The Research And Design Of GPS Receiver Baseband Processor Based On FPGA

Internet, mobile communication and satellite-based navigation are threemost important industries in the information society of 21st century. Meanwhile,the technical level and the development course of GPS system represent thedevelopment status of the satellite navigational system in the whole world. Atpresent, our country has already become a great user of GPS. The satellitenavigation industry chain has basically been formed. Nevertheless, we have notgone deep into the core GPS technology acquiring the satellite signal and keepingtrack of it, as the most important part of our GPS products are mostly import.So the research of the core GPS technology is very urgent.The principle of GPS position, the principle of GPS receiver—direct spreadspectrum communication, and signal structure and characteristic of GPS, wereintroduced in the paper. The methods of GPS receiver's signal acquisition andtracking were designed on these principles.On the process, firstly, the article compares acquisition algorithm in timedomain called traveling correlation and in frequency domain which is based onFFT, also the both were calculated using MATLAB. For the former which is suitablefor the hardware receiver in the article has good flexibility and can capturehigh-precision code phase and carrier frequency, so the article finallydetermines to use the traveling correlation acquisition algorithm.Then analyzes the characteristics of tracking loop, the tracking cell wasimplemented by using the coupling of code tracking loop and carrier wave trackingloop. For GPS system generally work in the environment which has very low ratio of signal to noise, but non-coherent ring has good tracking performance in theenvironment, so the code tracking loop was completed by DDLL. The phase detectorof the tracking loop is of energy, which is not sensitive in the data modulationand carrier phase, also it won't produce uncertainly phase. For the input signalexists phase reverse, but phase-locked loop called COSTAS allows data modulationand it isn't sensitive of the phase reverse, so frequency carrier tracking loopwas completed by COSTAS PLL. The algorithms above were calculated using MATLAB.The main parts of the circuit were implemented on Quartus II 8.0 by VHDL.And the function simulations were completed on EDA simulator calledModelSim-Altera 6.1g. So it can be concluded that this design can satisfy thefunction and performance requests of the system, and it may directly be usedin the real-time GPS receiver system and has established a foundation to designGPS receiver independently.Finally, in short electromagnetic environment, 32pps signal will be lostif acquisition is out of control. An algorithm of timing and punctuality to getthe strictly exact second pulse timing signal was designed to Synchronizationwith GPS time. And the algorithm was implemented.