Study On Key Techniques Of Distributed Spaceborne SAR Hardware-in-the-loop Simulation

Distributed spaceborne synthetic aperture radar (SAR) is an innovative spaceborne radar instrument based on a combination of the satellite formation technique and the SAR technique. It has more function and higher performance than simple integrated SAR constellations and can effectively generate high-resolution global digital elevation model (DEM) in all weather conditions both by day and by night, which has become the hot subject in all over the world. As the system has a lot of key techniques and complicated system integration and testing problem and close coupling among system error sources, the computer-based simulation verification can not completely meet the integration and testing requirements of the distributed spaceborne SAR. Using the hardware-in-the-loop simulation techniques is an effective means to deal with this problem. This thesis focuses on the hardware-in-the-loop simulation of the distributed spaceborne SAR system. The systematic research is carried out about three key techniques of the hardware-in-the-loop simulation in the signal synchronization error modeling, the hardware-in-the-loop raw signal simulation and the hardware-in-the-loop experiment applications. The research in each chapter is arranged as following:The synchronization method, error model and its influence on InSAR performance of the signal synchronization error of the distributed spaceborne SAR is studied in chapter 2. The influence of the phase synchronization error on InSAR performance is analyzed. The compensated phase model of the two phase synchronization methods by synchronous signal transmission, the direct-path echo method and the pulsed alternate method, is presented. And the characteristics and amount of the residual phase synchronization errors are analyzed comparatively. The phase synchronization processing method in alternating bistatic mode is studied, and an echo-domain phase synchronization processing method using correlation processing is proposed. The influence of the time synchronization error on the interferometric swath loss, the bistatic SAR imaging, and the interferometric height measurement is analyzed. The time synchronization method based on GPS disciplined USO (Ultra Stable Oscillator) is studied. A channel amplitude and phase mismatch model in frequency domain and its interferometric signal mode is presented, and the influence of channel mismatch error on the InSAR phase deviation and variance is analyzed.The accurate and effective digital raw signal simulation method of the distributed spaceborne SAR is studied in chapter 3. An accurate and fast raw signal simulation algorithm is proposed. The raw signal is expressed as the convolution of the transmitting signal and the scene modulation signal, and a interpolation technique is used to maintain the accuracy of the fast approximation algorithm. The scene modulation signal is decimated to the radar sampling frequency for real-time implementation in the simulator. The method has advantages such as wide applicability, high simulation accuracy and high computation efficiency. The raw signal calculated quantity concept is proposed to quantitatively evaluate the fast raw signal simulation algorithm performance and the hardware computing capability. A parallel raw signal simulation HPC (High Performance Computing) method based on the time and space decomposition, and a GPU accelerated raw signal simulation HPC method is studied.The design and implementation of the dual-channel raw signal simulator for the hardware-in-the-loop simulation application of the distributed spaceborne SAR is studied in chapter 4. The raw signal simulator is successfully designed and implemented, including the general design, the radio frequency subsystem, the medium frequency subsystem, the digital signal processing section, and the display and control software. A correlation and windowing method is proposed to accurately extract the simulator amplitude and phase frequency response, which can restrain the influence of the noise and spur in the recorded loop data. A complex FIR filter design and optimization scheme is proposed to design the complex FIR filter coefficients according to the estimated frequency response. A FPGA technique is used to implement the filter in real-time. The measured results of the work mode and the channel amplitude and phase frequency response show that the indexes of simulator satisfy the design requirements.The hardware-in-the-loop experiment and evaluation of the phase and time synchronization and the channel mismatch error of the distributed spaceborne SAR is studied in chapter 5. The computer simulation system architecture and the physical simulation system configuration is presented. An error characteristics extraction method based on data analysis is proposed, and an error isolated evaluation method by different hardware configuration is proposed. The hardware-in-the-loop experiment of phase synchronization error is designed to obtain the influence of the error on the InSAR height measurement accuracy, which validates the feasibility of the pulsed alternate phase synchronization method. The hardware-in-the-loop experiment of time synchronization error is designed to obtain the influence of the error on the InSAR height measurement accuracy, which validates the feasibility of the time synchronization method based on GPS disciplined USO. The hardware-in-the-loop experiment of channel mismatch is designed to obtain the influence of the error on the InSAR height measurement accuracy.