using GPS to measure ground movement

In order to measure ground movements of the order of a few centimetres or even millimetres, a different method is required. This is achieved using more sophisticated GPS receivers that store the data received from the satellites, allowing the signals to be processed by computer at a later stage.

These receivers use the code transmitted by the satellites but also make measurements of the carrier wave used to transmit the code. Because the carrier wave is of much higher frequency than the code, the positional accuracy is much higher. Using carrier wave observations taken over several hours, and advanced computing techniques, it is possible to measure the relative positions of two points anywhere on the Earth's surface to within 5 mm.

The COMET scientists have used GPS for a wide range of applications, such as measuring the movement of mountains and measuring ground movement during and after earthquakes.

The GPS constellation of satellites

GPS receiver

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