Study On GNSS Performance Evaluation And Related Auxiliary Technologies In The Space Service Volume

Satellite navigation technology has gradually entered into various fields of human's production and life.On the one hand,it provides a lot of conveniences for our daily activities;on the other hand,it is developed as one of the key technologies in the information age.Recent years,domestic and foreign scholars have carried out a large number of explorations and research on how to expand the fields of GNSS service and how to improve the quality of its service.And using GNSS to improve space service airspace(SSV)on-orbit navigation performance is one of the landmark projects in the field.Based on the discussion issue of "Interoperable Space Service Volume" organized by the International Committee on Global Navigation Satellite Systems(ICG),this paper reveals the existing status and problems of GNSS for SSV application by means of quantitative analysis and qualitative calculation,and puts forward a series of improving measures to meet the requirements of navigation accuracy,signal availability,service continuity and integrity at the largest extent.This thesis mainly studies the GNSS characterization in the space service volume and the corresponding advanced algorithms and auxiliary methods to cope with the deficiencies of using GNSS alone.The primary research contributions and results are presented as follows:(1)The SSV definition,background and development process are decribed in detail.Then the scientific problems in SSV application are classified into three main aspects.Firstly,the signal availability for upper SSV is insufficient;Secondly,the received signal power is weak and probability doped with high dynamic effects;Thirdly,the positioning solution is too poor or even can not be gotten at all.Accordingly,we can obtain three major scientific issues,i.e.,muilti-constellation GNSS interoperability to improve signal availability,weak dynamic signal acquisition and tracking by non-coherent maximum likelihood estimation(MLE),as well as robust orbit determination(OD)with improved GNSS/OP deep integration scheme.(2)The overall SSV characristics and received signal features of different orbit users are evaluated under single GNSS constellation,respectively.The key assessment parameters,or figure of merits(FOM)involves ranging accuracy,received signal power,valid half cone angles of main lobe and side lobes,signal availability,GDOP as well as outage time.The experiments are iterated under multi-constellation condition using sample point scanning method and complete voyage analyzing method,and the results compared with single-GNSS condition illustrate the advantage and necessity of interoperable GNSS in the SSV.(3)Referring to the principle of GNSS receiver tracking loop in signal processing,it is proved that the high dynamic attribute and low power attribute mutually restricts each other in the tracking loop.Conventional loop design is difficult to take both of them into account.To overcome this contradiction,the idea of open loop(OL)is put forward.Open loop structure can prevent the proble of losing lock of closed loop structure in high dynamic and weak signal environment while the aiding of inertial navigation system(INS)can ensure the signal is being tracked continuously.To solve the coupling effect between carrier phase estimates and Doppler frequency estimates,a non-coherent MLE is applied in the open loop.(4)Aiming at providing acceptable positioning accuracy on the basis of high reliability and high integrity,GNSS measurements are combined with orbit propagator(OP)in the proposed design.Referring to the implementation of GNSS/INS integration,GNSS and OP are deeply coupled in rang-domain which is superior to loose integration and tight integration.Besides,the traditional analytic OP or numerical integral OP is modified as a simplified fourth-order Runge-Kutta integrator based on the empirical acceleration model.A most typical SSV user,HEO spacecraft,is selected as the testing object,and the simulation results verified the GNSS/OP deep integration design performs much better.The key technologies and innovations in this thesis focus on the following three points:(1)We provide a systematic architecture for evaluating the service performance of single-constellation GNSS and multi-constellation GNSS,which is composed of the assessment paramters and their calculation methods.Taking the SSV user's requiments into consideration,the conclusions are fed back to system providers to promote coping strategies.The universality of the principle makes the assessment architecture can be extended to other applications of satellite navigation,or even non-satellite navigation related areas such as remote sensing and telecommunication.(2)It is revealed that the traditional standard spaceborne receiver,reference assumption model,is incompetent to track weak signal in high dynamic condition.Therefore,the adaptive Kalman filter(AKF)with adaptive loop bandwidth should be the priority way in this situation.However,in view of the particularity of the SSV application environment,there is an extremely high dynamic and extremely low power coexistence state,which makes the tracking loop using AKF out of lock,such as the lunar probe is taking maneuver above geosynchronous altitude.Under this circumstance,the INS aiding and other external aiding methods have to be utilized to ensure the normal signal acquisition and tracking in OL mode.(3)Comparing the pros and cons of different GNSS/OP integration modes,we find that the non-coherent deep integration is the most suitable mode for SSV application.If we want to save the computational load of the onboard processor and maintain a certain propagation accuracy level,simplifying the dynamic model in OP is necessary.By simplifying the solar/lunar gravity and solar radiation pressure at the same time,the empirical acceleration model is established.Once the empirical acceration model is embedded into the numerical integral OP,the balance between the precision and computational load is achieved.In a typical HEO user orbit analysis,the GNSS/OP deep integration can also significantly reduce the time required for signal acquisition,thereby reducing the time to first fix(TTFF).