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BISTATIC/MONOSTATIC SYNTHETIC APERTURE RADAR FOR ICE SHEET MEASUREMENT

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1BISTATIC/MONOSTATIC SYNTHETIC APERTURE RADAR FOR ICE SHEET MEASUREMENTS BY JOHN PADEN B. SC. COMPUTER ENGINEERING THE UNIVERSITY OF KANSAS, 1999 Submitted to the Department of Electrical Engineering and Computer Science and the Faculty of the Graduate School of the University of Kansas in partial fulfillment of the requirements for the degree of Master of Science. Thesis Committee: _______________________ Chairperson _______________________ _______________________ Date of Defense: April 18, 2003i ABSTRACT A ground-based monostatic/bistatic, broadband VHF synthetic aperture radar is being developed for the two-dimensional mapping of the basal conditions of polar ice sheets. The goal is to obtain knowledge of the morphology or ‘roughness’ of the bed, the distribution of the subglacial water film, and the amount of rock debris and silt in the basal ice. Since ice sheet basal conditions represent one of the boundary conditions of glacier flow, they are a fundamental component in ice flow models. The ability to establish basal conditions with a high degree of accuracy will fill an important gap in the knowledge base used to predict the origin and migration of ice streams, which drain the majority of the polar ice sheets. An improved understanding of ice sheet dynamics can be used to predict future sea level rise that threatens coastal regions throughout the world. A measurement model for the radar system is described. SAR processing algorithms based on this measurement model using a matched filter and maximum likelihood filter are then derived. The model and algorithms are used throughout the thesis to simulate various scenarios needed to identify certain constraints. The model and algorithms are also tested in the sandbox laboratory to assess their utility. Two methods are considered to accelerate the time taken to map a region. The first method found a sensor geometry that minimized measurement time as a function of frequency, backscatter characteristics, and ice thickness. The optimal position was very sensitive to the backscatter characteristics, but in general the transmitter may be placed further out than the optimal position without a significant increase in measurement time. This means that, while different basal characteristics may have different optimal sensor geometries, a compromise can be made between these geometries. Another method for improving traverse time uses an along-track array towed behind the transmitter vehicle. The maximum length of the array versus required bedrock resolution was found and several ideas for circumventing this maximum length limitation are proposed.iiA number of errors involved in SAR processing were analyzed using simulations. The matched filter is a fairly scalable algorithm, but it does not consider interference and its inability to estimate the reflectivity of a target that is adjacent to other targets is demonstrated along with a comparison to the maximum likelihood filter that does consider interference. The sensitivity of these filters to position errors is simulated. It was found that one tenth of a wavelength accuracy is needed to effectively SAR process targets. The sensitivity of the SAR processor to dielectric errors is illustrated. It was found that the image stayed focused even with fairly significant dielectric errors, but the target positioning accuracy was directly affected (as was expected). The effectiveness of the SAR processing algorithm for focusing smooth bedrock surfaces is considered as well. The sandbox laboratory was used to test a scaled model of the system. Several calibration measurements were taken to supply the system model with the parameters necessary to predict the response from targets buried in the sand. The SAR processor was able to accurately resolve each of the targets imaged to within a few centimeters. The results also showed that the reflectivities for identical targets were the same to within a 4 dB spread.iii TABLE OF CONTENTS ABSTRACT..................................................................................................................I TABLE OF CONTENTS .........................................................................................III LIST OF FIGURES ....................................................................................................V LIST OF TABLES ...................................................................................................XII CHAPTER 1: INTRODUCTION.............................................................................. 1 1.1 SCIENCE MOTIVATION ......................................................................................... 1 1.2 BACKGROUND...................................................................................................... 5 1.3 PRINCIPLES OF OPERATION .................................................................................. 8 1.4 OVERVIEW ......................................................................................................... 12 CHAPTER 2: SAR PROCESSING......................................................................... 14 2.1 INTRODUCTION................................................................................................... 14 2.2 MEASUREMENT MODEL..................................................................................... 19 2.3 SYSTEM MODEL................................................................................................. 20 2.3.1 Layered Medium (Chapter 3)..................................................................... 22 2.3.2 Homogeneous Medium (Chapter 4)........................................................... 22 2.3.3 Dielectric Half-Space (Chapter 5)............................................................. 22 2.4 MATCHED FILTER .............................................................................................. 25 2.5 MAXIMUM LIKELIHOOD FILTER ......................................................................... 27 2.6 PROCESSING DOMAIN......................................................................................... 28 2.6.1 Space-frequency......................................................................................... 29 2.6.2 F-k migration............................................................................................. 30 CHAPTER 3: BISTATIC SENSOR GEOMETRY............................................... 36 3.1


BISTATIC/MONOSTATIC SYNTHETIC APERTURE RADAR FOR ICE SHEET MEASUREMENT

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