Research On Ionosphere Scintillations Based On Space-based GNSS Occultation Sounding Technology

Ionospheric scintillation can cause the cycle slip and even lock-lose of the radio signal,which has a serious impact on the navigation positioning,satellite communication and regional communication of Global Navigation Satellite System(GNSS).Based on the rapid development of radar and Beidou navigation system and satellite(BDS)and the current situation of northern hump region of the magnetic equatorial anomaly in southern China,the timely and effective monitoring and early warning of ionospheric scintillation in this area is extremely important for national defense construction,South China Sea fishery,maritime shipping and other activities.Therefore,it is necessary to vigorously develop ionospheric scintillation monitoring technology for southern China and even the world,and promote space weather researcesh such as early warning and forecasting of the ionospheric scintillation.So far,there have been a large number of ionospheric scintillation detection methods in the world.Among them,the space-based GNSS occultation sounder(GNOS)detection technique has the advantages of global coverage,high vertical resolution,high precision,all-weather and long-term stability,thus it is one of the most important approaches for probing ionospheric scintillation at present,and it should be the focus of China's development in scintillation research.The GNOS payload developed by National Space Science Center(NSSC)onboard Fengyun-3C(FY3C)satellite obtained a large number of Global Positioning System(GPS)ionospheric scintillation data through occultation observation,which provides vital support for space weather research such as ionospheric scintillation monitoring and forcasting as well as the model of ionospheric scintillation occurrence probability prediction.Based on FY3C-GNOS,NSSC developed a new generation of GNOS mounted on Fengyun-3D(FY3D)satellite in 2017,which realized the scintillation observation compatible with both GPS and BDS and greatly increased the number of ionospheric scintillation indexes.In this work,the detection principle and research status of ionospheric scintillation are firstly introduced in detail,then the hardware design and implementation,ground simulation system design,ionospheric scintillation product precision verification and scintillation product applications are discussed focusing on FY3C/FY3D-GNOS payload,at last an outlook was made to the application prospects and development potential of scintillation products of FY3C/FY3D-GNOS.The main work carried out in this work is as follows:1.In Chapter 1,the dielectric properties of the ionosphere and the characterization method of the electron density irregularities are systematically introduced firstly,and the ionospheric scintillation effects on satellite communication,satellite navigation,radar and other systems are presented secondly.Then this work reviews the ionospheric scintillation detection methods at home and abroad,focusing on the FY3C/FY3D-GNOS payload,the technical characteristics and advantages of global coverage,high precision,all-weather and long-term stability of the space-based GNSS occultation detection method are analyzed.2.In Chapter 2,the hardware function of GNOS payload is realized by designing the control solution DSP and baseband processing FPGA.This work innovatively adopts the capture method based on matching filter and FFT frequency search and the variable bandwidth loop method,solving the difficulties of fast and accurate capture of GNSS signals and high-quality data output in short-term occultation events and a series of problems caused by high dynamic effects on satellites,which is the technical highlights of the payload design.3.In Chapter 3,we independently designed the ground simulation system of ionospheric scintillation based on Cornell model to test the function and performance of FY3C/FY3D-GNOS payload.The simulation software can flexibly configure the scintillation time,data update period and model parameters to obtain the GNSS signal amplitude fading and phase fluctuation sequence under the influence of scintillation.The L1 and L2 signals of GPS and B1 and B2 signals of BDS affected by ionospheric scintillation are generated by the Spirent dual-frequency GPS signal simulator and received and processed by the GNOS receiver.The results show that the ionospheric scintillation detection accuracy of FY3C/FY3D-GNOS meets the expected requirements.4.In Chapter 4,for the verification of the detection accuracy of FY3C-GNOS ionospheric scintillation in orbit,the maximum scintillation index S4 max obtained during in-orbit operation of FY3C-GNOS and the scintillation indexes observed by the space-based COSMIC occultation project were matched and analyzed for error by the cross-validation method.In the matching principle,except the conventional spacetime matching condtiion,the directionality of the occultation ray is also included in the matching criteria for the first time.Analysis shows that the average error and standard deviation(std)of the numerical differences of S4 max between FY3C-GNOS and COSMIC are less than 0.01 and 0.1,respectively.The magnitude of the aboved statistical error verifies the reliability and accuracy of the FY3C-GNOS ionospheric scintillation products.It also proves the unbiasedness,long-term consistency and stability between different ionospheric GNSS occultation projects,which lays the foundation for comprehensive and long-term application analysis of ionospheric occultation data.5.In Chapter 5,we can see that the space-based ionospheric occultation detection technology can observe changes of the global ionosphere and is capable of probing ionospheric effects caused by all kinds of phenomena.Based on the specailty of this technology in ionosphere,the ionospheric scintillation index S4 observed during the orbital period of FY3C-GNOS is applied to the event-specific magnetic storm for the first time to explore the feasibility for its application in magnetic storm research.The results show that the ionospheric scintillation data of FY3C-GNOS can effectively reflect the evolutionary features of one magnetic storm,including its initial phase,main phase and recovery phase,which is consistent with the existing scintillation observation results in previous researches,indicating the great potential of scintillation products of FY3C/FY3 D GNOS in studying various ionospheric phenomena.