Research On Key Techniques Of Detection And Weakening Of Ionospheric Scintillation In Satellite Navigation System

The signal delay during propagation in ionosphere and abnormal changes caused by scintillation have been one of the most significant errors which have negative effects on satellite navigation system as well as affecting the continuity and robustness of the system. Estimation and calibration of measuring errors caused by ionospheric scintillation and mitigating its effects have been an important issue in constructing a satellite navigation system. Therefore research on the navigation signals receiving techniques taking consideration of ionospheric background is of great strategic and practical importance. The main topics and results of this thesis are as follows:(1) An algorithm based on BOC modulation double sideband signal model for ionospheric delay estimation, in conjunction with the characteristics of the signals of the next generation GNSS adopt modulation of BOC and spectrum splitting, is proposed to solve the problem that traditional algorithm for estimation of ionospheric delay from the multi-frequency measurements cannot be applied to single frequency receivers. The expressions of the code phase delay and the carrier phase advanced value on nominal frequencies of lower side band and upper side band and the method for the ionospheric delays and its variation on the estimation of carrier frequency f0 are derived by Fourier series, and the accuracy of estimation is improved by smoothing algorithm. Simulation results show that the proposed algorithm can improve the accuracy for the estimation of ionospheric delay. For instance, compared with the single-frequency Klobuchar model, the optimal accuracy of estimation for the proposed algorithm is relatively higher(1.26 m, 0.90 m, 0.84 m respectively for BOC(10,5), BOC(14,2) and BOC(15,2.5)), which enables the receivers of next generation GNSS to accurately estimate the ionospheric delay.(2) An ionospheric scintillation detection algorithm based on code loop NCO adjustments is proposed to solve the needs of sufficient input data and the poor real-time ability of traditional algorithms. The code loop NCO adjustments are able to exceed the normal distribution and the fluctuations similar to ionospheric scintillations appear when signal power fades rapidly due to ionospheric scintillations. Based on this character the proposed algorithm can estimate whether ionospheric scintillation occurs and estimate the level of ionospheric scintillation coarsely by the changes of code loop NCO adjustments. Simulation results show that, the sampling period of proposed algorithm is milliseconds, and multiple times detection can be achieved in a short period of time. The detection rate is improved significantly, compared with the single detection, which is 4 times in weak scintillation scenarios, and 8~10 times in intensitive scintillation scenarios; the code loop NCO adjustments as the judgment variable, the new algorithm can achieve accurate detection for ionospheric scintillation within 10 seconds. Compared with the conventional subsequent statistical analysis, the real-time ability of proposed algorithm for ionospheric scintillation detection can be enhanced distinctvely.(3) Ionospheric scintillation may lead to fast fading of the satellite navigation signals and can even result in a complete loss of lock on a satellite, which has negative effects on the continuity and stability of systems and equipment such as monitoring receivers. A array signal processing based algorithm to diminish the effects of ionospheric scintillation is proposed in this thesis for the first time. Sensor arrays can steer the beam of the system to a certain direction with high gain and improve the carrier-to-noise ratio of the satellite signals in this direction, thus reducing the negative effects of ionospheric scintillation on receivers. The ionospheric scintillation is highly correlated in a short time(in milliseconds) while the propagation delays between antennas are always less than a nanosecond, so the array signal processing is considered to be segmented stationary. In this thesis, an array signal processing model that includes the effects of ionospheric scintillation is illustrated and further analysis reveals that under the condition of no interference, the weights calculated according to maximum signal-to-noise ratio principle are the same with that of a spatial matched filter and the signal-to-noise ratio gain is proportional to the number of sensors. What’s more, an availability based evaluation method for lessening the effects of ionospheric scintillation is proposed and an availability indicator for ionospheric scintillation is defined. Simulation analysis based on the data acquired by a signal antenna software receiver and a receiver with antenna array respectively has demonstrated that: the carrier-to-noise ratio using a 4-elements array and 7-elements array is 6d B higher and 8d B higher respectively than that of a receiver using a signal antenna, illustrating that the adoption of an antenna array can improve the signal-to-noise ratio in presence of ionospheric scintillation, thus mitigating the effects of ionospheric scintillation on receivers; the carrier-to-noise ratio and availability are slighted affected in presence of little ionospheric scintillation(scintillation parameter S4=0.2) when the carrier-to-noise ratio threshold is 33 d BHz but decrease significantly in presence of distinct ionospheric scintillation(scintillation parameter S4=1.2), however, the application of beamforming technique of antenna arrays can improve the carrier-to-noise ratio distinctively with an availability increase of 39%, verifying the improvements on mitigating the effects of ionospheric scintillation by using antenna arrays.(4) An improved beam tracking algorithm which is based on spatial correlation before code stripping is proposed to solve the low precision problem by conventional direction of arrival estimation of satellite signals according to ephemeris parameters. The resources needed for code stripping using existing tracking algorithms which are based on spatial correlation after code stripping are proportional to the number of sensors. And in this thesis, a new algorithm setting the spatial correlation before code stripping is proposed, using which only 3 code stripping modules are needed for any number of the antennas and significant improvement is made especially when the number of antennas is large. Under the condition that the relative direction of arrival of satellite signals changes slowly and both code loop and carrier loop are tracked stably, the algorithm proposed is equivalent to beam tracking algorithms after correlation, which is proved in the thesis. Further simulation analysis indicates that, the new algorithm can achieve a fast and accurate convergence of the beam, get a precise estimation of direction of arrival of satellite signals and increase the carrier-to-noise ratio by 6d B, thus tracking the direction of arrival of satellite signals stably and mitigating the influence of ionospheric scintillation effectively. Moreover, the technique can be extended to navigation applications with weak signals.(5) An evaluation algorithm based on space-time theory for ionospheric scintillation is proposed to give quantitative analysis and statistical methods for analyzing the effects of ionospheric scintillation on GDOP and system availability. The criteria that is based on GDOP deterioration factor, level of availability, deterioration function of scintillation, etc., is studied to evaluate the effects of ionospheric scintillation on GDOP and system availability. In comparison with analyzing the absolute value of GDOP, the evaluation algorithm based on GDOP deterioration factor is more likely to reflect the changes before and after ionospheric scintillation. According to the availability evaluation based on deterioration function of scintillation and space-time theory, it is indicated that regional effects of ionospheric scintillation always concentrate on the projected area of the center of scintillation. The system availability is affected by 10-2~10-4, which has limited effect on satellite navigation system.The main results in this thesis have been applied to projects such as the system design argumentation and development of ground equipment.