GPS Digital Intermediate Frequency Signal Simulation

GPS signal source can provide simulation environment for the development and testing ofreceiver, to test whether the receiver achieves the expected design goals. Because of satellite singalusing high frequency carrier wave, it is not easy to be filtered and demodulated. Before processingthe received data the receiver down-conversion the radio frequency signal (RF signal) into anintermediate frequency by the antenna front-end and by the A/D sampling and quantization intodigital intermediate frequency signal, then further signal processing. Therefore, all researches of theGPS receiver on the basis of the digital intermediate frequency signal, only the suitable intermediatefrequency digital signal can be processed by the GPS receiver. Therefore, the GPS digitalintermediate frequency signal simulation has important significance for the research anddevelopment of the GPS software receiver.This paper takes GPS intermediate frequency signal simulation as the research target. In theentire research process, the main contents are as follows:First understanding the related knowledge of GPS signal system and spread spectrumcommunication, introduces the structure of the pseudo random code and the navigation message indetail.The GPS signal has two pseudo random code——coarse acquisition code (C/A code) andprecision acquisition code (P code). Because, P code as military pseudo code, general users can’tuse, this paper takes L1carrier on C/A code as simulation object, realize GPS digital intermediatefrequency signal.In order to realize the subsequent simulation, this paper discusses the mathematical model aboutfrom the transmitter to the receiver converting the IF signal, and gives out the signal propagation,gives thesimulation model of error sources in signal propagation——ionospheric delay,tropospheric delay, noise and multipath effect, through iterative algorithm for signal transmissiontime, study Doppler frequency shift between satellite and receiver.All this provide the basic theoryand algorithm for signal simulation.The paper realizes the GPS C/A code, the navigation message and digital intermediate frequencysignal generation through MATLAB programming. At the same time, set up Simulink model torealize system level simulation of digital intermediate frequency signal generation, and then thesimulation result was given. Through the software simulation not only provides the foundation forreceiver algorithm research, such as satellite signal capture, tracking algorithm, and controllableparameters GPS intermediate frequency signal generator has good mobility by setting different parameters,which can provide different simulation environment, convenient and easy to operate.The research results will be able to provide some reference for the research of satellite simulator.Finally, research key parts of GPS signal source hardware implementation——carrier NCO andcode NCO generation using the DSP Builder tool, put forward relevant parameter setting method,finally completes the signal modulation generating, and in the Modelsim simulation tool forvalidation. It provides a good transition to hardware research and implementation in the FPGAplatform later.