What is GPS?
What is GPS?
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.
From any point on the planet, there are normally 5 to 8 satellites visible to an earth-bound GPS receiver allowing it to pinpoint its three dimensional position in space. The receiver determines accurate location by locking onto the signals of at least four satellites, reading their clock data, and measuring the distance to each. Normally, a point in space can be defined by measuring its distance from three known points, in a process known as “triangulation.” In theory, using three satellites as reference points and calculating the amount of time it takes their signals to reach the receiver then dividing by the speed of light (rate at which radio signals travel, equivalent to about 186,000 miles per second) should yield the exact distance from each of the three points, thereby pinpointing the location of the receiver. However, forces of the earth's atmosphere (which is constantly changing) slightly delay signals sent by the satellites, causing an error in distance calculations. Since a delay of only one-thousandth of second can mean a distance error of almost 200 miles, an accurate plot cannot be obtained using the slightly inaccurate data from the three satellites, and the information from a fourth satellite is required. The information from the fourth satellite enables the use of a simple error correction technique that yields a corrected signal-timing model for each satellite and ensures distance accuracy. Tracking the GPS time information from several satellites also allows the receiver to update its own clock and record time with atomic accuracy, without having an expensive on-board atomic clock.
Each satellite broadcasts in data on two different frequencies that penetrate the Earth's atmosphere at different speeds, allowing a more sophisticated means of measuring distance between the satellite and receiver by comparing signal arrival times. The processed dual frequency GPS data is so accurate, that with the use of specialized military equipment, physical position can actually be determined to within a centimeter. However, for National Security reasons the Department of Defense reserves access to the second GPS carrier frequency for military use only. The primary GPS frequency still provides civilian receivers accuracy inside of 100 feet, or better.
GPS technology is continually growing, and is now used worldwide for everything from commercial navigation, to agriculture. GPS technology applications are becoming so diverse that it is even used in the banking industry as a Universal Coordinated Time (UTC) reference in coordination of transactions across the globe. In just a short period of time, GPS has completely changed the way we measure distance, locate objects, and navigate the planet.
