The U.S. Government recently spent $18 billion on 12 additional GPS satellites for private sector use to help accommodate anticipated increases in GPS use.
In 1978 the first Global Positioning System (GPS) satellite was launched into space by the U.S. Department of Defense as a part of a satellite navigational system created, funded, and operated as a tool for the U.S. military. Although begun under the direction of the military, the GPS program was opened to civilian use not long after inception. Civilian use of the GPS system has grown so much so that recent figures estimate civilian GPS receivers outnumber military units about 10 to 1!
The GPS system is comprised of three basic parts, the ground based tracking and control stations, the network of GPS satellites orbiting the earth, and the GPS receivers that interpret the satellite data. Each of these components works together in precise unison to create a hyper-accurate means of plotting a location anywhere on the planet. Sophisticated processors and surface map overlays built into GPS receivers visually interpret the satellite data for use in navigation or tracking applications.
The U.S. military employs a network of tracking stations located around the world that monitor signals from the GPS satellites and track the precise orbit of each satellite. Although the orbit of a satellite is very predictable, forces such as the gravitational field of the moon, and solar radiation can have a slight affect on the speed and path of the satellite. These forces create minute deviations from a satellite's planned orbit and are called “ephemeris” errors. The ephemeris errors are processed by the GPS Master Control station in Colorado, which then uploads minute corrections to the GPS data and atomic clocks of each satellite for re-transmission to GPS receivers.
Navigation is made possible by the constellation of 24 GPS satellites orbiting the planet every 12 hours from a distance of about 12,000 miles, and transmitting radio signals from space that allow earth-based GPS receivers to calculate their X/Y/Z axis position, relative speed, and time. Each GPS satellite is equipped with 4 atomic clocks that keep pace with the atomic Universal Time clock on earth in order to accurately record the length of time it takes a signal to reach the earth from space. This measurement is particularly important since the length of time it takes a signal to reach the earth is the key element in calculating the satellite's distance from earth. In addition to transmitting the code used for tracking the satellite (known as "pseudo-random code") the satellites also send GPS time and ephemeris data used to update the receiver's internal almanac, which precisely details where each satellite should be at any given time.