Study On The Threat Models For BOC Signal Distortions And Ionosphere Dispersion Effects

The quality of Global Navigation Satellite System?GNSS?signal-in space?SIS?can be easily affected by various unpredictable anomalies occurring in the process of signal generation,emission,transmission and reception,thus decreasing the GNSS service performance for positioning,navigation and timing?PNT?.GNSS users can be most easily influenced by evil waveforms of navigation signals,since evil waveforms can result in translation or deformation of correlation peaks,and therefore lead to the poor ranging accuracy.The International Civil Aviation Organization?ICAO?adopted the 2^nd-order step?2OS?model for description of evil waveforms of binary phase shift keying?BPSK?signals.With the modernization of GPS and the advent of new GNSS such as Galileo and BDS,new signal waveforms such as Binary offset carrier?BOC?,multiplexed binary offset carrier?MBOC?and alternative binary offset carrier?AltBOC?modulations have been introduced to improve the GNSS service performance.The issues about characterization and tracking of BOC signals have been discussed a lot.However,BOC signal distortions are seldom referred to.On the other hand,the new generation GNSS signals occupy the wider frequency band than the bandwidth of the GPS C/A code.Because the ionosphere is dispersive,different frequency components in the wideband spectrum suffer different delays as they traverse the upper atmosphere.The simplification that each incoming signal is a single frequency tone represented by the center frequency is ineffective for wideband signals.Therefore,the ionosphere dispersion effects should no longer be neglected in the wideband signal case.In this thesis,some issues about evil waveforms and ionosphere dispersion effects on wideband GNSS signals are referred to.It is meaningful to establish threat models for simulation of anomalous waveforms,establishment of anomaly signal model database and signal quality monitor?SQM?.The work can be summarized as follows.?1?A 2OS evil waveform threat model for BOC signals is proposed.The evil waveforms of BOC signals are first defined in time domain.Starting from power spectral density?PSD?of lead/lag signals in the digital failure,the frequency domain mathematical expression of digital deformation signals is then derived.Secondly,the linear relationship between the correlation function of ideal signals and that of digital distortion signals is clarified.On this basis,the analytical expression of the correlation function of digital distortion signals can be derived.Finally,the mathematical models in frequency and correlation domain for the analog failure and combination of digital and analog failure modes can be obtained according to the frequency response function of the 2^nd-order filter.?2?Starting from the 2OS threat model,the analytical expression of the signal-to-noise ratio?SNR?and code tracking error for distorted signals are deduced in the coherent and non coherent processing case,clarifying the relation between the two metrics and the deformation parameters of the 2OS threat model.It is shown from theoretical analysis that the SNR is affected by both the receiver precorrelation bandwidth and distortion parameters,and the code tracking error is influenced by the correlation spacing besides the receiver precorrelation bandwidth and distortion parameters.The simulation experiments are performed to evaluate the effects of evil waveforms on the SNR and code tracking accuracy.The results indicate that evil waveforms in the digital failure can result in SRN attenuation,and SRN may be attenuated or enhanced by the digital failure and combination of digital and analog failure modes,relying on deformation parameters.The SNR cannot therefore be used as a metric for assessing the evil waveforms effects,and neither be an indictor to monitor signal integrity.In addition,the tracking error induced by the digital failure can be ignored.But the analog failure and combination of digital and analog failure modes can lead to the tracking error varying with the distortion and receiver parameters.?3?A general double sideband?DSB?model is proposed for BOC signals to take into consideration all frequency elements of wideband GNSS signals rater than treating them as a single tone.The simulation results demonstrate that the ionosphere delay varies with the band.In other words,upper sideband?USB?signals and lower sideband?LSB?signals suffer different ionosphere delays.The ionosphere dispersion effects on wideband GNSS signals should thus no longer be neglected as in the narrowband signal case.The DSB model for BOC signals is useful in helping us receive BOC signals and simulate ionosphere dispersion effects.?4?An analytical expression is derived from the nonlinear ionosphere phase model for analyzing the ionosphere dispersion effects on the ranging of wideband GNSS signals.Theoretical analysis shows that the ionosphere dispersion within wide band results in complex correlation results which shift the phase and cause correlation power loss but does not lead to an additional delay in the estimated code delay.The wideband GNSS signals such as BOC?15,2.5?and AltBOC?15,10?are taken as an example to quantitatively evaluate the effects of ionosphere dispersion on the phase shift and correlation power loss.The results demonstrate that the phase shift and correlation power loss resulting from ionosphere dispersion become lager as the total electron content?TEC?and signal bandwidth increase.For AltBOC?15,10?signals,the phase shift and correlation power loss caused by ionosphere dispersion are about7^O and 0.02dB in the ionosphere quiet period,respectively,and in the ionosphere activity period,the phase shift and correlation power loss can reach 32^O and 0.35dB,respectively.