Analysis Of Multicomponent LFM Signals And Imaging Of Inverse Synthetic Aperture Radar

Inverse Synthetic Aperture Radar (ISAR) has the capability of getting fine resolution images of noncooperative targets (such as airplane, ship, satellite etc.) all day and all night in all weather from long range, so it has great military and civil values. Thus, ISAR theory and technique has gained increasing attention and been greatly developed, it has been applied to image complex moving targets from the original targets with stable flight, the conventional Range-Doppler (RD) algorithm is no more applicable because the non-stationary signal, especially the multicomponent LFM signals, is involved frequently. It is necessary to acquire the correct time-frequency distribution of the target for the subsequent ISAR imaging processing. ISAR imaging of high speed targets, maneuvering targets, and multiple moving targets are studied in this thesis, around the multicomponent LFM signals.The high speed of the target can lead to range dispersion, to solve this problem, on the feature of high speed target echo which consists of multi-component LFM signal with the same chirp rates, a method based on instant range profile (IRP) using STFT and minimum entropy is presented, it solves the problem of range dispersion. By instant range-Doppler imaging processing, a two-dimensional (2-D) image will be achieved finally.During the short time of ISAR imaging, for inert maneuvering targets such as aircraft, the echoes of the same range cell approximately consist of multicomponent LFM signals with different chirp rate. For this case, a method based on Local Wave Decomposition (LWD) and Wigner-Ville transformation (WVT) is proposed, thus 2-D imaging of the target is realized. It can not only avoid the cross-term of WVT but also keep high time-frequency resolution and meanwhile decrease the computational load effectively.For multiple moving targets which are flying within the same antenna beam and cannot be separated in range direction, the conventional ISAR imaging algorithms cannot be directly used to obtain a focused image. Echo of every target at the same sampling time approximately consists of multicomponent LFM signals with same chirp rate during the short observation time, which has different chirp rate due to different target. For this feature, two methods are addressed. Motion parameters are estimated and echoes are separated in the first method based on Reassign-Smooth-Pseudo-Wigner-Ville Distribution (RSPWVD) and Hough transform. The second method separates the echoes using LWD. Then every single target echo is processed to get the clear 2-D ISAR image.In summary, this dissertation has acquired some helpful research results in ISAR imaging. But up to now, there are still a lot of opening problems in ISAR imaging and worth further efforts.