Multipath Mitigation Based On Strobe Pulse Design For Modern Global Navigation Satellite Systems

The Global Navigation Satellite System(GNSS)with high precision has the characters of wide covering area and all-weather.In recent years,the use of GNSS is becoming increasingly wider.The US mordenlized GPS,Eurpean Galileo system and Chinese Compass system has been developed.The modern GNSSs adopt BOC modulated signals,which significantly increase the usage of the spectrum band.On the other hand,the autocorrelations of BOC signals are multi-peaked and suffer from tracking ambiguity.As the development of modern GNSS,most of the errors,such as ionospheric error,clock error and orbital error,have been eliminated by modeling and differing.Multipath has become the dominant error in modern GNSS.This thesis studys the multipath methods based on the concept of strobe techniquein for BPSK and BOC modulated signals,including the contributions as follows:i)The conventional strobes are affected by short-delay multipath.An enhanced multipath mitigation s-curve based on the linearization of the optimal multipath s-curve is proposed,and the corresponding strobe is designed.The proposed strobe has a higher discriminator gain while the pull-in region is unchanged.The corresponding multipath mitigation strobe has better multipath performance and lower code tracking error in the presence of thermal noise compared with conventional strobes.When the front-end bandwidth is 10.23 MHz,the proposed strobe exhibits better multipath performance than conventional strobes by approximately 40.93% at the cost of doubling the sampling rate and the quantization bits.ii)The conventional strobes have false lock points in the s-curves and suffer from tracking ambiguity in BOC(1,1)signal.This thesis proposed a novel bipolar unambiguous strobe.An extra element rectangular pulse is added to the conventional W4 strobe,and the s-curve has no false lock point.The proposed strobe is a trade-off between unambiguous tracking,multipath mitigation,tracking precision in the presence of thermal noise and hardware complexity.The strobe is bipolar and can be realized easily using 1 bit quantization.The assistance of bump-jumping mechanism is not needed any more,which decreases the hardware and computional complexity.iii)The existing receiving methods for high-ordered BOC signals cannot effectively eliminate the medium-and long-delay multipath.This thesis proposed novel multipath mitigation methods which are the combination of strobe technique and Double Side Band(DSB)technique and the Double Estimator Technique(DET),respectively.The strobe technique is applied to the DET to replace the narrow correlator in the code delay lock loop,and an improvement of multipath performance by 39.90% is obtained.Three kinds of unambiguous strobes are designed according to the characteristics of high-ordered BOC and BOCc signals.The prompt component is obtained by the introduction of DSB technique,and the non-coherent s-curves are shaped,which can efficiently eliminate the medium-and long-delay multipath.Compared with conventional methods,the proposed scheme is less sensitive to front-end bandwidth,and performs satisfactorily under narrow front-end bandwidth conditions.iv)The existing MBOC multipath mitigation methods do not use the BOC(6,1)component and the tracking performance is not fully exploited.This thesis proposed double-strobe based unambiguous methods for multipath mitigation for TMBOC and CBOC signals,respectively.The time-multiplexed double strobe technique for TMBOC signals generates two kinds of strobes to track the codes modulated by BOC(1,1)and BOC(6,1)subcarriers,respectively.In the joint double strobe technique for CBOC signal,the incoming signals are correlated with replica BOC(1,1)waveform and two kinds of strobes.The resulting correlation outputs are linearly combined,and the shaped s-curves are unambiguous and unbiased.The s-curves obtained by the two proposed methods have greater discriminator gain than conventional strobe techniques.The proposed techniques fully use the BOC(6,1)component,and exploit the potential of MBOC signal compared with pure BOC(1,1)signal.The tracking precision in the presence of thermal noise and multipath is increased.When the gate width of the element pulse is 1/8 chip and the front-end bandwidth is 20.46 MHz,the proposed methods exhibit better than conventional strobe techniques by 9.64% and 26.21% for TMBOC and CBOC signals,respectively.Finally,convlusions are drawn in Chapter VI.