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UBC Theses and Dissertations

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UBC Theses and Dissertations

Image formation from squint mode synthetic aperture radar data Davidson, Gordon W.

Abstract

The objective of this thesis is the investigation of image formation from squint mode, stripmap synthetic aperture radar (SAR) data, and the extension of the recently developed chirp scaling algorithm to accommodate problems in this type of imaging. In squint mode SAR, the antenna is pointed forward or backward of the perpendicular position used in conventional SAR, allowing different azimuth viewing angles of the surface. Squint mode has been used previously in spotlight SAR imaging, but signal characteristics and efficient signal processing for a spaceborne, strip-mapping squint mode SAR have not been thoroughly understood. Several SAR processing algorithms are reviewed and analyzed to compare their processing errors at high squint and the type of operations they require. This includes the range-Doppler, squint imaging mode, polar format, wave equation and chirp scaling algorithms. In contrast to other algorithms, the chirp scaling algorithm does not require an interpolator in either the two-dimensional frequency domain or the range-Doppler domain, and it removes the range dependence of range cell migration correction (RCMC) efficiently by taking advantage of the properties of uncompressed linear FM pulses. Also, it achieves accurate processing for mod erate squint angles by accommodating the azimuth-frequency dependence of secondary range compression (SRC). Next, the properties of the squinted SAR signal are investigated to determine their effect on processing. A solution is presented for the yaw and pitch angles of the antenna which minimize the Doppler centroid variation with range and terrain height. The residual variation for a satellite platform is found to be negligible for an L-band SAR, while for C-band the variation was moderate and some accommodation in processing may be required. Then, the squinted beamwidth, which determines the azimuth bandwidth, is derived, and it is shown that choosing the yaw and pitch angles to minimize Doppler centroid variation results in an azimuth bandwidth that is independent of range. The resulting azimuth bandwidth and pulse repetition frequency (PRF), as a function of squint angle, is used to derive a fundamental limit on the squint angle such that a received echo fits between adjacent transmitted pulses. For spaceborne SAR. and a 40 km slant range swath, the squint angle is limited to about 35 degrees for L-band, and 50 degrees for C-band. The chirp scaling algorithm is then investigated by analysis and simulation, and extended for processing high squint SAR data. The side-effects of chirp scaling include a range dependent range-frequency shift which may result in a loss of range bandwidth if frequency components are allowed to be shifted outside the window of the range matched filter. The original chirp scaling algorithm approximates the range dependence of RCMC by as suming a constant B parameter in the distance equation for an orbital geometry. This causes a noticeable degradation in the point spread function for squint angles above about 15 degrees for L-band and 30 degrees for C-band. To provide accurate RCMC at high squint angles in an orbital geometry, the chirp scaling algorithm is extended so that the range dependence of the B parameter is accommodated in RCMC, by including a higher order term in the chirp scaling phase function. Finally, the original chirp scaling algorithm neglects the range dependence of SRC, and this affects the quality of processing for squint angles above 10 degrees for L-band and 20 degrees for C-band. To solve this problem, the concept of nonlinear FM chirp scaling is introduced, in which a nonlinear FM component is incorporated into the received range signal which interacts with chirp scaling to remove the range dependence of SRC. This allows accurate processing of strip map SAR data for squint angles up to the limitations imposed by the SAR imaging constraints. Two methods of nonlinear FM chirp scaling are introduced. The nonlinear FM filtering method introduces the nonlinear FM component by an extra filtering step during processing, and is more accurate. The nonlinear FM pulse method incorporates the component into the transmitted pulse, thus requiring no extra computation, although it is slightly less accurate. The processing errors of both methods are analyzed and the expected performance is verified by the processing of simulated point scatterer data. In addition, conventional spaceborne SAR data from Seasat was skewed to emulate the signal from a high squint SAR, and processed with the original chirp scaling algorithm and the nonlinear FM chirp scaling algorithm. The resulting images show the improvement in range resolution with nonlinear FM chirp scaling.

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