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    GPS

    GPS (Global Positioning System) is a satellite based location system which works around the earth. You'll find an extraordinary and diverse range of GPS applications as it becomes both simpler and cheaper to add satellite-based locations. GPS sensitivity is a complex subject. Receiving the position information form satellites is a complicated task. The jobs of a GPS receiver are to identify as many GPS signals as possible from the separate satellites and to obtain a precise time of arrival for the start of the code sequence for each one. This code-sequence time is measured relative to the receiver's internal clock. The microwave signal strength itself is at best very low. With a typical received signal of approximately -130 to -140 dBm, the GPS receiver identifies the signal in the noise by comparing it with the expected PRBS in a correlator. When the signal is received and decoded, you need to do geometrical calculations to compute position. To grossly oversimplify, they involve solving time-of-flight measurements from three satellites with known positions for a 2-D fix and four or more satellites for a 3-D fix to yield a position for the receiver, once you convert the time-to-flight data to distance. Vendors are focusing on the E-911 locating mandate for cellular phones, which requires that cell-system operators be able to identify with high confidence and accuracy the location of a phone. Thus, improving the basic sensitivity of GPS is perhaps the single biggest story in emerging chip sets. Early GPS receivers had a single signal processing channel that wasswitched (multiplexed) between the various satellites the receiver wasusing to compute a fix. If a satellite went out of view momentarilywhen the receiver was trying to get data from it, the receiver couldlose the fix, and take some time re-acquiring satellites.Current receivers have multiple processing channels (often 12), sothey can process data from multiple satellites simultaneously, andwill usually have data from more satellites than are really needed tocompute a fix. This makes the fix more accurate, and tolerant of lossof signal from one or two satellites.


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