In 1978, the Department of Defense launched its first satellite that was to be the beginning of the Global Positioning System, or GPS. The military called it NavStar. The program called for the launching of 24 satellites to be positioned roughly equidistant from each other, so that they would uniformly cover the surface of the earth. Three extra satellites were to be launched as backups, in case any of the others failed.

Device Works By Triangularization

The purpose of these satellites was to be able to allow the US military to determine its exact position through a system of triangularization (actually called trilateration). A hand-held, or vehicle mounted device, would receive signals from at least three satellites. The GPS unit would then process that information to inform the user exactly where they were located. By receiving signals from at least four satellites, the device could give not only the latitude and longitude, but also the altitude.

Satellite System Completed

The GPS system of having 24 satellites in position was accomplished in 1994. Each of the satellites, weighing more than 3,000 pounds each, and designed to last for ten years, orbits the earth twice a day at an altitude of about 12,000 miles. Years before this system was completed, the military permitted civilians to begin using it.

Deliberately Inaccurate Signals

When civilians began to use the system, the GPS satellites deliberately transmitted a degraded signal. This deliberately inaccurate signal was designed to prevent an enemy from using our own signals against us, and it was called Selective Availability (SA). While using the SA, the signals generated a rather generally accurate location to within about 100 meters. This system of generalized readings was turned off by the military in May 2000.

Current Accuracy of GPS Devices

Today, however, the GPS system works very accurately - but it does depend on where you are in relation to the satellites. If they are either more or less lined up, or bunched together, then the reading won't be all that accurate. Other factors can cause an increased inaccuracy such as being near tall buildings, dense foliage, atmospheric disturbances, etc. On the other hand, since satellites are continually moving (about 7,000 mph), if you allow the GPS device to get a couple of readings from the satellites, then your position will be more accurately determined. The accuracy today, depending on how new your device is, and the features it has, could be accurate to with a 5 to 10 meters. The enhanced features of some particular brands can bring this down to within 3 meters, while the military devices can put them within a millimeter, or two.

Satellites Transmit Three Pieces of Information

Your device receives three different pieces of information from each satellite that it picks up communication from - a pseudorandom code, ephemeris data and almanac data. This data deals with the number of the satellite, the position of the satellite, and the status of it, which concerns whether or not it is working properly. This last one includes the transmission of the time that the signal was sent, and then your GPS device measures how long it took to receive it. By this means, then, along with signals from other satellites, it will calculate your position.