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UT EE 381K - Inverse Synthetic Aperture Radar Imaging

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Literature Survey for EE381K ProjectInverse Synthetic Aperture Radar ImagingJunfei LiElectrcial and Computer EngineeringUniversity of Texas at Austin10/16/19982Table of ContentsAbstract 3Introduction 4Objectives 5Review 5Plans 8References 9Figures 103AbstractHigh-resolution radar images can be achieved by employing SAR or ISAR techniques. Itcan be shown that SAR and ISAR have the same underlying theory but differentconfiguration. Here the specific problem of aircraft ISAR imaging using ground-basedradar is addressed.As motion plays a critical role in ISAR, we study how the motion compensation shouldbe done to focus the echoed data into a 2-D image. Besides the normal motioncompensation, which uses the data sets themselves, here GPS aided motion compensationis also studied in detail, which uses GPS motion data of the aircraft as additional input.Comparisons of these two cases help to expose problems of the normal motioncompensation and to form a better understanding of ISAR imaging process.After giving problem definition and objectives of the project, this literature survey mainlysummarize the existing work on ISAR imaging, with emphasis on the system model andmotion compensation techniques. Then the implementation plans are put forward as theguideline for future work of this project.Key words: ISAR, motion compensation, GPS data41. IntroductionHigh-resolution radar imaging is interdisciplinary and has wide interests among manydifferent areas. In radar remote sensing, synthetic aperture radar images are usually usedto map the land of the terrain. In defense industry, inverse synthetic aperture radarimaging of moving objects is an important tool for automatic target recognition. Theproblem of radar imaging of an aircraft using ISAR is addressed in this project, withemphasis on motion compensation.Both SAR and ISAR have the same underlying theory, the main difference is thegeometry configuration. In SAR imaging, the radar is flying in the space, and the objectis stationary, while in ISAR imaging, the object is moving and the radar is stationary. Butonly the relative movement between the object and the radar is important. So the ISARimaging problem can be found to be equivalent to the more easily understood SARimaging problem.From signal processing viewpoint, radar imaging is a 2-D signal processing problem. Toform an image, 2-D resolution must be defined for radar imaging. Here the twodimensional discrimination is realized by compression in range direction and syntheticaperture in the cross range direction. Actually, radar echoes are just 1-D time series, but itis convenient to format this 1-D signal into 2-D signal.Radar images can be called as motion-induced images. Hence, in SAR and ISAR, motionis the problem and the solution (Carrara et al. 1995). In ISAR, the motion compensationis more challenging as we have no priori knowledge about the object, and in some casesthe object like the aircraft can exert complex movement. It can be observed that thenormal motion compensation might fail during some time of the flight.5To assess the normal motion compensation, we compared it against the GPS aidedcompensated images. We expect to get a better image with the GPS motion data as anadditional input, we also expect to get a better understanding the ISAR imaging process.2. ObjectivesThe objective of this project is to get a better understanding of the ISAR imaging bycomparing the GPS aided motion compensation technique with the normal motioncompensation technique.With GPS data as an additional input, the GPS aided motion compensated image shouldbe better, at least not worse than that of the normal compensated image. If this is not thecase, a reason should be given regarding to the accuracy of the GPS data.Also the reasons for the failure of the normal compensation technique might be exposed.And with the GPS truth data, some idea might come out to construct a modified motioncompensation algorithm.3. ReviewRadar is important because it can provide all weather, day/night capability to generate aspatial, visual representation of the radar reflectivity of an illuminated scene. Thus it hasa wide use from land use survey to automatic target identification.Imaging radar typically provides a two-dimensional representation of scatters in theilluminated volume. Generally, we speak of radar resolution in the range and cross-rangeor azimuth directions. Range resolution is inversely proportional to the bandwidth of thetransmitted signal. A wide bandwidth can be implemented by employing a frequency-6modulated pulse instead of a simple pulse. In conventional real aperture radar, azimuthresolution is large, as it is inversely proportional to the electric size of the physicalantenna aperture. A synthetic aperture array technique is used to improve the azimuthresolution. In synthetic aperture radar (SAR), the radar illuminates the object many timesas it passes by. Then with subsequent processing, it is equivalent to illuminates the objectone time but with the longer synthesized aperture.Inverse SAR imaging, in its narrowest sense, refers to the use of target motion alone togenerate a synthetic aperture for azimuth resolution. In practice, ISAR often includesantenna platform motion. The ISAR application of imaging a maneuvering aircraft is ofprimary interest in this project. Figure 1(a) illustrates the ISAR concept and geometry. Itshows a stationary radar sensor illuminating a passing airborne target. The target'svelocity and attitude changes (roll, pitch, and yaw) each contributes to the angularinterval over which the radar sensor collects data from the target. Since only the relativemotion is important for radar imaging, it is common to model the ISAR data collectionand processing by treating it as an equivalent maneuvering SAR illuminating a stationarytarget. Figure 1(b) illustrates this perspective. Thus ISAR is very similar to a spotlightmode SAR. Historically, literatures on SAR tended to be inaccessible to researchers outside the radarcommunity. Munson and Visentin 1989 derived the strip-mapping mode SAR imagingequations using a signal processing approach. It is believed that future development inSAR/ISAR benefits from contributions from researchers in signal and image processing.Soumekh 1992 treated the SAR/ISAR imaging problem as an inverse problem, and afterpresenting an analytical expression of the source/object interaction, he gave the inversion7(imaging) strategy in the wavenumber domain.


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