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Sources Of GPS Signal Errors |
By:
Dagfinn Rognerud |
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Factors that can degrade the GPS signal and thus affect
accuracy include the following: * Ionosphere and troposphere
delays -- The satellite signal slows as it passes through the
atmosphere. The GPS system uses a built-in model that calculates
an average amount of delay to partially correct for this type of
error. * Signal multipath -- This occurs when the GPS signal is
reflected off objects such as tall buildings or large rock
surfaces before it reaches the receiver. This increases the
travel time of the signal, thereby causing errors. * Receiver
clock errors -- A receiver's built-in clock is not as accurate
as the atomic clocks onboard the GPS satellites. Therefore, it
may have very slight timing errors. * Orbital errors -- Also
known as ephemeris errors, these are inaccuracies of the
satellite's reported location. * Number of satellites visible --
The more satellites a GPS receiver can "see," the better the
accuracy. Buildings, terrain, electronic interference, or
sometimes even dense foliage can block signal reception, causing
position errors or possibly no position reading at all. *
Satellite geometry/shading -- This refers to the relative
position of the satellites at any given time. Ideal satellite
geometry exists when the satellites are located at wide angles
relative to each other. Poor geometry results when the
satellites are located in a line or in a tight grouping. *
Degradation of the satellite signal -- Selective Availability
(SA) is an intentional degradation of the signal once imposed by
the U.S. Department of Defense. SA was intended to prevent
military adversaries from using the highly accurate GPS signals.
The government turned off SA in May 2000, which significantly
improved the accuracy of civilian GPS receivers.
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Article Source: http://www.friendsofvista.org/articles/article40844.html |
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