The Global Positioning System, commonly called GPS, is a navigational system that uses satellites and computers to determine the latitude and longitude of a receiver on Earth. This is done by computing the time difference for signals from different satellites to reach the receiver.GPS is the only satellite navigation system that is fully functional. It uses a collection of more than twenty-four satellites to send radio signals to electronic GPS receivers. By doing so, the satellites are able to find the correct location of the receivers. They get the exact longitude, latitude and altitude of the receiver. GPS systems work around the clock under any weather conditions.The GPS system became fully functional in 1985. Today it is used for navigating on land, sea and air. It is also used for mapmaking, land surveying and scientific research.Over the last fifteen years, the accuracy of the GPS signal has improved steadily. Today, a GPS system can accurately pinpoint the receiver on Earth to almost five meters of its exact location. However, further improvements and developments may increase the accuracy to about 1 cm of the receiver. This can be done using techniques such as differential GPS. A GPS also gives an extremely precise time reference which is useful in some scientific experiments.The GPS system was devloped by the United States Department of Defense. It was officially named NAVSTAR GPS, which was an abbreviation for Navigation Signal Timing and Ranging Global Positioning Systems. Out of the twenty-four satellites revolving around the Earth, the last one was launched in 1994. New satellites are periodically launched to replace aging ones. With the improvement in technology, GPS devices have become less costly. GPS is available for free use in civilian applications.