The position coordinates are obtained with the help of 24 satellites that beam down precise signals through radio to GPS receivers all over the world. This helps the receivers to find their location in terms of longitude, latitude and altitude, with great accuracy, irrespective of weather conditions.
Behind this seemingly simple function is a $400 million annual investment of the US Department of Defense. The department maintains all the satellites that make this system possible. The actual coordination is done by the 50th Space Wing based at Schriever Air Force Base. This wing has its hands full making sure that all the satellites work in tandem and are accurate to the last micro second.
Besides this, there are five monitoring stations located at Hawaii, Kwajalein, Ascension Island, Diego Garcia and Colorado Springs. They report to the master control station which is at Schriever AFB. Interestingly, the US government makes the system available free of cost for civilian use.
The system itself has been designed very carefully. It has 24 satellites that have been put up in space in six orbital planes. Each plane has four satellites, each in its own orbit. The positioning has been done in such a manner that at any point of time, no matter where you are on this planet, there will be four satellites in your line of vision. These satellites circle the Earth twice daily at an altitude of 20,200 kilometres and send down packets of information.
These packets include information about their orbital position, and an almanac of the approximate position of every other active GPS satellite. The almanac lets GPS receivers use this data from the strongest satellite signal to locate other satellites. After that the receiving set does a series of calculations using the concept of trilateration and throws up the exact location.
The popularity of GPS can be gauged from the fact that it being used in all modern forms of navigation -- ships, planes and surface transport. The system also helps in land surveys and cartography besides being an extremely precise time reference for telecommunications and scientific research.
In fact, in the latter part of 2005, a series of next-generation satellites were added to the existing group. These satellites had a second civilian GPS signal called L2C for enhanced accuracy. In the years to come the satellites that will go up will add the third and fourth civilian signal.