Navigation - GPS

Global Positioning System

The Global Positioning System (GPS) is an electronic radio navigation system that uses orbiting satellites to calculate a triangulated position.

The GPS network comprises 24 satellites orbiting at around 11000 miles up (plus a number of land base reference stations). To get an accurate fix you require reception of three satellites for triangulation and a forth to correct any time descrepencies. With this system you generally can get positional accuracy down to metres (but with the right equipment can get centimetre accuracy).
Using a GPS is no substitute for knowing how to use a map and compass.

A GPS can get poor reception under trees and in built up areas.
What do you do when the battery fails
The GPS sattelites can be switched off or degraded in accuracy and performance in times of troubles
A GPS can guide you from A to B but generally (if you have a mapping GPS the maps are usually limited in detail) will not tell you the type of terrain or what obstacles may be in your path.