Research On The Integrity Monitoring Of GPS Local Area Augmentation System

In order to make GPS support aircraft precision approach and landing of civil aviation as the main means,the system needed a further improvement in positioning accuracy,continuity,integrity and availability.In the nineties of twentieth century,the local area augmentation system which using carrier smooth and difference positioning technology was put forward,It was so greatly enhanced the accuracy of GPS that addressing the related fields scholars and civil aviation bodies extensive attention.Practice had proved that LAAS could easily meet the performance requirements of category I precision approach,but it needed a further improvement on the system structure and function for CATâ…¡/â…¢.All of the performance indicators required in precision approach,the most difficult to meet was the integrity.The main purpose of integrity monitoring was to guarantee the system operating at the normal state and improve the credibility of navigation data.The means of integrity monitoring included LAAS ground facility integrity monitoring and receiver autonomous integrity monitoring.The research purpose of this paper was about the integrity problem of GPS local area augmentation system,and the main researches were as follows:1.This thesis researched on the detailed methods of standard deviation inflation factor determination when the true distribution of LGF Broadcasting correction error was non-zero mean Gaussian or non-Gaussian.Under the non-zero mean Gaussian distribution,inflation factor of standard deviation and mean were given by vector norm inequality.Under the non-Gaussian distribution, Gaussian mixture model was used to fit the probability density of LGF correction error,which would be determining the inflation factor under non-Gaussian model into Gaussian mixture model.Then EM algorithm estimated Gaussian mixture model parameters,BIC and SEM algorithm modelled the structure of selection and optimization.Simulation result was proved that inflator coefficient determinated in this way could meet requirement,and estabilished the calculation formula of Sigma inflation factor.2.The total inflation factor of LGF broadcasting standard deviation was determinating with the limited sampling restriction.Vertical protection level was calculated with pseudolites augmentation,the relative accurate model of standard deviation and standard deviation inflation,and got the following conclusions:If standard deviation was not inflated,regardless of with the augmentation of pseudolites,GPS could meet the performance requirements of CATâ… /â…¡/â…¢,but it was difficult to guarantee that positioning error was less than protection level.If standard deviation was inflated,regardless of with the augmentation of pseudolites,GPS only met the performance requirements of CAT I,but the availability was significantly improved with pseudolites.If VPL was calculated with the standard deviation relative accurate model and the augmentation of pseudolites,the system can meet the availability requirements of CATâ…¡,but it could only be used for theoretical analysis.3.Standard deviation and mean monitoring method was carried out detailed research.Because CUSUM scheme could not detect a larger standard deviation and mean displacement within a step,Combined Shewhart-CUSUM scheme was put forward to solve those defects.The detailed performance of CUSUM, Shewhart and Shewhart-CUSUM was analysed as the indicator of the average run length,Result was shown that the detection threshold of combination scheme was only slightly larger than that of CUSUM,we could get the same performance for checking cumulative anomaly.Finally,the performance of standard deviation and mean monitoring was anslysed by simulation test,which showed that the combinated scheme could detect the larger anomaly within a step,and had the same performance with CUSUM for detecting cumulative anomaly.4.The "snapshot" RAIM algorithm of parity space was restricted by the mathematical model of measurement equation,when the measurement error was non-zero mean Gaussian distribution or had an unknown disturbance input,the false alarm rate and misleading alarm rate of fault detection were increased.The measurement equation was improved by inputing the uncertainty model,which added the robustness and anti-jamming capability of the parity space algorithm. The uncertainty input was estimated to turn into certainty input,and the double optimal Decoupling method was used to separate between uncertainty input and fault vector.Finally,simulation was proved that the improved RAIM algorithm could reduce false alarm rate and misleading alarm rate of satellite fault detection.5.The multivariable cumulative RAIM(MCRAIM) was presented in this paper,the multivariable cumulative sum algorithm was adopted to handle parity residual error,which solved the defect of "snapshot" RAIM not detecting cumulative fault within a step.In allusion to the special application case of GPS, it respectively detected cumulative anomaly of system and cumulative anomaly of satellite.Simulation result was shown that MCRAIM algorithm had a unique advantages and performance for GPS fault detection.A further improvement for MCRAIM algorithm formed combined RAIM-MCRAIM algorithm,it not only had capability of "snapshot" RAIM detecting larger fault within one step,but also could detect cumulative fault.Finally the characteristics of combined algorithm were proved by the way of simulation.