Global Positioning Systems in a Nutshell

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Since the Department of Defense placed the first satellite into earth’s orbit in 1978, an entire network of 27 satellites, 24 currently transmitting and 3 emergency replacements  as needed, have launched to form today’s Global Positioning System (also called NAVSTAR). Solar-powered and always active, the satellites allow the pinpointing of any location at any time on earth’s surface.


How is a GPS location calculated?

It’s all three-dimensional trilateration. The communication between a GPS receiver and GPS satellites in orbit uses radio signals to accurately determine a location on earth. First, the distance in space between the position on earth and one initial satellite of at least four is calculated. The satellite’s highly accurate atomic clock resets the receiver’s down to the correct nanosecond so that both are on the exact same time, which in turn syncs the clocks of the other three (or more) satellites alongside them to that time as well. When a signal is sent from satellite to receiver, the time (lag) difference between the two locations is recorded and used by the receiver to generate the distance from earth to satellite and, in turn, an approximate point on earth.


Then the other satellites also generate, in the same manner, approximate points near the first, and the meeting point of all satellite locations is determined to be most precise position on earth. Once it has a starting point, this system can calculate longitude and latitude, as well as elevation. When combined with other active satellites, it can also be used to find direction, velocity, and the distance to a predetermined end point.


Is GPS ever wrong?

Most errors occur because signal communication with the receiver is interrupted or lost. For instance, huge buildings or large natural formations on earth’s surface can hinder signals from hitting the receiver at the correct time. Certain conditions in the atmosphere also slow a signal. Inaccuracies in the receiver’s clock and satellite orbital shift may be taken into consideration and recalculated. Satellites are built with a 10 year average life span; in some cases perhaps the equipment itself has merely worn out.


What if GPS is wrong? 

It’s not that likely. The GPS satellites are not only armed with an atomic clock and interior almanac but also are constantly updating themselves with receivers monitored by Department of Defense to check for data errors. A high-tier Differential GPS satellite, not permitted to be used by the general public, receives around-the-clock accurate information from a permanently placed receiver and sends updated corrections to all other local public satellites.

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Claire Richards

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