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Global Positioning System (GPS)

History of GPS

Historically humans have strived to figure out their position on earth be it land, water or air and plot a route from present position to destination. In times gone by this has always proven to be cumbersome. In the quest to simplify this basic necessity people experimented with various technologies that proved in-adequate.

Seafarer's used angular measurement to the sun and stars to calculate locations while their descendents used technologies such as radio navigation. This technology utilized man-made satellites that transmit precise line of sight radio navigation signals used to determine position information in a two dimensional system.

Present day necessities of the military and civilians for precision in Time synchronization, Positioning, Navigation and Tracking of mobile assets led to the implementation of "Global Positioning System" and revolutionized GPS enabled services.

Photo courtesy Wikipedia

Global Positioning System (GPS)

A constellation of 24 satellites, setup by the Department of Defense, U.S.A allows determination of position in three dimensions, timing and velocity using precise radio navigation signals. The GPS signals are available 24/7, free for civilian use worldwide. The 3 components that constitute GPS are the satellite constellation orbiting earth, any GPS receiver and control system on earth.

The GPS satellites in a precise orbit circle the earth once every 12 hours and continuously transmit radio signals. The transmitted signal information includes transmit time based on the inbuilt atomic clock, the ephemeris that contains parameters to calculate location of satellite and the almanac which is the general system health. The signal travels at the speed of light through the outer space and at a slightly lower speed through the atmosphere.

The GPS receivers on earth pick up this transmitted signal uses the received time to calculate distance to the satellite and the receiver's on-board processor by way of triangulation calculates its precise location. The GPS receiver has to have a view of at least three to four satellites to enable three dimensional geographic position information (latitude, longitude and altitude), timing information and velocity.

The earth control system consists of 6 control & monitoring stations and four ground antennas geographically spread across the earth. The monitoring stations passively track the navigation signals from the GPS satellites which are then passed for processing at the master control station operated by the USA Air force. The master station via the ground antennas send updated navigation signal back to the satellite. All GPS satellites on a daily basis checks, computes and corrects its own position and time with respect to the ground station.

Accuracy and Errors

The position accuracy is directly dependent on the satellite position and the signal delay. Various methods are used to improve accuracy in commercial and consumer GPS receivers. Typically most such receivers have an accuracy of up-to 10 to 30 meters.

The error in accuracy can be attributed to atmospheric effects, Multipath effects, ephemeris, clock, selective availability, relativity, sagnac distortion, natural and other artificial sources.

Applications

Commercial and civilian applications include timing, terrestrial tracking & navigation, aviation, marine, rail, GIS, agriculture, environment, survey, public safety, disaster & emergency rescue and recreation.

U.S.A Defense forces utilize GPS receivers in all systems such as ground vehicles, ships, aircrafts, space crafts and specialized munitions.