Research And Simulation Of ISAR Imaging Algorithm Borne By Aerostat

The Inverse Synthetic Aperture Radar (ISAR) system carried by aerostat,like airship, has advantages of large detection range, low-cost and highlyflexible flight. The impact of platform drifting on the final result ofaerostatics ISAR imaging is studied in this thesis. And it proposes a methodthat adopting the Compressive Sensing (CS) algorithm during azimuthcompression of ISAR imaging, which is able to reduce the main lobe width inthe azimuth dimension, in situations where the angular velocities of airshipchanges quickly. Moreover, this algorithm can also effectively suppress thesidelobe of azimuth effectively, eliminate the false scattering points andenhance the focusing effect, so as to diminish the negative influence ofvibration on unstable platform, under the condition of sparse-apertureimaging.First of all, the radar turntable imaging principle of ISAR imaging and theclassic Range-doppler (RD) imaging theory, together with the azimuthcompression algorithm based on CS are elaborated on in detail. Then thereasons of defocusing and several commonly used motion compensationtheories are discussed. After that, a ship model, which including geometricmodel of scattering points and three-dimensional motion function, is bulit asthe imaging target in this article. By comparing the image results of differentmotion model, the3d perspective projection and azimuth resolution areexplained in detail.Secondly，this thesis sketches the areostatical ISAR imaging system, andmodels the platform drift motion. The simulation results have varified theinfluence of the changes in velocity and angular velocity of aerostats duringthe image procedure. It came up with the conclusions that, in the case of thetranslational motion have been completely compensated, the image result havebeen less affected by velocity of the airship so it is negligible, while theangular velocity will affect the two-dimensional projection plane, especiallywhen the pitching and yaw angle change too fast, the scattering points faraway from the rotation center would be defocusing. Then some experimentswere done to testify the advantage of CS algorithm on reducing the main lobewidth.In the end, this thesis shows the simulation of sparse-aperture imaging when part of the echos were lost due to platform drifting. On theoretical aspect,it analysed how to design the sparse matrix and measurement matrix when theCS theory was employed to accomplish the azimuth compression, and severalexperiment were done to testify the effect of the sparsity and the number ofmeasurement on the final image result. After confirming the parameter settings,it shows the images getting from RD algorithm and CS algorithm respectively,then it compare several parameters, such as main to side lobe ratio and3dBmain lobe beamwidth of isolated scattering point.The new algorithm shows itsconsiderable advantage in suppressing the sidelobe in azimuth dimensionwhen loss ratio of echos is high.