CSI Wireless Seres, Reference Manual

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Seres Reference Manual
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1.1.2 GPS Services
The positioning accuracy offered by GPS varies depending upon the type of
service and equipment available. For security reasons, two GPS services
exist: the Standard Positioning Service (SPS) and the Precise Positioning
Service (PPS). The SPS uses a code modulated onto the signal for
measurements and is referred to as the Coarse Acquisition code (C/A
code). The US Department of Defense (DoD) reserves the PPS for use by
its personnel and authorized partners. The PPS uses a different code than
the SPS, referred to as the Precise Code (P-code) and contains more
resolution than the C/A code. The DoD provides the SPS free of charge,
worldwide, to all civilian users.
In order to maintain a strategic advantage, the US DoD used to artificially
degrade the performance of the SPS so that the positioning accuracy was
limited to 100 meters 95% of the time. This intentional degradation is
called Selective Availability (SA). The effect of SA has been turned to zero
since mid-2000, however, it has not been officially ‘turned off’.
Currently, autonomous GPS is able to provide accuracy on the order of 10
meters, depending on the sophistication of the GPS engine. For many
positioning and navigation applications, this level of accuracy is not
sufficient, and differential techniques must be employed.
1.2 Differential GPS
The purpose of differential GPS (DGPS) is to remove the effects of SA,
atmospheric errors, timing errors, and satellite orbit errors, while
enhancing system integrity.