| Global Positioning System provides global, all weather and high precision based information of time and space position. It is widely used in the fields of military, transportation, life and so on, and making the research of the GPS system become the hot spot.The main work and innovation points of this paper show as following:(1) Built the GPS software simulation system that consist of the GPS software signal simulator and the GPS software receiver based on the matlab programming. User setting his position and time of signal be received by setting interface, signal generator calculates speed and position of satellite with the setting parameter, and then selects the satellites which visible to the receiver, calculates the ionosphere error, troposphere error, satellite clock error and the doppler shift, generates C/A code and navigation message, finally, add the relative time delay into the signal with the interpolation algorithm is proposed in this paper to complete the signal modulation, then receive a specific signal which can be used in receiver performance testing. Software receiver can implement the processes of capture, tracking, synchronization and positioning of GPS signal, output the results figure in each stage of baseband signal processing and the final positioning result latitude and longitude coordinates.(2) It is find that in the research of software GPS signal source, C/A code doppler frequency shift which often be ignored in generation of the intermediate frequency baseband signal, has an important influence on the final positioning results. The above problems, instead, are given close attention by this paper. In this paper, the GPS software simulation system is used to generate two kinds of signals of the added and non-added C/A code doppler shift to acquire the positioning graph and the GDOP graph through simulation, which is compared and analyzed with the simulation result of real signals. The results show that the signal with C/A code doppler shift have smaller location error, smaller GDOP, higher positioning accuracy, and the simulation result is closer to the real signal.(3) GPS signal have time delay cause by the ionosphere, troposphere, clock error and so on during the spread process from satellite to the receiver, that scholars pay much more attention to the optimization and calculation of the time delay module, but in this papper, the focus is the realisation of time delay modules with the matlab language. This paper describes the implementation steps of the signal modulation algorithm and the relative time delay among satellite signals in detail. In view of the relative time delay signal, a new interpolation algorithm is proposed in this paper, with this interpolation algorithm that signal frequency be change to 1/ts, and then adds the relative time delay into the signal with the precision of ts. Compare with the experimental data, it is found that the deviation between receiver and signal generator is less than the algorithm accuracy ts, so the algorithm can successfully realize the relative time delay of signal.(4) A large number of related operation involves in GPS signal tracking process, it leads a longer tracking time and lower work efficiency of the software receiver. In order to speed up the tracking and reduce the time to first fix (TTFF), This paper makes the tracking processing parallelization by calling the functions of MatlabMPI of pMatlab, it makes full use of multi-core parallel processing performance of the CPU, and it greatly improved the performance of software receiver. |
