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How
Does GPS Work?
The basis of GPS technology is precise time and position
information. Using atomic clocks and location data, each
satellite continuously broadcast the time and its position.
A GPS receiver receives these signals, listening to three or
more satellites at once, to determine the users position on
earth.
By measuring the time interval between the transmission and
the reception of a satellite signal, the GPS receiver
calculates the user and each satellite. Using the distance
measurements of at least three satellites in an algorithm
computation, the GPS receiver arrives at an accurate
position fix. Information must be received from three
satellites in order to obtain two-dimensional fixes(latitude
and longitude), and four satellites are required for
three-dimensional positioning (latitude, longitude and
altitude).
Under normal conditions, the GPS signal will provide a
civilian user an accuracy of better than 25 meters. Civilian
GPS receivers are sometimes subject to Selective
Availability (SA) interference by the United States
Government to maintain optimum military effectiveness of the
system. When engaged, SA inserts random errors in the data
transmitted by the satellites. As a result, signal accuracy
can be reduced to 100 meters. However, using a technique
called differential GPS (DGPS), the user can overcome the
effect of SA interference and increase the overall accuracy
of the GPS receiver. With DGPS, one GPS receiver unit is
placed in a known location and the position information from
that receiver is used to calculate correction in the
position data transmitted to other GPS receivers in the
area. The resulting real-time accuracy is in the 10 meter
range. Sub-meter accuracy can be obtained by using DGPS and
post-processing calculations in static
positioning.
 
see also:
GPS
= Global Positioning System
Who
Uses GPS?
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