What is the Global Positioning System (GPS)?

The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.

Today's GPS receivers are extremely accurate, thanks to their parallel multi-channel design. Garmin® GPS receivers are accurate to within 15 meters on average. Newer Garmin GPS receivers with WAAS (Wide Area Augmentation System) capability can improve accuracy to less than three meters on average. Users can also get better accuracy with Differential GPS (DGPS), which corrects GPS signals to within an average of three to five meters. This system consists of a network of towers that receive GPS signals and transmit a corrected signal by beacon transmitters.

The 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us. GPS satellites are powered by solar energy. Small rocket boosters on each satellite keep them flying in the correct path. The first GPS satellite was launched in 1978 and full constellation of 24 satellites was achieved in 1994. Replacements are constantly being built and launched into orbit. A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.

GPS satellites transmit two low power radio signals, designated L1 and L2. A GPS signal contains three different bits of information - a pseudorandom code, ephemeris data and almanac data. You can view this number on your Garmin GPS unit's satellite page, as it identifies which satellites it's receiving. Ephemeris data, which is constantly transmitted by each satellite, contains important information about the status of the satellite (healthy or unhealthy), current date and time. Each satellite transmits almanac data showing the orbital information for that satellite and for every other satellite in the system.