Study Of Parameter Estimation And Imaging For ISAR Targets

Microwave imaging radar has an ability that can provide a high resolution radar image in all-weather, day/night and long range condition. Because the imaging radar can be competent for some missions that are difficult for some general surveillance systems, such as optics and infrared systems, it is being more and more widely used in military and civil fields. With the increased exploitation and utilization of ocean and space resource, the surveillance and safety protection of ocean and space resource will become more and more important. So, the detection and ISAR imaging of moving targets will play an important role in inspecting and identifying targets and have an important military and scientific research value.An emphasis will be put on the research of the detection and ISAR imaging of sea or space targets in the paper, which proposes a method for detecting wideband signal based on Crosscorrelation Function, builds the echo signal model of complexly moving targets, and proposes two methods based on the scale transform in time-frequency distribution plane (STTFD) and the time-chirp Clean (TC-DechirpClean) for targets ISAR imaging. Finally, a shown method is used for achieving ship imaging in airborne radar system. The main work of this dissertation is as follows:[1] Chapter 1 is the introduction. It reviews targets detection and ISAR development, then introduces the dissertation's research background and main work.[2] Chapter 2 overviews conventional ISAR imaging principle and various technological methods used for ISAR imaging.[3] Chapter 3 studies a targets detection technology in wideband signal. The proposed parameter estimation algorithm is composed of the following steps: Firstly, do adjacent cross correlation between received signals to eliminate target's translational motion effect, and reduce the range cell migration; then apply Keystone transform to eliminate the couple effect of range and cross range; next detect moving targets with coherent accumulation of correlation function after an scale transformation to each range cell data. The acceleration and the change of acceleration in range direction of targets can be estimated based on the peak position of the processed adjacent correlation function of echo signals. The velocity of target is estimated based on the peak position of the cross range accumulated signal after the high order phase term having been compensated, which is calculated based on the estimation acceleration.[4] Chapter 4 proposes an ISAR imaging algorithm based on linearly modulated frequency stepped (LMFS) signal. The method makes full use of the information of range envelope and azimuth Doppler to estimate the radial velocity and acceleration and makes compensation for motion parameters in the raw data. At the same time, this method was also applied to the imaging of ISAR in low signal to noise rate (SNR). At the end, the detailed imaging steps of LMFS ISAR are put forward and the simulated results with different SNR confirms this method.[5] Chapter 5 presents a new method based on a scale transform in time-frequency distribution plane (STTFD). After range envelop alignment, the data of each range cell is transmitted into the new ones in a time-frequency domain. A scale transform is applied to the time-frequency domain data to remove the coupling effect between signal time and the correlation function delay. After the above transforms, the bias distribution in the time-frequency plane of each scatterer signal result of the cross-range quadratic phase term is changed to a beeline distribution parallel to the time axis. Therefore, a high resolution ISAR image for a maneuvering moving target is obtained by the Fourier transform to the processed data. The proposed ISAR imaging algorithm is verified by simulation and raw radar data results.[6] Chapter 5 studies a new method based on the time-chirp distribution for imaging complexly moving targets. we first model the complex motion of ship target with cubic phase terms (parameterized on chirp rate and its change rate), then a new ISAR imaging method, referred to as TC-DechirpClean, is proposed, which estimates the chirp rate and the change rate of chirp rate of all scatters in the time-chirp distribution plane. Both numerical and experimental results are provided to demonstrate the performance of the proposed method.[7] Chapter 7 presents an imaging algorithm for achieving ship imaging in airborne radar system. An effective method based on an idea that uses motion compensation, general second-step keystone transform twice and at the same time, finds an isolated point and adopts WVD transform to estimate Doppler parameters for removing range walk and curvature. And what is more, the detailed deductions are given in this paper. Finally, the SAR image of moving ship is acquired in the azimuth frequency field. The simulation result of three moving point-targets and the imaging result of real ship data confirm this method.[8] Chapter 8 is the summary of the dissertation. It also discusses future research areas to be further studied.