How Reliable Is a GPS Unit?
A Global Positioning System unit utilizes transmission signals from satellites to calculate an object's position on the ground. The accuracy of this measurement depends on a range of factors, including the type of GPS receiver, atmospheric conditions, location of the satellites and natural obstacles.
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The Global Positioning System
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The Global Positioning System, developed by the U.S. Department of Defense, is a constellation of 24 satellites, which transmit precise signals containing their position and time, operated by the U.S. Air Force and coordinated with atomic clocks located at the U.S. Naval Observatory. In 1983, the system was opened for civilian use by President Reagan after a civilian Korean airliner was shot down by Soviet interceptors upon mistakenly entering Soviet airspace.
How Position Is Calculated
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GPS satellites are continually transmitting their time and position. A GPS unit calculates distance to a satellite based on the time required for the signal to reach it, and coordinates its position based on the mathematical principle of trilateration. Given the radii and centers of multiple spheres (the distance and location of the satellites), the unit calculates its position at the point where these mathematical spheres intersect. Because it is calculating its position in three dimensions, a GPS unit must be in contact with four satellites.
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Accuracy of Civilian GPS Units
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Most hand-held GPS devices have an accuracy of between 10 to 20 meters, coordinating position based on the location of four satellites. However, some receivers, utilizing a method called differential GPS, which coordinates position with a second fixed land receiver, provide a position which is accurate to one meter.
Variables Affecting Accuracy
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Because GPS units require contact with four satellites to calculate position, accuracy is affected by the location of the satellites and the atmospheric conditions through which the signal must travel. In addition, natural obstacles such as mountains, tall buildings, or forested areas can disrupt signals, and so accuracy will be greatest in open spaces.
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