Influence Of Doppler Effect On The PMF-FFT Algorithm For High Dynamic GNSS Signal Acquisition

It is need to search the Doppler frequency in addition to the pseudo code when the receiver captures the signal.But Doppler frequency offset of hundreds of kilohertz and the rate of change several kilohertz per second in a high dynamic environment will not only increase the acquisition time of the receiver,but also greatly reduce the correlation peak when the pseudo code is synchronized.In particular,when the received signal is a weak signal,the excessive Doppler frequency offset and the rate of change are likely to cause the receiver to catch no signal.Therefore,it is of great significance to realize the fast acquisition of satellite signals,especially weak signals in high dynamic conditions.Although the algorithm of partial matched filter combined with fast Fourier transform(PMF-FFT)is ideal for applications in highly dynamic environments,but its application is severely limited in these cases.To solve this problem,the effect of Doppler effect on PMF-FFT algorithm is analyzed by using high dynamic GNSS signal in this paper.And three Doppler compensation methods based on Keystone transform are proposed then for improving the performance of the algorithm for long time coherent accumulation under high dynamic conditions.The main work and achievements of this paper are as follows:1.The factors that influence the performance of PMF-FFT algorithm when the coherent capture time is short are analyzed in detail under high dynamic conditions.In order to solve the problem of frequency domain attenuation characteristics of PMF,the method of windowing is studied,and two methods of zero padding and windowing are studied for the effect of FFT scallop loss.In addition,the influence of the number of PMF and the length of pseudo-code on the performance of the algorithm is studied.It is pointed out that the more the Doppler frequency offset can be captured by the algorithm,the more the number of segments is,and also the more the number of FFT points is.If the length is not enough,it can not be solved by fixing the zero after the pseudo code.This will reduce the performance of the algorithm and bring new problems.2.The influence of Doppler effect on PMF-FFT algorithm under high dynamic condition is studied in detail when the coherent acquisition time is long.Based on the principle of Doppler effect,the model of high dynamic GNSS signal is deduced.Then,the influence of pseudo-code Doppler frequency and carrier Doppler frequency on PMF-FFT coherent acquisition is analyzed.In this paper,the compensation method based on Keystone transform is proposed for the influence of pseudo code,and the compensation method based on discrete chirp-Fourier transform(DCFT)and polynomial chirplet transform(PCT)is proposed for the influence of the first order and second order change rate of carrier Doppler,and a simplified method of delay autocorrelation plus FFT is proposed for DCFT.3.The performance of the algorithm is verified by the experiment,which mainly includes the test of probability and Doppler frequency acquisition accuracy.The results showed that:In the detection performance,the performance of the compensation method based on Keystone transform and DCFT is the best,the method based on Keystone transform and delay autocorrelation plus FFT is the worst;In the Doppler frequency acquisition accuracy,the compensation method based on Keystone transform and PCT has the best performance with the increase of coherent integration time,the method based on Keystone transform and delay autocorrelation plus FFT is the worst.The research of this paper can provide a theoretical basis for improving the acquisition performance of Beidou navigation system under high dynamic conditions.