Research On Solution Of Doppler Shift Affect In Pseudo-code Ranging System

The subject of this paper is the Doppler shift impact in pseudo-code ranging system and its capture and elimination methods. As a widely used method, pseudo-code ranging system has occupied an important position in the measurement and control areas with its simple structure, reliablility and anti-interference ability.In pseudo-code ranging system, due to the fast motion of the target, there is a large Doppler shift in the system which will cause severely affect on the ranging signal. In the satellite communication system, the maximum Doppler shift is±100kHz,which can not be tolerated by the system. In practice, the pseudo code ranging system usually use the spread spectrum communication mode in which the acquisition performance of PN code will be seriously affected by Doppler shift. In this case, it even cause the fault of Dispreading PN code. So it must be compensated before demodulation.The background and significance of the research is introduced at the first of this paper, then introduced the PN code ranging methods, the concept of Doppler shift and the current research at home and abroad. After that two typical pseudo-code sequence, m sequences and Gold sequences, were analyzed, including their production methods, balance performance, self-correlation and cross correlation, and the simulation results is also given. After that we introduced the principle of pseudo-code ranging , four typical pseudo-code ranging program and the most unambiguous distance of pseudo-code ranging method.In the next the Doppler shift impact on the pseudo-code ranging system is analysed, including the impact on PN code acquisition performance, dispreading and demodulation performance of carrier.Followed that the maximum likelihood estimation theory principle is introduced. Then three typical algorithms of maximum likelihood estimation theory which called L&R algorithm, Fitz algorithm and the M & M algorithm are deduced. Take QPSK signal as an example to simulate and analyse the three frequency offset estimation algorithms.Finally, traditional Doppler shift compensation method which based on DMF and AFC loop is researched in which its shortcomings are pointed out. Then a new and effective method to estimated the Doppler frequency shift is used which is combined with the FFT and In-phase - quadrature loop( COSTAS loop). Then the estimated value is used to compensate the carrier and bit rate. At the end the theoretical analysis is verified by simulation by which the correctness and feasibility can be confirmed.