Research On The Methods Of Three-Dimensional Imaging For Rotor Synthetic Aperture Radar

As a new kind of imaging mode, Rotor Synthetic Aperture Radar (ROSAR) hasbeen used widely for natural disasters rescue, mineral resources exploration,nondestructive testing, battlefield surveillance and low-altitude fire support due to suchadvantages as all-time, all-weather, electromagnetic penetration, short revisiting timeand omnidirectional imaging. Helicopters and other rotorcrafts are the most typicalplatforms for ROSAR technology. The antenna is fixed at the end of a rigid arm, pointsaround the platform and travels along the circular motion with the constant velocityrotor. In this way, an observation of the surrounding scene is completed while theantenna rotates a full circle. Using the blade rotation characteristic, ROSAR technologycan generate a circular synthetic aperture in the azimuth direction with one antenna, andrealize the two-dimensional (2-D) imaging of surrounding scene for guaranteeingrotorcraft safety. However, the acquired2-D image may show spatial information of thepotential hazards incorrectly in a low-altitude and complex environment, and thus it isabsolutely necessary to carry out research on the three-dimensional (3-D) imaging ofROSAR.To make low-altitude aircraft secure, and to solve the key problems in3-D imagingof ROSAR, we expand the work from signal models and imaging methods of3-DROSAR. The relevant work is supported by the National Basic Research Program ofChina (973Program) under Grant2011CB707000and the National Natural ScienceFoundation of China under Grant61101242. The main work and contributions of thisdissertation are presented as follows:1. The model of2-D ROSAR is introduced, which lays the foundation for thesubsequent research on3-D imaging. To improve the weakness of time-consuming andazimuth defocus in the existing algorithms, a new2-D ROSAR algorithm based onspectral reconstruction is presented. The expression of ROSAR echo signal is analyzed,and then the precise2-D spectrum is obtained by using of the Fourier transform directly.Due to the complex form of2-D spectrum, it is difficult to realize the later imagingprocess. Therefore, considering the accuracy and operability, an approximate andinformative spectral is acquired by the method of fourth-order spectral reconstruction,and based on which, the Range Migration Algorithm (RMA) and the Chirp ScalingAlgorithm (CSA) of ROSAR is proposed. Finally, simulation results show the accuracyand rapidity of the proposed method with the azimuth angle of less than90degrees.2. An interferometric imaging model of ROSAR is established, and the problem of axial deviation of platform is analyzed emphatically under non-ideal circumstance.By means of ROSAR technique and interference method, a set of surrounding data arecollected in a certain “hover” position, while another set of data in a new “hover”position. After imaging and interferometric processing, we can get the omnidirectional3-D imaging of the surrounding scene. As to the non-ideal motion of the platform, wediscuss the influence of axial deviation on azimuth bandwidth, image position andinterferometric phase. In particular, the deviation leads to an additional term in theinterferometric phase. To correct this additional phase, we derive its expression,construct the relevant compensation function, and define a weighted factor as a result ofheight information inadequate and nulling in the compensation function. In thesimulation experiment, the influence of axial deviation is analyzed and the digitalelevation model (DEM) of3-D scene is generated.3. A model of spiral synthetic aperture radar (SSAR) is established and acorresponding algorithm is proposed. Based on the existing2-D ROSAR technology,SSAR utilizes the ascent of platform to form a second synthetic aperture, and then itachieves the ability of3-D imaging due to the formation of cylindrical array, which isthe result from the interaction of horizontal rotation and uniform ascent, and thewide-band signal transmitted in the range direction. In Chapter four,3-D imagingalgorithm is presented for the SSAR model. Firstly, to reduce the difficulty of imagingprocess, we analyze the impacts of azimuth sampling and height ascent on antennaposition, construct the compensation function for spatial offset, and simplify the “spiral”mode into the “hover-and-go” mode in the height direction. Secondly, through thedetailed analysis of phase history of simplified model, the spectrum shift in the heightdirection is calculated, the bulk compression function and interpolation kernel in thewavenumber domain is derived, and thus the3-D imaging is implemented. Finally, theperformance analysis of SSAR model is performed, and the effectiveness of theproposed algorithm is validated through simulation results.4. A model of forward-moving ROSAR (FMROSAR) is established, and a3-Dimaging algorithm is proposed for this model. In the forward-moving state, the antennatransmits the wide-band signal to the front area; meanwhile, it can form a plane arrayunder the influence of platform movement and blade rotation. Therefore, FMROSARcan realize3-D forward-looking imaging owing to the high-resolution in the rangedirection and the2-D resolution of the plane array. The imaging algorithm forFMROSAR includes model simplification and3-D imaging for the simplified model.First of all, with the azimuth and along-track offset compensation, the “forward-moving” mode is simplified as “stop-and-go” mode in the along-trackdirection. In “stop-and-go” mode, the algorithm can be further divided as therange-azimuth imaging and the range-along-track imaging. As for the latter, the azimuthangle variation and its impacts on the matched filter are discussed, and an improvedRange Doppler Algorithm (RMA) is proposed to focus the data with high squint anglein the along-track direction. Finally, the performance of FMROSAR is analyzed, and thevalidity of this algorithm is verified.