Design Of The Bei Dou Pseudolite System With Time Division Pulse Modulation

With the completion of the BDS-3,the importance of the BeiDou system in various fields has become increasingly prominent,but its limitations are gradually exposed.In the indoor environment,the satellite signal is obscured,and the navigating and positioning cannot be performed.Even in an outdoor environment,it is sometimes affected by the tall topography and building,and the accuracy and continuity of positioning will decrease accordingly.Pseudolite technology,as the mainstream auxiliary positioning technology of the GNSS system,can not only be used for indoor high-precision positioning by independent networking,but also can be used as a ground-based augmentation system of the BeiDou system to seamlessly integrate positioning with the BeiDou system.Therefore,the limitations of the BeiDou system can be make up by the combination of pseudolite technology and the BeiDou system,which comprehensively can improve the accuracy,reliability and continuity of indoor and outdoor positioning.The key problems of pseudolite technology,including time synchronization,network layout,near-far effect and multipath effect,must be considered when using pseudolite technology for positioning.This article focuses on the pseudolite system transmitter and its key technologies.Based on the existing theory,a set of BeiDou pseudolite system that can achieve time synchronization,has the ability to resist near-far effects,and has a reasonable layout is developed.Firstly,referring to the technical system of the BeiDou satellite and according to the requirements of the pseudolite technology,the system structure and working mechanism of BeiDou pseudolite are determined,and a suitable navigation message is designed for the pseudolite.According to the characteristics of the C/A code,the C/A code suitable for the pseudolite is selected.Thereby the design of the pseudolite signal is completed,and the working parameters and technical indicators of the BeiDou pseudolite are determined.The near-far effect,time synchronization,and network layout of pseudolites are analyzed to provide technical support for system design.Secondly,aiming at the near-far effect of BeiDou pseudolites,a solution of TDMA pulse modulation is proposed.The duty cycle and pulse pattern of the pulsing scheme are designed,and the solution is simulated and verified.The clock module of the pseudolite is designed in detail,and the 1PPS of the timing receiver is used to tame the crystal oscillator,which solves the problem of the accuracy of the pseudolite clock.The reproduced 1PPS and the signal of spread spectrum starting are used to control the synchronous starting between the pseudolites,so as to realize the time synchronization of the pseudolite system.Based on the precision factor,a simulation analysis of the pseudolite network layout is carried out to provide a basis for optimizing the pseudolite system layout in practical applications.And then,according to the related theory of pseudolites and the requirements of the overall technical plan,using software radio design ideas and modular design methods,the pseudolite transmitter is divided into three units,which includes the baseband signal processor,the baseband controller and the RF Front-end.The software and hardware of each module are designed and debugged in detail.And the design of BeiDou pseudolite transmitter is finally completed.Finally,a test environment is seted up.Firstly the various functions of the pseudolite transmitter's baseband board and RF front-end are tested.Secondly,the reception test of the entire transmitter is conduct.Finally,test the ability of the BeiDou pseudolite to resist the near-to-far effect,which include signal propagation Attenuation experiment,pulse modulation weakening strong power signal experiment,compatibility experiment between pseudolites and between pseudolites and satellites.The test results show that the BeiDou pseudolite transmitter meets the relevant design requirements.