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Global Positioning Systems What is GPS? |
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GPS (Global Positioning System) is a constellation of about 25 Department
of Defense satellites that orbit the earth approximately every 12 hours. The
position and time information transmitted by these satellites is used by
a GPS receiver to trilaterate a location on the earth. GPS was developed to provide a continuous, 24 hour, 3D (position and elevation) coverage anywhere on the earth. It provides reliable, repeatable information that is unaffected by rough terrain and bad weather, and is highly resistant to multipath errors and interference.The satellites broadcast on two carrier frequencies in the L-band of
the electromagnetic spectrum. One is the "L1" or 1575.42MHz and the other
is "L2" or 1227.6MHz. On these carrier frequencies are broadcast codes, much
like a radio or television station broadcast information on their channels
(frequencies). The satellites broadcast two codes, a military-only encrypted
code (PPS) and a civil-access or Standard Positioning (SPS) code.
All commercial and consumer GPS receivers are SPS receivers. There are
two basic types of SPS receivers, those that use the broadcasted code to
do their positioning (code-phase) and those that do carrier phase measurements
(carrier-phase). PPS, Precision-code or P(Y)-Code (PLGR's) receivers utilize the P(Y)-code broadcast on the L1, L2 carrier frequencies for positioning. These type of receivers are only available to the military and some government agencies. Positional accuracies for the code-phase, resource grade or C/A-code receivers range from 100 meters to less than 2 meters. Accuracy for carrier-phase units (commonly referred to as geodetic receivers) can be measured in millimeters.
Positional accuracy for both types of receivers strongly depends on a process
called differential correction. MORE->>
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