Garmin eTrex Summit Owner's Manual - Page 60
Appendicies, Appendices
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Appendicies Appendices Appendix B What is GPS? By now the overall picture of how GPS works should be getting much clearer. Each satellite transmits a message which essentially says, "I'm satellite #X, my position is currently Y, and this message was sent at time Z." Of course, this is a gross oversimplification, but you get the idea. Your GPS receiver reads the message and saves the ephemeris and almanac data for continual use. Now, to determine your position, the GPS receiver compares the time a satellite signal was transmitted with the time it was received by the GPS. The time difference tells the GPS receiver how far away that particular satellite is. If we add distance measurements from a few more satellites, we can triangulate our position. This is exactly what a GPS receiver does. With a minimum of three or more satellites, your GPS can determine a latitude/longitude position-what's called a 2D position fix. With four or more satellites, a GPS receiver can determine a 3D position fix which includes latitude, longitude, and altitude. By continuously updating your position, a GPS receiver can also accurately provide speed and direction of travel (referred to as 'ground speed' and 'ground track'). How accurate is your GPS? There are several things that can have an affect on GPS accuracy, satellite geometry, multipath errors, atmospheric delay, and the United States Department of Defense just to mention a few. The descriptions of these items are covered in depth on our web site www.garmin.com. In general you can expect your GPS to be accurate from 60 to 255 feet depending on the number of satellites available and their geometry. 58