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Understanding GPS Navigation System

Global Positioning System (GPS) is the worldwide radio wave positioning system which was developed by U.S. Department of Defence and has been jointly operated by U.S. Department of Transportation.

GPS consists of 24 satellite groups in total, and they transmit the information necessary for positioning.
GPS antenna and GPS receiver are necessary for positioning by GPS satellite.
GPS receiver receives the positioning information from the satellite, and calculates the current location by measuring the distance to the satellite.

The distance between the receiver and the satellite can be calculated by the difference of the time transmitted by the radio wave from the satellite and the time of the current receiver, and if the radio wave is received from a minimum of 4 satellites, the current location can be calculated.

By now, there is no satellite fee. As for the accuracy of the GPS, SPS opened to the public had a big range of error because it was developed during the confrontation with the Communist bloc.

200m~300m of error occurred due to Selective Availability (SA). Because the SA has disappeared, such a great error does not occur now, and the greatest error is only 30~40m of error. The average error is about 50m.

The pros of using GPS for positioning are
• it can continuously position at real time.
• it can be used under all weather conditions.
• accumulated error does not occur because it is a sort of absolute positioning designed not to have any relation with the previous location.

However the problems are
• the error is greatly increased in tunnels or underground roads where the satellite signal cannot be received.
• it also relates to the topology of the satellite; the receiver using a triangular survey method has weak points so satellites are shown as being clustered and a deflection error occurs if the height of the satellite is low.

One cause of the important positioning error is jamming of the satellite signal by the action of the solar spot, which has the power to make the satellite communication network including GPS paralyzed at the peak of 11-year cycle.