A Study And Implementation Of Range Image Of A Multimode Circular Trace Scanning SAR System

Traditional linear synthetic aperture radar(SAR),working in only one fixed mode,has limited detection range in azimuth and has inflexible maximum detection range and range resolution.It can not meet users’ needs in some practical applications.While multi-mode circular trace scanning synthetic aperture radar(CTSSAR)has the ability of omnidirectional detection and can work in many independent modes.Each mode has different detection ability.Hence,users can choose the optimal working mode according to the situation where they are in.According to the demands of high resolution navigating radar program,this work is based on CTSSAR.Details of research are presented as follows:(1)The programmable ability of FPGA can implement the circuit of analog-to-digital chip configuration,multi-mode digital down conversion(DDC),multi-mode image in range and recombination of data in range.The multi-mode CTSSAR works in ten modes with frequency modulated continuous wave(FMCW).The analog-to-digital chip is configured to sample first and then ten modes DDC is realized in field programmable gate array(FPGA).The intermediate frequency signal is converted to base-band after DDC processing.Then image in range domain is realized after the process of range image submodule.The data needed to image in azimuth is picked out and recombined before sent to rapid IO submodule.(2)Functional simulations and closed-loop test in circuit board are conducted.The results are correct and access the validity of the image technology in range of multi-mode CTSSAR.The functional simulation is performed by providing a single frequency sine wave as an excitation signal with Modelsim tool.The submodules’s functions can be verified by the simulation results.After that,the analog front end is connected with the circuit board and the close-loop test is conducted.And each submodule’s function and total function of each mode in real noisy background can be verified with the online debugger tool in Quartus software.(3)The simulations and the corresponding solution is presented at last to solve the problem of interference signals which are out of the intermediate frequency band.There are interference signals,including second harmonic,third harmonic and narrow-band sweep signals in the intermediate frequency,outputed by the receiver component of the analog front end.By system simulations in MATLAB and analysis,it is found that second harmonic has no influence on the baseband signal while third harmonic and narrow-band sweep signals have a bad effect on the final baseband signal.In order to solve this problem,a band-pass filter is followed with the receiver component and the value of stop band attention is given according to the system simulations.