Global Positioning System is the only fully functional Global Navigation Satellite system. The G.P!S uses a constellation of at least 24 Medium Earth Orbit Satellites that transmit precise microwave signals that enable GPS receivers to determine their location, speed, direction and time.

It was developed by the United States Department of Defence whose official name is NAVSTAR-GPS. A similar satellite navigation system includes the Russian “Glonass”, the European Galileo Positioning System, and the proposed Compass Navigation of China.

The era of Global Positioning System began following the shooting down of Korean Airlines Flight 007 in 1983. The President of USA Ronald Reagan issued a directive making the system available free for civilian use as a common good. Since then the GPS has become a widely used aid to navigation worldwide and useful tool for map making, land surveying, scientific uses etc.

GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time. The GPS operational constellation consists of 24 satellites that orbit the earth in 12 hours. This system has many application advantages. Such as:

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(i) Civil:

It can determine the exact location of person or a vehicle any where in the world. It is also used to find out the shortest distance between two places. Aircrafts equipped with GPS can fly and land in an airport without any help from the ground. A ship can also sail without external assistance.

(ii) Military:

GPS allows soldiers to find objectives in the dark or in unfamiliar territory and to coordinate the movement of troops and supplies. Various military weapon systems use GPS to track potential ground and air targets.

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This GPS also helps to engage the target accurately. Military aircrafts those used in air to ground roles use GPS to find targets. GPS allows accurately targeting of various military weapons including ICBMS, cruise missiles and precision-guided missiles.

Downed pilots can be located faster if they have a GPS receiver. GPS satellites also carry a set of nuclear detonation detectors consisting of an optical sensor, on x-ray sensor, electromagnetic pulse. Now it is also being used in CDMA cellular which is very helpful in positioning of mobiles for emergency calls and other applications.

Finally GPS has enabled researchers to explore the earth environment including the atmosphere, and gravity field. GPS survey equipment has revolutionized tectonics by directly measuring the motion of faults in earthquakes.