A Multipath Mitigation Method Based On Satellite Carrier Frequency Sweeping For Pseudo-Random Code

The error of satellite navigation receivers depends on many factors,most of which can be eliminated by differential technology and using high accuracy models.However the multipath effect correlates to the modulation of signal,the algorithm adopted by receivers,the performance of antenna and the surrounding of receivers as well,which cannot be mitigated by differential technology,therefore,multipath error has become one of the main errors in high accuracy performance of satellite navigation receivers.Based on the observation of Beidou satellite navigation system,it can be figured out that the multipath error of MEO(Medium Earth Orbit)satellite is approximately random,which can be mitigated by data average methods.However,because of the static orbit of GEO(Geosynchronous Earth Orbit)satellite,the multipath error period of it increases to about 1 day compared to that of other kinds of satellite.Therefore,the average of the multipath error will not be zero unless the processing time would be long enough,which appears to be repetitive and dependent and cannot be mitigated by traditional methods,so called Static Multipath.This thesis focuses on the research of a multipath mitigation method based on carrier frequency sweeping,which contains 3 main sections:1)This thesis has confirmed the effectiveness of carrier frequency sweeping for pseudo random code multipath mitigation.The variation of GEO satellite multipath can be increased by carry frequency sweeping,which can enhance the traditional multipath mitigation methods based on data filtering and average.The simulation results shows that when modulated on a carrier sweeping from 1541.098 MHz to 1581.098 MHz,the multipath error of GEO satellite signal can be decreased by 74.7%~92.6%.2)This thesis has done the theory deduction of the multipath mitigation performance of this method.It has been proved that when carrier sweeps in a line or sine mode,the multipath signal remains modulated after demodulation of receivers,while the direct signal has been successfully demodulated.The multipath code contains in a sine envelop,and after correlation the multipath error can be decreased.The performance of this method depends on the period of sweeping,the frequency range of sweeping,the multipath delay and the correlation time of receivers as well.when the correlation time approaches the period of the sine envelop,the multipath error can be decreased by 96%.3)This thesis has analyzed the parameters of the sweeping frequency carrier,and done the deduction of the effect of modifying the sweeping period or the frequency ranges,which may be instructive for the design of the sweeping frequency carrier.In consider of the limitation of bandwidth for navigation signal,this thesis has discussed the short period of frequency sweeping in narrow bandwidth in particular,and analyzed its performance by using error envelop.It has been proved that when the parameters are properly chosen,the error envelop can be diminished by 97%,which means the multipath error has been almost mitigated.There is a review and conclusion of this thesis,as well as perspective application of this method in the end of this thesis.