Global Positioning System Term Paper

Global Positioning System

GPS is a navigation system that gives our exact location on earth at any time and anywhere. It is a high precision tracking system that provides accurate location coordinates mapping even the remotest of locations. Though it was initially designed, developed and controlled by the U.S. department of defense for its military purposes, it soon became available for the civil users worldwide. This navigation system covers the entire surface on earth using 24 satellites (and 3 additional) that orbit around it at a height of 11,000 nautical miles. [The Aerospace Corporation]. Ground stations based in five different locations on earth receive signals transmitted by these satellites and send data, which are used to monitor and control the satellites. Considered a revolutionary breakthrough in the field of navigation, this technology is finding newer applications everyday. A brief overview would provide us a better insight into the subject.

Global Positioning System

GPS makes use of high frequency radio wave transmitters stationed in space making it possible to cover the entire globe accurately. Locations can be tracked to precision up to a height of 300 ft above earth. The GPS system has three different segments namely the space segment, ground-based control stations and the user segment. As discussed above the space segment is composed of 24 satellites moving in fixed orbits taking 12 hours to complete one full circle across the earth. The GPS communication system with all the 24 satellites was completed in 1994. The orbits of these satellites are arranged such that at any point of time at least 4 of them are in visual range from any point on earth. [The Aerospace Corporation]

GPS Control Stations

The control stations are based in strategic locations across the globe. There are a total of 5 such control stations that are based in Hawaii, Kwajalein Island in the South Pacific Ocean; Schriever Air force base in Colorado Springs, the Diego Garcia Island in the Indian Ocean and Ascension Island in the Atlantic Ocean.The Schriever airbase station serves as the central control station. These stations perform the clock corrections for all the GPS satellites and also upload the ephemeris data as and when required. [Peter H. Dana]

GPS Receivers

The GPS user segment is nothing but receivers or gadgets that are equipped to receive the signals from the satellite. It is the receivers that perform the calculations to deduce the location. To do this the receiver must locate four of the GPS satellites and find out the distance from them. Then using a geometrical principle called trilateration the location is calculated. Trilateration is similar to the triangulation function of location determination except that it does not take into account the angle factor. Using a minimum of three reference points (satellites locations) and the distance between each reference point to the affixed point, it is possible to identify the location of the fixed point anywhere on earth. Thus by knowing the position of three satellites (4 satellites for three dimensional mapping) and the distance to them, GPS receivers can locate themselves on earth. The positions of the satellite at any given time is easily known because of their standard orbits. Every GPS receiver also has an almanac, which stores the predicted path of the satellites. [Marshall Brain]

GPS Satellites have highly accurate atomic clocks that can differentiate up to billionths of a second. To find the distance from the reference points (satellites) the receiver calculates the time taken for the radio waves to travel from the satellites to itself. This is done by means of a simple calibration. The satellite generates and transmits a particular digital pattern that the receiver can also generate. However, there is a time lag associated with the satellite's signal pattern compared with the similar pattern generated by the receiver. The receiver measures the time lag and multiplying it with….....