| Summary: | 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. === Applied Science, Faculty of === Electrical and Computer Engineering, Department of === Graduate
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