Range-doppler synthetic aperture radar processing at VHF frequencies

Doctoral Thesis


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University of Cape Town

This thesis focuses on the application of range-Doppler processing at VHF frequencies for wide azimuth beam, airborne, stripmap SAR systems with zero Doppler centroid and modest pulse bandwidths relative to the carrier frequency (simulations less than 30 percent). Such a system is the South African SAR (SASAR) VHF sensor. The theory of such SAR operation is addressed. In general, closed form analytical expressions for range compressed, range-Doppler domain signals do not exist. Thus, in this work, extensive use is made of simulation. Using simulated SAR signals with severe range curvature, the regions of applicability of standard range-Doppler processing, when applied without Doppler frequency dependent secondary range compression, are investigated for a range of processing parameters in the frequency range of 100 MHz to 200 MHz. The effects on the range impulse response of centre frequency, target closest approach range and nominal range resolution are investigated, each for a range of processed azimuth resolutions. Information is presented in the form of plots showing the degradation in the range resolution and in the form of tabular results which also include the range peak- and integrated side lobe levels and the non-linear phase error in the Fourier domain. An extension to range-Doppler processing, suggested to the candidate by Michael Jin, is demonstrated to provide significantly improved performance over the standard range-Doppler processor for signals with severe range curvature. The basic idea of the extended algorithm, first published by Raney and Vachon in 1989 and applied in the context of a narrow beam, squinted SAR, is to make an initial correction to a reference range through a multiplication with a reference function in the 2-D frequency domain. This is followed by a residual range curvature and azimuth focusing operation in the range-Doppler domain. Airborne motion compensation strategies for flight path reconstruction are discussed. In addition, an ERIM-developed approach for efficiently including an azimuth dependence for wide beam motion compensation is discussed in the context of the SASAR system. An overview of the SASAR VHF project, the experience gained, and the encouraging results from the processing of the data from the first radiating flights, is presented. A full Statement of Originality is given.

Bibliography (pages 144-150).