GPS units use the Global Positioning System, a series of 27 Earth-orbiting satellites designed to give signals to GPS receivers (24 satellites are functioning, with three in reserve in case one fails or is disrupted). Orbits are calibrated so at any given time, anywhere on Earth, at least four are visible to GPS receivers.
As the GPS receiver receives information from the visible satellites, it uses advanced calculations to determine its position on the Earth. This process and is called trilateration.
Knowing your distance from something such as a city is worthless without knowing your exact position in relation to it. The satellites use 2-D trilateration to give the GPS receiver two coordinates, such as 650 miles from point X and 400 miles from point Y. While not precise, it is the first step to giving an exact location.
The satellites send the GPS receiver a third coordinate, such as 200 miles from point Z. When added to the 2-D coordinates, the GPS receiver will give a location accurate to within several feet.
The information needed for the receiver to pinpoint location is sent from the satellites via radio waves. The information needed is the location of at least three of the available satellites and the distance of each from the GPS receiver.
Article Written By Eric Cedric
A former Alaskan of 20 years, Eric Cedric now resides in California. He's published in "Outside" and "Backpacker" and has written a book on life in small-town Alaska, "North by Southeast." Cedric was a professional mountain guide and backcountry expedition leader for 18 years. He worked in Russia, Iceland, Greece, Turkey and Belize. Cedric attended Syracuse University and is a private pilot.