GPS
From Trail-Finder -- Dedicated to getting you outside
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Overview
GPS devices, short for Global Positioning System, utilize a set of at least 24 medium earth orbit global satellites to determine your exact position. The dominant system in use today receives data from US-based satellites, however additional, independant systems are being developed by Russia (GLONASS) and the European Space Agency (Galileo Positioning System).
GPS receivers are commonly used for hiking, canoeing, and kayaking. They are also the central component of geocaching. Although a GPS is no replacement for a good map or compass, and if you're traveling far in backcountry, you should still be comfortable with map reading and compass use, a GPS can make navigation much easier.
Technical Operation
Each satellite in a GPS system is aware of its exact time and position in orbit, as well as its health. Each satellite continuously broadcasts this information via microwave to anyone who cares to listen.
A GPS receiver, commonly called a GPS, receives signals from multiple satellites. By comparing the time signals from multiple satellites, and applying basic trigonometry, it is possible to determine your location. A bare minimum of three satellites is required to determine a 2D location. A bare minimum of four satellites is required to determine a 3D location.
The actual number of satellites, and the accuracy of position, is determined by the relative positions of the satellites, called geometry. If the satellites are all in a line, or all clustered near one point, you may not be able to get a location at all. The more spread out the satellites, the more accurate your location. With good geometry, you can commonly achieve accuracy of around five meters (fifteen feet) horizontal distance.
The other factor impacting accuracy is terrain. In order for your GPS to pick up a signal from a satellite, it must have a direct, line-of-sight path to the satellite. There's no problem with clouds, and newer GPS receivers have no trouble with tree cover. GPS signals will not, however, travel through most buildings very well, and won't travel through rock or dirt at all. This limits their accuracy in valleys and, to a lesser extent, very hilly terrain.
