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published_or_final_version / Electrical Engineering / Doctoral / Doctor of Philosophy

Thesis (M.S. in Applied Physics)--Naval Postgraduate School, June 2009. / Thesis Advisor(s): Borden, Brett H. "June 2009." Description based on title screen as viewed on July 14, 2009. Author(s) subject terms: radar imaging, moving targets, point spread function, ambiguity function. Includes bibliographical references (p. 73-75). Also available in print.

For a synthetic aperture radar (SAR) system, the Doppler centroid is the azimuth Doppler frequency received from a point scatterer centered in the azimuth antenna pattern. This parameter is required by the SAR processor in order to properly focus SAR images. Since the azimuth Doppler spectrum is weighted by the azimuth antenna pattern, the Doppler centroid can be determined by locating the peak of the Doppler spectrum. This measurement, however, is ambiguous because the azimuth Doppler spectrum is aliased by the radar pulse repetition frequency (PRF). To resolve the ambiguity, the antenna beam angle, which determines the Doppler centroid, is measured; the accuracy of this measurement must be high enough to determine the Doppler centroid to within ±PRF/2. For some SAR systems, such as the future Radarsat system, the beam angle measurement must be very accurate; this can be technically infeasible or too costly to implement. This thesis examines an alternative approach to resolving the Doppler centroid ambiguity which does not require accurate beam angle measurement In most SAR processors, several partial azimuth aperture "looks" are processed, rather than a single long aperture, in order to yield a final SAR image with reduced speckle noise. If the Doppler centroid is in error by an integer number of PRFs, then the SAR looks will be defocussed and misregistered in range. The degree of misregistration depends on with which Doppler centroid ambiguity the data is processed. The new method for Doppler centroid ambiguity estimation measures the range displacement of SAR looks using a cross-correlation of looks in the range direction. The theoretical background and details of the new method are discussed. The effects of differing terrain types, wave motion, and errors in the azimuth frequency modulation (FM) rate are addressed. The feasibility of the approach is demonstrated by testing the cross-correlation algorithm on available Seasat data processed with simulated Doppler centroid ambiguity errors. The Seasat analysis is extrapolated to the Radarsat system with favourable results. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Three key areas of controversy in synthetic-aperture radar (SAR) imaging of ocean surface waves are considered: first, the nature of Bragg scattering; second, the role, magnitude, and calculation of the scene coherence time; and third, the relevant ocean wave velocities for coherent Doppler modulations. This work begins with a re-derivation and extension of existing SAR imaging theory for point and diffuse targets. Generic, relatively simple, closed-form expressions for the impulse response, the resolution, and the image bandwidth summarize this unified treatment. Theoretical differences between the imagery of point and diffuse targets are pointed out. Based upon these fundamental differences, a statistical testing procedure is formulated to address the question of scene target density. Background ocean surface wave theory is outlined in preparation for discussions of SAR ocean imaging. Of central importance is the role of the phase velocity, which is the speed of translation of the mean pattern of reflectivity, and the orbital motion, which leads to coherent (phase) modulation, and hence to velocity bunching, acceleration defocus, and target decorrelation. Based upon this theoretical background, one- and two-dimensional simulation models are developed. The one-dimensional simulation addresses the effects of various parameters upon the mean image contrast in a velocity bunching model and guides the development of the two-dimensional simulation. The two-dimensional simulation is unique because each target which constitutes the scene is explicitly considered. This leads to a degree of control and flexibility which is not available from actual SAR imagery. Qualitative and quantitative comparisons are drawn between the simulated and actual SAR imagery to address the key areas of controversy. The assertion that Bragg scattering is a coherent process is defended, despite inability to conclusively verify this using SEASAT data. Comparisons between simulation and C-SAR imagery of waves propagating into ice verify the roles of the scene coherence time and the wave phase velocity. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate