Study On Key Technique Of Signal Processing For BDS-3 Based Radio Occultation Receiver

Atmospheric radio occultation receiver based on Global Navigation Satellite System(GNSS)is called as GNSS radio occultation receiver,and it is applied for global neutral atmosphere and ionosphere detection.The GNSS radio occultation receiver not only works well during all weather,but also has high precision,low budget,and longterm stability.Therefore,it has become one of the most advanced equipments in global atmosphere detection field,and been widely used for numerical weather prediction,climate change research,and ionosphere detection.In addition,the key task of the GNSS radio occultation receiver is processing received GNSS signals including refractive and direct signals in real time.Recently,the third-generation Bei Dou Navigation Satellite System(BDS-3)officially started to operate and provide open service for global civil users by broadcasting a series of navigation signals whose carrier frequencies are separately located at several different frequency bands,which implies the BDS-3 has become one of the most important member of the GNSS and an important signal source of GNSS radio occultation receivers.On the one hand,the BDS-3 has paid attention to the compatibility and interoperability with other GNSS systems like GPS,and therefore GNSS signals broadcasted by the BDS-3 are similar with GNSS signals broadcasted by other GNSS systems in terms of modulation modes.Consequently,in terms of GNSS radio occultation receivers,the signal processing techniques designed for the BDS-3 can be easily applied for other GNSS systems,which facilitates the multi-constellation compatible design and then improves the number of occultation events.On the other hand,the BDS-3 broke the monopoly of European and American countries in the field of the GNSS,which protects the interests of Chinese GNSS atmosphere radio occultation receivers in defense,science,and economy from foreign threats.As a consequence,this thesis mainly focuses on the studies on key techniques of signal processing for BDS-3 based atmospheric radio occultation receivers,and its main works and innovations can be summarized as following:1.This thesis first introduces the development history of the GNSS radio occultation technique and the development status of GNSS radio occultation receivers and related signal processing techniques.Then,the development tendancies of GNSS radio occultation receivers and of related signal processing techniques are given.2.With respect to sounding the Earth atmosphere by means of GNSS radio occultation techniques,this thesis separately describes the GNSS,basic principles of GNSS radio occultation techniques,and the system and signal processing flow of the GNSS radio occultation receivers.All descriptions focus on revealing the specific relationship among that three aspects and then indicating the performance of the GNSS radio occultation receiver has a decisive effect on the product quality of the GNSS radio occultation technique.3.This thesis systematically introduces GNSS signal acquisition principles and present acquisition algorithms.Based on short-time coherent integration plus an FFT,an Improved Serial-Parallel Matched Filter(ISPMF)is proposed in this thesis to carry out two-dimension parallel search algorithm.In comparison to the Serial-Parallel Matched Filter(SPMF)and Partial Matched Filter(PMF),the proposed ISPMF has lowest system complexity and simplest implementation,and requires fewest hardware resources.Further,based on the proposed ISPMF and the BDS-3,a low-complexity and multi-constellation compatible acquisition scheme is present in this thesis.This scheme is capable to acquire all GNSS signals having low PRN code rates,which contributes to increasing the number of occultation events and improving the positioning accuracy.4.After thoroughly studying characteristics of the secondary code of modern GNSS signals represented by the BDS-3 B1 C signal,methods for quickly determing secondary code phase,and secondary code acquisition principles and techniques,this thesis proposes a novel method for quickly determing secondary code phase,which is based on the proposed characteristic length vector and sovles the ambiguity problem concerned with the secondary code chip sign.Furthermore,this thesis proposes the Partial Correlation Method(PCM)to acquire the secondary code.The PCM not only consumes less hardware resources and acquires secondary code faster than existing methods,but also has no cost in detection probability,which significantly improves the performance of the GNSS radio occultation receiver in synchronization and weak signal processing.5.This thesis first presents GNSS signal tracking principles and techniques,and then discusses the reason of selecting the specific tracking technique for GNSS radio occultation receivers.Based on the thoroughly studies on the adaptive notch filter and Frequency-Locked Loop(FLL),an adaptive-filter based novel FLL is proposed to tracking GNSS signals.A set of parameters specially designed for the BDS-3 B1 C signal is obtained to carry out experiments so as to verify the effectiveness of the novel FLL.Experiments show that the proposed novel FLL is much better than the conventional two-order FLL usually used for GNSS radio occultation receivers in tracking sensitivity,tracking accuracy,ans convergence rate.Consequently,this novel FLL significantly improve the tracking performance of the GNSS radio occultation receiver.