Global Positioning System (GPS)

The US NAVSTAR global positioning system is a global navigation satellite system, comprising a group of 24 - 32 satellites owned and maintained by the U.S. Department of Defence. The satellites are equipped with extremely precise atomic clocks. Each satellite emits digital radio signals that give the satellite's location and the exact time to a GPS receiver on Earth. The quartz clock in a GPS receiver is calibrated to a satellite's atomic clock via information in the GPS signal. With this information the receiver can measure how long the radio signals take to arrive from each satellite - and since the speed of light is known and (effectively) constant, the distance to each satellite can readily be calculated. A precise Earth location can be obtained by solving a trigonometric equation for the intersection of four spheres with known radii. A minimum of four accurate distance measurements is therefore required to determine a precise three-dimensional location (latitude, longitude and altitude) on the surface of the Earth. GPS data are usually expressed as latitudes and longitudes using the World Geodetic System 1984 datum.

Up to 1 May 2000, the main source of GPS error was selective availability. This referred to deliberate errors induced by the U.S. Department of Defence which restricted high potential accuracy to those authorized by the U.S. military and its allies. The combination of selective availability and other sources of error meant that most civilian GPS positions were only within 100 m of the true position. A US presidential decision in 2000 to remove selective availability improved civilian GPS accuracy to 10-30 m, although further correction, known as differential correction is still necessary to compensate for the other error sources.

A more recent development, of great value for wildlife biologists, is the miniaturization of GPS receivers for use in radio collars (for example see Nielsen (2004)). Radiocollars can be programmed to acquire locations very few hours. The information is then retrieved, either when the collars are retrieved, or by remote downloading from the collars. This is clearly much more efficient than having to obtain regular ground-based fixes on the animal.

In 2009 the US global positioning system is the only fully operational global navigation satellite system. A Russian system (GLONASS) was operational in the 1990s but fell into disrepair as the Russian economy collapsed. It is now being restored in partnership with the Indian government, and is expected to provide worldwide coverage by 2010. The European Galileo positioning system should become operational by 2013, and the Chinese Compass navigation system should do so by 2015.