FROM SAR DESIGN TO IMAGE QUALITY
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In this chapter, we shall first look at SAR design from a radiometric perspective. The preceding chapter dealt with a geometrical approach to radar processing. It seemed to leave us with a paradox (Sect. 2.2.4): if D is the length of the radar antenna in the azimuth direction, the azimuth resolution obtained after SAR synthesis is D/2, which seems to contradict the common understanding of the separating power of a microwave antenna (Sect. 22.214.171.124). This is not really so surprising: if D is reduced, the azimuth radiation pattern is broadened, which increases the illumination time for a target, and thus the width of the Doppler spectrum. Hence the azimuth resolution is naturally improved. So, why should we try to give imaging radars long antennas? There are several reasons. A ‘geometrical’ reason, discussed in Sects. 3.7.1 and 3.7.2, concerns the conditions under which ambiguities appear in the image. But there is also a ‘radiometric’ reason: the shorter the antenna, the lower its directivity and thus the weaker its gain. The amplitude of the radar echo is attenuated proportionally to the square of the antenna’s gain (because the wave crosses the antenna’s main lobe twice). It is thus necessary to be careful when designing the antenna to ensure an adequate level of collected power, which requires a minimal value for D. In these few words we have just described the trade-off between the image’s geometric qualities (i.e., its resolution) and its radiometric qualities (which depend on the amplitude of the radar echo). This is the central issue in system design. We shall keep this in mind when considering essentially radiometric issues in the following sections.