What is GPS?
The GPS, or Global Positioning System, is a system of 24 satellites in orbit above the Earth. The satellites are divided into six groups of four satellites, each group traveling on a different orbital track. The orbital track for the satellites is roughly 20,000 miles above the surface of the earth. That allows them to cover the most area possible with their radio transmitters. Because the satellites operate 24 hours a day, 365 days a year, GPS data is always available to those who have a receiver. The satellites completely surround the planet, and can provide location information anywhere on the planet to within a few meters.
What do the Satellites do?
The satellites send radio signals down to Earth. The signals contain information about each satellite, including the precise time. The GPS receiver, which can be either a hand held unit, a vehicle-based receiver or a static ground station receives and decodes that signal. These receivers were, until the early 1980s, only available to military personnel. The system was designed and implemented by the Department of Defense in order to coordinate attacks with large numbers of troops. While the system is still in use worldwide, manufacturers such as Garmin and Magellan rose to the top of the industry by offering quality civilian units. While the civilian units are very accurate, often to within just a few feet, the military models, which sometimes use multiple antennas are accurate to within inches.
How Does it Know Where You Are?
The time code included in the signal is compared to the precise time kept in the ground unit. By comparing the data contained in the transmitted signal to that stored in the unit, a location can be determined to within 10 to 20 meters. With a secondary antenna, or another receiver slaved to the first, accuracy can be reduced to inches. As long as the GPS receiver can contact at least three satellites on the GPS network, a position fix can be determined; of course the system is designed so that at least three satellites are above the horizon at any given time. The more GPS satellites that a receiver can detect, the greater the accuracy. Often, as many as 8 or 10 satellites can be detected, and the civilian units can offer near one meter accuracy.