Garmin eTrex Camo Owner's Manual

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Garmin eTrex Camo Owner's Manual Manual pdf 57 page image

43Appendix BWhat is GPS?The global positioning system is a satellite-based navigation system consisting of a net-work of 24 orbiting NAVSTAR satellites that are eleven thousand nautical miles in space andin six different orbital paths. The satellites are constantly moving, making two complete orbitsaround the Earth in less than 24 hours.The orbital paths of these satellites take them between roughly 60 degrees North and 60degrees South latitudes. What this means is you can receive satellite signals anywhere in theworld, at any time.One of the biggest benefits over previous land-based navigation systems is GPS works inall weather conditions. No matter what your application, when you need it the most, yourGPS receiver keeps working.The GPS signal contains a ‘pseudo-random code,’ ephemeris (pronounced: ee-fem-er-is),and almanac data. The pseudo-random code identifies which satellite is transmitting—inother words, it is an I.D. code. We refer to satellites by their PRN (pseudo-random number),from 1 through 32, and this is the number displayed on a GPS receiver to indicate whichsatellite(s) we are receiving. Ephemeris data contains important information such as satellitestatus (healthy or unhealthy), and the current date and time. The almanac data tells the GPSreceiver where each GPS satellite should be at any time throughout the day. Each satellitetransmits almanac data that provides orbital information for all of the satellites in the GPSnetwork.Your GPS receiver reads the message and saves the ephemeris and almanac data for con-tinual use. Now, to determine your position, the GPS receiver compares the time a satellitesignal was transmitted with the time it was received by the GPS. The time difference tells theGPS receiver how far away that particular satellite is. If we add distance measurements from afew 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/longitudeposition—what’s called a 2D position fix. With four or more satellites, a GPS receiver candetermine a 3D position fix which includes latitude, longitude, and altitude. By continuouslyupdating your position, a GPS receiver can also accurately provide speed and direction oftravel (referred to as ‘ground speed’ and ‘ground track’).