Phung

Today's GPS receivers are extremely accurate, thanks to parallel multi-channel design. Garmin's 12 parallel channel receivers are quick to lock onto satellites when first turned on and they maintain strong locks, even in dense foliage or urban settings with tall houses. Certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers. Garmin GPS receivers are accurate to 10-15 meters on average. Newer Garmin GPS receivers with WAAS (Wide Area Augmentation System) capability can improve the accuracy to less than three meters on average. No additional equipment or any fees are required to take advantage of WAAS. Users can also get even better accuracy with Differential GPS (DGPS), which corrects GPS signals to within an average of three to five meters. The U.S. Coast Guard operates the most common DGPS correction service. This system consists of a network of towers that receive GPS signals and transmit a corrected signal by beacon transmitters. In order to get the corrected signal, users must have a differential beacon receiver and beacon antenna in addition to their GPS.

The 18-24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are travelling at speeds of roughly 7,500 miles an hour.

GPS satellites are powered by solar energy only. They have backup batteries onboard to keep them running in the event of a solar eclipse, when there's no solar power. Small rocket boosters on each satellite keep them flying in the correct path.

16 years as an officer in the Norwegian army. See my pages: http://www.gps-info.org/ and http://www.lincoln-navigator.org/