Research On High Frame Rate Radar Imaging Approach

Radar imaging has the advantage of working all day and all-weather,which makes it especially useful for applications such as remote sensing,military surveillance and security check.Achieving better resolution has been the goal of developing advanced imaging radar since the beginning of this technique.In the early stage of the radar imaging,new techniques had been developed,such as Doppler beam sharpening(DBS)and synthetic aperture radar(SAR),to achieve better resolution.Up to now,the resolution of imaging radars is comparable with that of charge-coupled device(CCD),which makes it an essential tool for remote sensing.Nowadays,the emerging needs for real time reconnaissance are driving this technique toward developing imaging radars with both high resolution and high imaging frame rate.There are mainly two ways to achieve the high frame rate radar imaging.One utilizes the emerging millimeter and sub-millimeter wave band RF technique to acquire a high angular resolution under the constraint of the limited system volume or speed of the platform.The other one utilizes advanced processing concepts and algorithms,such as the overlapped subaperture processing technique,to produce a dense image stream.The former approach is represented by the ViSAR program and the advanced scanning technology for imaging radar(ASTIR)program of the defense advanced research agency(DARPA).While the latter one is represented by the Video-SAR concept of Sandia national laboratory.This dissertation focuses on developing the advanced millimeter wave imaging radar and high frame rate imaging algorithms of the Video-SAR.The main contributions are as follows:1.On the issue of Video-SAR imaging,computational burdens of two well-known processing approaches,which are overlapped subaperture imaging and recursive backprojection,are compared under different system parameters.Then based on the stripmap mode or quasi stripmap mode of SAR,a very high efficient Video-SAR imaging algorithm named recursive subaperture imaging is proposed in this dissertation that combines advantages of both the overlapped subaperture imaging and the recursive backprojection.The imaging simulation validates the proposed algorithm.2.On the condition of high frame rate radar imaging in the medium or short range,various approaches are analyzed that based on millimeter wave technique.A millimeter wave beamscanning system that consists of a parabolic main reflector and a reflectarray sub reflector is proposed Based on the concept of ASTIR program,which has the advantages of low cost and low complexity.The proposed imaging system is capable of steering a high directional beam in multiple directions with a high switching speed,which makes it suitable for imaging applications such as security check,hazardous site monitoring,and self-driving system of vehicles,etc.The feasibility of the proposed system is validated through the electromagnetic simulation and the imaging performance of the system is verified in the point and complex targets imaging simulation.3.Based on the concept of the ASTIR program,a novel radar imaging approach through an electronically-displaced-phase-center antenna(eDPCA)is proposed.In this approach,a reflectarray is used to directly scan a beam across a parabolic reflector,where the antenna phase center is displaced electronically.The proposed eDPCA-based imaging radar can provide very high frame rate at a low cost and low complexity and does not depend on the motion between the target and the radar platform.Compared with the beam scanning imaging systems,the eDPCA approach could steer a wide beam that covers the scenario during the integration,which is favorable for the micro motion target detection.The imaging frame rate and signal to noise ratio of such a system is described in detail.While the feasibility of the eDPCA-based imaging radar is investigated and validated through a designed example eDPCA at 157 GHz and the electromagnetic simulation,respectively.The effects of practical impairments such as errors in phase centers and position dependent fluctuations in antenna patterns on the imaging quality are examined.4.Considering patterns fluctuation in the eDPCA,a range-Doppler algorithm with the precompensation method is proposed to focus the radar image in the azimuthal frequency domain,which has the advantage of a much lower computational burden compared with traditional imaging algorithms.The proposed algorithm is validated through the point as well as the complex target simulations,which makes it a candidate for using in the practical scenario.5.To overcome the degradation of the SAR image quality caused by the inherent pattern fluctuations of the eDPCA,a modified virtual array method is proposed to correct the irregularities in the eDPCA-based system.The effectiveness of the proposed approach is validated through experimental examples on a simulated eDPCA imaging system at 157 GHz,where the imaging results indicate a significant improvement on the image quality.Then,a modified virtual array method is proposed to be operated under the condition of a significant range cell migration.At the end of this dissertation,a virtual array based supper resolution algorithm is proposed to transform the pattern fluctuations into external Doppler frequencies to achieve a higher resolution than the Rayleigh limit,where imaging simulations based on electromagnetic data validate the approach.