| Global Positioning System(GPS)has achieved great success in civil and military applications by deploying navigation satellites in Medium Earth Orbits(MEO),since the 1970 s.Subsequently,the navigation satellites of Galileo of Europe,Bei Dou Navigation Satellite System of China,and GLONASS of Russia are also deployed in MEO.With the emergence of some new applications,such as autonomous driving,the requirement for accurate navigation and positioning gradually increases from meter level to centimeter-level.However,the geometry of the traditional MEO navigation systems varies slowly due to the characteristic of the orbit,resulting in a long time of integer ambiguity resolution for Precise Point Positioning(PPP).Usually,ambiguity resolution that can last about 20 or 30 minutes restricts precise applications for the real-time case.By broadcasting the enhancement signals from LEO satellites,the ambiguity resolution time can be rapidly decreased due to the fast movement of the LEO satellites,resulting in the possibility for global real-time precise positioning applications.The introduction of the LEO navigation augmentation system in BDS will greatly improve the positioning accuracy and the integrity of the Bei Dou‐3 navigation satellite system.To ensure the global navigation service of centimeter-level,the compatibilities of the broadcasted signal,the signal landing power,and the service model of the LEO enhancement system with the existing GNSS system must be carefully considered.Further,how to ensure the quality of the signal broadcasted by LEO satellites and the real-time determination of the LEO satellites are also challenging.This paper focuses on the following key technical problems of real-time centimeter-level enhanced service for LEO satellites.(1)Aiming at the core problem of which signal structure should be adopted for LEO navigation augmentation system to meet the requirements of high precise navigation augmentation,user terminal succession,and most importantly the compatibility and interoperability issue with the existing GNSS systems,this paper proposes a signal structure framework for LEO navigation augmentation satellite system(the closer the framework to the existing GNSS systems,the smaller impact on the terminal industry chain).This paper analyzes and evaluates the navigation signal frequency plan,message rate,landing power,and other characteristics of LEO navigation augmentation signal structure,based on the constraints and inter-system interference threshold proposed by ITU.To meet the requirements of message rate and ranging accuracy,as a result,this paper proposes to broadcast navigation signals on L1(1575.42MHz±20MHz)and L5(1176.45MHz±35MHz)bands respectively,with the offset of the center frequency to be either 2.046 MHz or 4.092 MHz,landing power about-153 d BW,the power ratio of pilot and data channels of 1:1.The compatibility performance of the proposed signal structure with BDS-3,GPS,and Galileo is evaluated based on the method of spectral separation coefficient(SSC).The analysis results indicate that the signal framework meets various constraints of ITU resolutions and shows acceptable frequency feasibility,which can be used as a reference for the design of the LEO navigation augmentation system signal structure.(2)By focusing on the impact of the broadcasting channel distortion on the enhanced signal quality,the detailed analyses on the generation of the enhanced signals,indicators of evaluation of signal quality,the analytic relationships between the indicators and the channel distortion,and the corresponding pre-distortion method are presented,and the high precision control of the broadcasting signals is achieved.A multi-dimensional evaluation index system for navigation enhancement signal quality in the time domain,frequency domain,and correlation domain is constructed.A method of evaluating the S-Curve zero-crossing point bias based on deducting the reference bias is presented,and the problem of inaccurate evaluation of the S-Curve bias affected by the data modulation and the multiplex is solved.Accurate results of S-Curve zero-crossing point bias can be obtained from sampled data of milliseconds instead of from sampled data of seconds.By taking the key factors that affect the navigation service as the starting point,the analytic models between these important indicators(correlation loss,phase bias between signal components,S-Curve zero-crossing point bias)and channel distortions are presented,and the mechanism of the impact of modulation error and group delay distortions on these indicators is also revealed.Finally,the nonlinear compensation method is provided for the broadcasted navigation signal to solve the problem of decreasing the effect of group delay pre-distortion due to the nonlinear effect of the power amplifier.These presented analytic models provide the basis for the optimal design of satellite payload and the broadcasting of high quality navigation signal.(3)To ensure the hardware compatibility of the user terminal,the enhanced signal broadcasted from the LEO navigation augmentation system should have the same polarization and a similar frequency as the GNSS navigation signal.These requirements bring serious interference within the bandwidth to the Global Navigation Satellite System(GNSS)receiver.The architecture of the GNSS receiver for the LEO enhanced navigation satellite and the real-time orbit determination technique based on the observations from the GNSS receiver have been studied.The architecture of the GNSS receiver that adopts high isolate transmitting and receiving antennas,adaptive Radio Frequency(RF)interference suppression,and digital method of interference suppression is presented to resolve the problem of receiving GNSS signal with the existence of in-band interference.An effective frame synchronization method for the BDS-3 signal with Non-binary LDPC codes is presented to resolve the frame synchronization under a low signal-to-noise ratio.The performance of the presented GNSS receiver is validated in the Electromagnetic darkroom.The obtained results reveal that the quality of the observations of the GNSS receiver and the decoded performance both meet the requirement of autonomy orbit determination and high precise synchronization,which indicates the breakthrough of precise performance of the GNSS receiver under strong in-band interference.By focusing on the impact of the filter’s parameters on the orbiting performance,a method of orbit determination based on the LEO satellite dynamic model and the observations of the GNSS receiver is presented to improve the real-time and stability of the orbit determination performance. |
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