Research On Acquisition Algorithm And Implementation Of High-sensitivity GPS Receiver

GPS (Global Positioning System), as one of the world’s fastest growinginformation industries in recent years, is playing a more and more important role in ourdaily life. However under the environment of cities, forests and mountains, blocking,multipath and interference phenomenon becomes more serious, causing weakness anddegradation of the signal. The existing ordinary GPS acquisition algorithm will not beable to acquire GPS satellite signals under weak SNR environment. So GPS acquisitionalgorithm under weak SNR environment becomes a hot topic in recent years, and thepaper is just focused on this issue.Firstly, the GPS signal structure is introduced and some basic characteristics of theGPS signal are discussed, including the Doppler frequency shift and Power, then asimulation model of digital IF GPS signal is established in order to study signalacquisition algorithm. Afterwards, the basic theory of GPS signal acquisition and twobasic acquisition algorithms: including serial and parallel acquisition algorithms, aredescribed. their performance is simulated at the end of this chapter.Secondly, three methods of weak signal acquisition, including coherent integration,non-coherent integration and differentially coherent integration, are discussed withemphasis. In order to get the final acquisition performance for the coherent andnon-coherent integration algorithms, the statistical distribution of the decision variablesand some ways to determine the threshold are analyzed carefully. Finally, we got thecell probabilities for each searching cell, and the simulation results are presented at theend of this chapter.On the basis of the cell probabilities, we discussed a CFAR detector, the hybridpeak-to-average ratio algorithm, from both the cell probabilities and the decisionprobabilities. A detailed analysis is carried out and Monte Carlo simulations arepresented, It shows that the computation of the new algorithm is only half of thetraditional max peak-to-average ratio algorithm with nearly the same decisionprobabilities.Finally, A FFT-based parallel acquisition algorithm for FPGA implementation is selected, the parallel acquisition algorithm can calculate the same Doppler frequency onall code phase value at a time, enhancing the acquisition speed greatly. A detailedrealization of the parallel acquisition algorithm, including the working principle andflow charts of each module in the circuit, is also given in this paper. The FPGA projectis developed under the ISE software development environment with Verilog HDLhardware description language in programming. The project was tested with simulatedand measured data first, and then a field experiment was carried on, it shows that theprogram can work very well.