GPS stands for Global Positioning System: or sometimes people say 'Global Positioning by Satellite' instead. A GPS
allows you to find your position on the earth's surface, using a system of grid co-ordinates. The most common way
of doing this in Britain is to use the Ordnance Survey national grid system. It is possible to locate a position
with an accuracy to within one metre.
GPS is an excellent tool for locating archaeological finds, earthworks or other field evidence without having to
lay out a conventional survey grid and measure a position off known features. This is especially useful where there
is little fixed topography marked on maps, such as in open moorland or even in large fields. Some professional
surveyors use a 'differential' GPS, which has two receivers, one fixed and the other mobile. These are the most
accurate, but hand-held single-user GPS's that are almost as accurate are now available at a fairly cheap price.
They are about the size of a mobile phone and display the position in co-ordinates on an LED screen. These are
easily accessible to archaeologists and are becoming essential survey 'kit'. Even metal-detectorists are encouraged
to purchase and use them to locate their finds more accurately.
GPS developed out of US Navy technology, using coded signals from four or more satellites to calculate the receiver's
relative distance from them, and therefore its position on the earth's surface. There have to be at least four
satellites within signal range (orbiting on our side of the world), for the angles of their signals to be enough
for the GPS to be able to calculate its position on the globe. If less than four satellites happen to be available,
the ability of the GPS to determine position can fade for short periods. Where ground-to-sky visibility is a problem,
such as within buildings or under heavy woodland cover, the GPS signals can be difficult to receive accurately.
Research and development efforts are under way to reduce and eliminate these factors. Another factor affecting
accuracy has been the tendency of the US Military to deliberately restrict public access to the satellite linkages
for security reasons. Until September 11th 2001 this seemed likely to become less of a problem, but new security
fears may possibly lead to future restrictions. As an alternative to the US system, a European system called
'Galileo' is under development, backed by the EU and a number of European governments.
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