A GPS or Global Positioning System is a collection of two dozen satellites, which revolve around the earth. It is used to determine the location of a receiver on earth.This is done by sending radio signals to the electronic GPS receivers from the satellites. Every satellite has a receiver on earth. These signals contain data on the location of the satellites and the time taken for signal transmission. The satellites are placed twelve thousand miles above the surface of earth and complete 2 orbits every day. The functions of the satellites are periodically checked on and new ones are sent to replace the old ones.With the improvement in technology, accuracy has increased. GPS systems have become more affordable as compared to earlier times. Accuracy differs depending on various factors. Sometimes it can even be as good as a few meters. The GPS systems can be used in a large number of areas. Further, decreases in costs have increased the number of users. Fishermen and hikers can use portable GPS to navigate in the wild. Cars are being equipped with GPS systems, which give drivers better navigation abilities and better protection against theft. It is used by the military to guide cruise missiles to pre-specified targets. Personal hand held devices like wrist watches are equipped with GPS to help in tracking people out and getting their exact location.The system was developed by the Department of Defense of the US. It was initially named NAVSTAR GPS. Of the twenty-four satellites, the first one was launched in 1974 and the last in 1994. The Department of Defense switched off selective availability (SA) on 2 May 2000. The selective availability had been a cause for less accurate readings. The department reserves the right to turn it back on when they see the need to do so. The GPS systems are now about 10 times more accurate than they were when selective availability had been active.