Performance Analysis And Imaging Tecchniques Of Airborne Synthetic Aperture Radar TOPS Mode

The TOPS(Terrain Observation by Progressive Scans)mode is a new working mode proposed to solve the contradiction between the high resolution and wide scene imaging capability of spaceborne synthetic aperture radar(SAR).This mode overcomes the scalloping phenomenon that exists in ScanSAR.When the SAR is working in the TOPS mode,a wide range of azimuth observations is achieved in a short time by the beam's rapid scanning in the azimuth direction;the observation scope in the range direction is further improved by beam switching among the sub-section in the range direction.In addition,this mode can be combined with ground moving target indication(GMTI)and electronic reconnaissance technology.TOPS mode has the advantages of wide imaging range and low probability of intercept,therefore is one of thehot topic in the field of remote sensing.In order to obtain a sufficiently large azimuth imaging scope,the TOPS mode requires a larger scanning angle for airborne SAR,compared with spaceborne SAR,thus new problems arise in system performance and signal processing: as the scanning angle increases,the beam gain decreases,and the echo signal power at the edge of the azimuth scene is attenuated relative to the center of the azimuth scene,resulting in a change in image intensity within the imaging area;in addition,the azimuth amgihuity within the scene also deteriorates as the scanning angle increases.Finally,increasing the scanning angle makes the echo aliasing seriousily in azimuth time domain/frequency domain,which also makes imaging processing more difficult.These problems make the research results and conclusions for spaceborne TOPS mode no more applicable to the airborne TOPS mode.In this paper,based on the airborne digital array SAR,the theoretical research and simulation verification of the TOPS mode in system performance analysis,parameter design,performance optimization method and imaging processing are carried out.Since the beam switching of the TOPS mode in the range direction is similar to the case where the stripmap mode operates in different depress angles,and the research results are relatively mature.Therefore,this paper mainly studies the problems existing in the azimuth beam scanning of the TOPS mode..The research work and achievements of this paper can be summarized as follows:1.The characteristics of the array antenna pattern are analyzed,and the airborne TOPS mode echo signal model is established.The specific work is as follows:(1)By considering the weighting effect of the array element pattern on the array pattern,the variation characteristics of the beam pattern with large azimuth scanning angle are studied.(2)Based on the working principle and imaging geometry of the TOPS mode,the time-frequency characteristics of the echo signals are analyzed.The azimuth time-frequency domain aliasing problem of the echo signals is discussed.(3)For the airborne FMCW-SAR system,the echo signal model of the TOPS mode under Dechirp reception condition is established,and the signal forms in the range Doppler domain and the two-dimensional frequency domain are analyzed.The content in this chapter lays a theoretical foundation for the subsequent study of performance analysis and imaging processing of the airborne TOPS mode.2.The system performance of the airborne TOPS mode operating at a large scanning angle is analyzed.The specific work is as follows:(1)By discussing the beam scanningsequence,the constraint relationship between the performance of the airborne SAR TOPS mode and the system parameters is analyzed and derived.(2)The signal-to-noise ratio model in airborne TOPS mode is established.The influence of beam gain variation on the signal-to-noise ratio at different azimuth locations is discussed.(3)The residual Scalloping phenomenon when working at large scanning angle is discussed,and the influence of beam quantization scanning on this phenomenon is analyzed.(4)A theoretical model describing the azimuth ambiguity of the airborne TOPS mode-Azimuth Ambiguity Signal Ratio(AASR)is established,and the factors affecting the azimuth Ambiguity signal are analyzed.(5)The influence of beam scanning angle on AASR is analyzed.(6)The influence of pulse repetition frequency(PRF)on AASR is analyzed,and the selection of PRF is discussed from the perspective of optimizing AASR.(7)Under the condition that the azimuth aperture size and the number of azimuth elements of the antenna are fixed,the influence of the number of azimuth subarrays on the performance of the airborne TOPS mode is analyzed.3.The radar system parameter and performance optimization for airborne TOPS mode are studied.The specific work is as follows:(1)By analyzing the connection between the airborne TOPS mode radar parameters and system performance,the system parameter design-flow is proposed in combination with an examples.(2)Aiming at the problem that the residual Scalloping phenomenon in the airborne TOPS mode is seriously deteriorated with the increase of the scanning angle,a non-uniform beam scanning scheme is proposed.By adjusting the beam scanning angular rate in the beam scanning process,the deterioration of signal-to-noise ratio at the edge of the azimuth scene is well compensated.(3)At the large scanning angle or when working in the squint TOPS mode,the Doppler bandwidth of the target at the edge of the azimuth scene will decrease as the scan angle increases,resulting in inconsistent azimuth resolution.In response to this phenomenon,another non-uniform beam scanning scheme is proposed.By analyzing the relationship between the target Doppler bandwidth and the scanning angle,the beam scanning rate required at the next moment is calculated at the current scanning angle,thereby reducing the error introduction due to the quantized scanning.The error is compensated and an approximately uniform azimuth resolution is obtained over the entire azimuthal scene.(4)For the system performance deterioration at the edge of the azimuth scene at the large scan angle,a new mode combining the TOPS mode and the SPOT mode—the TOPS-SPOT mode is introduced.The application of the SPOT mode reduces the maximum scanning angle required by the conventional TOPS mode in the beam scanning process,and improves the system performance at the edge of the azimuth scene due to the increase of the scanning angle.Besides,the performance of this mode in terms of signal-to-noise ratio and azimuth ambiguity is analyzed.4.For the airborne array FMCW-SAR system,the problem of full aperture imaging of TOPS mode echo data is studied.The specific work is as follows:(1)Aiming at the Doppler aliasing problem of echo data in TOPS mode,a Doppler aliasing removal method based on azimuth preprocessing is proposed,and the related parameters setup is discussed.(2)For the echo data received by Dechirp,the influence of the intrapulse continuous motion of the SAR platform on the echo data is analyzed,and the focusing process in the range domain of the echo data of the TOPS mode is discussed based on the Frequency Scaling(FS)algorithm.(3)Aiming at the azimuthal time-domain aliasing problem of echo data in TOPS mode,an improved Baseband Azimuth Scaling(BAS)algorithm is proposed to realize non-aliasing focusing of echo data.By adjusting the compensation function in the traditional BAS algorithm,the algorithm can be combined with the azimuth preprocessing method and realize the full aperture processing of the TOPS mode echo data.(4)Verify the proposed methods by the simulated data.This paper studies the TOPS mode of airborne digital array SAR from four aspects: system performance analysis and optimization,parameter design and imaging processing methods.The study in this paper is of great significance for airborne digital array SAR to achieve simultaneous multi-mode imaging.