3.1 Introduction

Following the SAR image formation procedures outlined in Chapter 2 should, in principle, result in focused images with a resolution independent of range or wavelength and depending only on the length of the real aperture. In practice, however, a number of factors can degrade image quality by causing either amplitude or phase modulation of the signal. The former is a consequence of a failure of antenna stabilization. Platform roll or yaw can change the pointing angle of the antenna, as mentioned in Section 2.7, resulting in varying illumination. This is more significant at the extremes in both range and azimuth. The latter is a consequence of an unpredicted change in the range between the object and antenna. This gives rise to unknown Doppler modulation of the received signal, which impinges considerably on the pulse azimuthal compression, as we shall show.

Unpredictable amplitude modulation is mainly evident in the image through:

• Radiometric distortion;

• Increased sidelobe levels;

whereas unwanted phase modulation leads to:

• Image defocus;

• Geometrical distortion;

• Radiometric distortion;

• Increased sidelobe levels.

Image defocus, radiometric distortion, and increased sidelobe levels restrict target detection and recognition capability, including enhanced performance using super-resolution, as discussed in Chapter 10. Geometrical distortion prevents direct pixel-to-pixel comparison of SAR images, which is required for techniques such as multichannel registration, relevant to Chapter 12, and target change detection, discussed in Chapter 10. It also prevents the direct image/map comparison or matching of SAR with other forms of imagery, such as optical data. In terms of significance...