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Computing Time, Velocity and Position
GPS stands for Global Positioning System, which was first developed by the military from funding by the US Department of Defense. Though there are millions of civilian users today, the primary goal of GPS was for it to be operated by the military for which it was designed, in the first place. It enables receivers to compute position, velocity and time from satellite signals that have been encoded mainly for processing by GPS receivers. GPS can thus be used as a Satellite Navigation System.
GPS has some other uses as well and these include the Space segment, Control segment and User segment. Space segments have space vehicles which are used to send radio signals from space while the control segment is made up of a number of tracking systems located all around the world. The user segment is made up of the GPS receivers and the community of users. Such a receiver can convert the satellite vehicle signals into time, velocity and position estimates.
The main use of GPS is to navigate in three dimensions and navigation receivers are built for ships, airplanes and ground vehicles as well as for hand held devices, used by individuals. Special purpose GPS receivers have been used in astronomy, telecommunications as well as in laboratories. Atmospheric pressure has also been calculated through use of GPS, especially in research projects.
The Precise Positioning System is used by the U.S. military and some Government agencies and has 22 meter horizontal accuracy, 27.7 meter vertical accuracy as well as 200 nanosecond time accuracy. For the civilian user there is the Standard Positioning Services, the use of which does not incur any cost and is unrestricted.
Most receivers can receive and use the Standard Positioning Services. But, it has less accuracy which is purposely done at the will of the Department of Defense, having 100 meter horizontal accuracy, 156 meter vertical accuracy and 340 nanosecond time accuracy.
The GPS Navigation Message is made up of timed data bits that mark the time of transmission for each sub-frame as they are transmitted across by the space vehicles. Common errors that occur are due to noise, bias and blunders. GPS receivers are priced differently, depending on their capabilities and small receivers cost barely US$200 and may even be capable of accepting differential corrections. Other receivers having greater capabilities can cost thousands of dollars, depending on their unique features.
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