HOW IT WORKS
The Global Positioning System (GPS) consists of 24 satellites orbiting the Earth every 12 hours and transmitting a sequence of numbers back to its surface.
The system was developed by the U.S. Department of Defense. Handheld GPS receivers pick up the satellites' signals.
Standard recreational GPS receivers have 12 channels, meaning that they can track up to 12 satellites at once. Reception from three satellites triangulates a "fix" on your location, giving your precise position on Earth in longitudinal and latitudinal coordinates. Reception from four satellites is required to calculate altitude. A receiver's "view" of the satellites can be blocked by dense foliage, terrain, buildings, or even thick fog.
A GPS unit, like a good pair of boots, is something you appreciate only by using it. Get out the instruction manual, power it up, and hit the trail. Before setting off for the great unknown, though, take it on a few test trips in familiar areas. Use it on a weekend backpacking trip or on short walks around your neighborhood. Purposely get "lost" and let the GPS help you find your way home. Or have a GPS-savvy friend create a route for you to follow.
For more tips on using GPS, check out GPS Made Easy: Using Global Positioning Systems in the Outdoors, by Lawrence Letham (The Mountaineers, 800-553-4453; www.backpacker.com/bookstore; $14.95).