The Key Techniques Research In Ionospheric Oblique And Oblique Backscattering Sounding System

With the rapid development in technologies of electronic, digital signal processing, radar and modern network communication, the ionospheric sounding has been changed from analog instruments to digital instruments, from sounding with one instrument at one single station to networking sounding with a lot of instruments in large area. Furthermore, the automated sounding has been improved greatly and the obtained ionospheric parameters have been more rich and accurate. In addition, the application of network communication has made the ionospheric sounding data can be transferred and shared at a long distance. Miniaturization, digitization, automation and network have been the inevitable trend for ionospheric sounding technique’s development.The Wuhan Ionospheric Oblique Backscattering Sounding System (WIOBSS) developed by the ionosphere lab of Wuhan University, adopted with the techniques of pseudo-random code, pulse compression, coherent spectral integration, and can achieve the ionospheric backscattering sounding at the range of 1200km with transmitting power of 200～500W. Also, it can achieve ionospheric vertical sounding with only tens of watt transmitting power. Moreover, the Doppler with high resolution can be attained along with 3-D ionogram (time delay versus operating frequency with color-coded echo amplitudes) in WIOBSS. On the general architecture design, according to the thought of software radar, high performance components, such as digital signal processor, Field Programmable Gate Array (FPGA) were used to ensure the openness, generality, functional extensibility, and improve the operational reliability in WIOBSS. Moreover, it was intended to be designed as a vehicular sounding system with the character of small-size, light, low-power and multi-function.To these drawbacks existed in the previous WIOBSS, the advanced techniques of software radio, Pci eXtensions for Instrumentation (PXI) bus, modeling and simulation with Advanced Design System (ADS) were applied to the new generation of WIOBSS and made the WIOBSS be designed as a vehicular ionospheric sounding system with high flexibility, openness and easy-upgrade ability in this paper. Furthermore, the methods, such as time and frequency synchronization based on Global Positioning system (GPS), were used to design and develop a set of ionospheric oblique sounding system based on WIOBSS, and to increase the sounding functions of WIOBSS. The main contribution of my work is as following:1. A digital transmitting channel based on software radio was designed and implemented for a new generation of WIOBSS. In this transmitting channel, high performance FPGA and digital upconverter were used to develop an all-purposed hardware platform. Moreover, the analog parts were decreased as large extent as possible and all functionalities were implemented with programmable software. With this transmitting channel, it was flexible to change the coding manner, modulating manner and sounding system to achieve various sounding modes just through loading appropriate software in WIOBSS at practical sounding.2. A new set of ionospheric oblique sounding system, which named Wuhan Radio Ionospheric Oblique Sounding System (WRIOSS), was designed and implemented. Based on the sounding system of WIOBSS, several technologies, such as the time and frequency synchronization based on GPS, were added to achieve ionospheric oblique sounding in WRIOSS. Compared with traditional ionospheric oblique sounding system, the oblique backscattering sounding can be achieved in WRIOSS at the time of oblique sounding. With this design, the advantage of multifunction can be acquired in WRIOSS. Furthermore, the information of Doppler with high resolution in oblique sounding can be got with WRIOSS in time. Therefore, it provided conditions for studying the change of ionosphere along with sounding path with Doppler. In addition, transmission and reception with each other can be realized between WRIOSS’s transmitting site and receiving site to study the features of high frequency channel3. A new kind of time and frequency synchronization method based on GPS was introduced and implemented to meet the requirements of WRIOSS. In this design, the pulse per second signal from GPS receiver was used to discipline the time and frequency reference from a high performance oven controlled crystal oscillator. On one hand, the calibrated frequency of 10MHz with high stability and accuracy was used as the reference frequency for the transmitting site and receiving site to achieve frequency synchronization in WRIOSS. On another hand, the calibrated pulse per second signal with high precision was attained to use as the signal of time synchronization for WRIOSS. With it, receiving site and transmitting site can be accurately controlled to work at the scheduled time by single chip microcomputer in WRIOSS. Compared with traditional method for time and frequency synchronization, the timing precision at the level of ns and the frequency precision at the level of 1E-12 can be attained with this method. Furthermore, whether the GPS signal was received or lost, it can all provide stable signal of time and frequency synchronization for WRIOSS, and make WRIOSS can be operated at any time.4. A set of PXI interface was designed and implemented for the new generation of WIOBSS. In this design, FPGA and special interface ship PCI9054 were adopted to design the hardware platform. Furthermore, two kinds of data transmission modes, such as peripheral component interconnect target mode and block direct memory access mode, were realized to meet the requirements for communication between different modules and computer.In addition, Driverstudio was used to develop the interface driver to achieve the plug-and-play function of various modules in WIOBSS. The experimental results demonstrated that with this PXI interface, WIOBSS’s dimension was 177.8×431.8×457.2mm except for the antenna systems and power amplifier, and it was suitable for using in vehicle. In addition, the swept-frequency ionogram and Dopplerionogram including the information of ionospheric oblique backscattering sounding and vertical sounding can be clearly obtained in the range of 2200km with WIOBSS in practical sounding.5. A new kind of method for WIOBSS’s hardware platform modeling and simulation with ADS was introduced. Moreover, this method was applied in the modeling and simulation of WIOBSS’s transmitting channel. Compared with other simulation methods, the simulation results of this method can be more similar to the practical test results. Therefore, it can be used to support for the practical hardware design in WIOBSS.