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GPS receivers


The received GPS signal is woefully ill-suited for computer processing. Even though we might like to, we cannot simply connect the GPS antenna to a fast computer. First, the signal power must be increased by approximately ten orders of magnitude in power, or 100 dB. Second, natural noise and man-made radio-frequency interference (RFI) must be removed to the extent possible. When the signal arrives, these competing signals are much stronger than the desired GPS signal. Third, the received carrier frequency is 1.5 billion cycles per second and most computers would have a difficult time coping with such a rapid variation. Consequently, we must down convert the frequency to something more manageable. Finally, we must convert from an analog signal to a digital signal.

After conditioning, the signal has been amplified to a level that ignites the analog to digital (A/D) converter and creates numbers inside the digital portion of the receiver. Even though the signal is now well suited for digital processing, we have much work left to do. We must estimate the arrival time, τ, because it contains the basic range and clock information required to compute user position and clock offset. We must also estimate the Doppler shift, fD, because it contains the pseudorange rate information used to compute the user velocity and clock frequency. If we desire the ultimate in precision, the carrier phase, θ, must also be estimated and tracked.

The estimation of our key triplet, {τ, fD, θ}, proceeds in two stages. The first stage is a global search for approximate values of {τ, fD}.

Needless to say, the signal acquisition and tracking processes suffer from interference if strong competing signals are present or the satellite signals are blocked. After all, the GPS signals travel 20,000 kilometers from medium earth orbit, and the received signal powers are approximately 10–16 watts. Signals from terrestrial sources are generally much stronger. If they fall in the GPS portion of the radio spectrum, then the GPS signal acquisition and tracking situation can quickly become bleak. Physical obstructions that block the GPS signals are also appreciable challenges. Fortunately, a growing portfolio of countermeasures exist to mitigate the deleterious effect of GPS signal obstructions or competing radio signals.





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