Study On Signal Processing Of Moving Target For Long Range Distance Airborne/spaceborne SAR

Synthetic aperture radar(SAR)is of great importance in military applications such as wide-field monitoring and battlefield reconnaissance because of its weather and daylight independence,remote working advantages along with the ability of moving target detection,imaging and locating.However,affected by the movement of a target,coherent accumulation of the echo signal is unachievable expecially in a long during time and remote working pattern.Severe smearing of the moving target due to noncoherent accumulation leads to low signal to clutter and noise ratio(SCNR),which makes the detection and parameter estimation as well as the following imaging and relocation of the moving target difficult.In this paper,the recent progress in research on Synthetic Aperture Radar-Ground Moving Target Indication(SAR-GMTI)is deeply analyzed.Aming at improving the performance of SAR-GMTI in engineering application,the key problems in moving target detection,imaging and relocation under air-borne remote SAR are investigated.In addition,centre on the requirements of the State Key Development Program for Basic Reserch of China,researchs on ships and warships detection by Geosynchronous orbit SAR(GEOSAR)are conducted.The main content of the dissertation is summarized as follows:By enough transmitting power,wide-swath can be obtained by increasing the range distance between SAR platform and the observation scene.In the premise of unchaged platform velocity and beamwidth,the synthetic aperture time is accordingly increased.Long observation time is benificial for enengy accumulation of the moving target,but the coherent accumulation is difficult to achieve due to its movement.Noncoherent accumulation leads to the smearing of the moving target in the SAR image,which inevitably causes the degradation of the detection performance.Aiming at solving this problem,in chapter 2,a new moving target detecting method for long range distance air-borne SAR is proposed.Firstly,an azimuth sliding window is used to accumulate the amplitude of the target signals,then the differences of accumulate amplitude is obtained to detect the range focused targets.In the last,the detected target is refocused for moving target confirmation.The processing results of measured data illustrate the effectiveness of the proposed method.Smearing of a moving target not only brings negative effects to the detection,but also causes inaccurate interferometric phase estimation in multichannal SAR.Interferometric phase is critical for velocity estimation and relocation,even small interferometric phase error will lead to considerable relocation error in the condition of long range distance.Aiming at imaging and relocating the moving target in long range distance SAR,a new method using instantaneous interferometry is proposed in chapter 3.After a moving target is detected and extracted,it is transformed into azimuth-time domain for instantaneous interferometry.In order to raise the accuracy of the instantaneous interferometry phase,an iterative method is implemented.Then the equivalent cross-track velocity(ECV)of the moving target is estimated by the accurate instantaneous interferometry phase and the ambiguity number of cross-track velocity.The equivalent cross-track velocity is utilized to perform precise imaging and relocation.Instantaneous interferometry overcomes the smearing effects of the moving target,the processing results of measured data illustrate the effectiveness of the proposed method.By a significant rise of the orbital altitude in Geosynchronous Earth Orbit SAR(GEOSAR),breakthrough have been made in practical application comparing with that in the low-orbiting satellite.GEOSAR can provid the capability of global coverage and continuous observation,which enable great advantages of GEOSAR in marine observation.For the purpose of ship detection,basic analysis of system paramter design is shown in chapter 3 firstly.Due to the over-long synthetic aperture time of GEOSAR,ship detection in sub-aperture is proposed.By sub-aperture division,not only the complexity of the relative motion between the satellite and the ship are reduced,but also the timeliness of detection is achieved.Besides,according to the radar equation and measured data processing,it is illustrated that proper sub-aperture division has no effects on the signal to noise ratio(SNR)accumulation.Due to the Doppler and position shift induced by cross-track velocity,sub-aperture imaging algorithms implemented in azimuth time and frequency domain are also analyzed,respectively.Even in the sub-aperture time of GEOSAR,the ship is still moving,which leads to the range difference from a stationary ship.SAR imaging algorithms are designed for stationary target,without motion compensations,moving ships will be smearing in the sub-aperture SAR image.The smearing of the ship will cause a SNR loss,consequently degrades the detection performance.However,the SNR loss can be compensated by the system design.In chapter 5,we focus on the study of detection SCR of the ship in GEOSAR,the defocusing effects caused by translation and rotation of the ship is particularly analyzed,then the accordingly SCR loss is calculated.This provide the basis of SCR compensation by system design.