If you've seen GPS equipment, you've seen only a part of what is GPS. Because GPS (Global Positioning System) is a satellite-based navigational network that makes it possible to find your position anywhere on earth. Commercially available GPS receivers are capable of accuracy to within a few meters or yards. And with GPS devices at a relatively low cost, accurate navigation or positioning is available to most anyone.

The GPS Satellite
The heart of the GPS is a group of 24 satellites in unique orbits approximately 11,000 miles above the earth. The satellites are maintained by the U.S. Department of Defense (DoD) which conceived of the system in the 1970s. GPS was created to provide highly accurate positioning for submarines before they fired their missiles. The first satellite was launched in 1978 and replacements are launched periodically as needed. The satellites are monitored by five ground stations to assure they maintain their precise positioning.

The Satellite's GPS Information
Each GPS satellite has a ultra-high-frequency radio transmitter that sends out a unique stream of information. A GPS receiver uses the signals from three or more satellites to determine its location by means of triangulation. The satellite signal has three types of information; a pseudorandom code, ephemeris data and almanac data. The pseudorandom code identifies which satellite is sending the information. Ephemeris data consists of the current date and time as well as whether the satellite is functioning properly. And the almanac data relays the expected position of all the satellites during the day.

GPS Accuracy
Satellites emit two signals, one for military use and a less accurate signal for civilian use. When the military first allowed civilians access to GPS in the 1980's it didn't anticipate how accurate commercial GPS receivers would be. So, out of security concerns, the DoD created "Selective Availability (S/A)" which randomly decreased the civilian signal's accuracy. However, other ways to maintain security were developed and Selective Availability was stopped on May 2, 2000. Current uncorrected GPS accuracy using the civilian signal is less than 15 meters or 49 feet.

The GPS position calculations are based on comparing the reading of a clock in a GPS satellite with one in a GPS receiver. Ideally, the comparison is made almost instantaneously, since radio waves travel at the speed of light. But since the signal passes through charged particles and water vapor in the atmosphere, it slows by a variable amount. And anything which slows the speed of the signal affects the position calculations. The signal also takes longer to travel if it bounces off a building or other obstacle (multipath error) before reaching the GPS receiver. Further errors can come from a satellite slightly out of position, an inaccurate receiver clock or other radio signals causing interference. Compensation is built into the system to help correct for these possible errors, but some inaccuracy will always be present.

Differential GPS
Differential GPS is a simple method to improve GPS accuracy to within a few meters or yards. The U.S. Coast Guard created it as a response to the accuracy loss caused by Selective Availability. Differential GPS relies on the use of two GPS receivers. One is permanently located at a precisely determined spot. This static receiver can compare its known position with the position calculated from the GPS satellite signals. Signal timing error corrections can then be transmitted to the mobile GPS receiver. Differential GPS can eliminate most accuracy problems except multipath errors or signal interference.

The two usual sources for corrective Differential GPS signals are the free transmissions provided by the Coast Guard and the more widely available private subscription services. A separate receiver is required to pick up the correction signals and pass the information to the GPS receiver. And, of course, the GPS receiver must be capable of processing the correction signal. Although the accuracy possible with Differential GPS is attractive, for most uses the gain is unnecessary.