As a consequence, the wider the transmitted bandwidth, the finer the slant range resolution. On the other hand, the resolution in the azimuth direction (or cross-range direction) is inversely proportional to the variation of the aspect angle and, hence, to the synthetic array aperture. In fact, the idea behind SAR is to synthesize the effect of a large physical aperture antenna, the realization of which is impractical because of the size that should have to get a fine enough cross-range resolution, by exploiting the platform motion. In this way, the effects of a long physical antenna are synthetically created in post-processing by coherently processing the echoes received during the syn-
thetic aperture. The cross-range resolution is inversely related to the synthetic antenna size or array size along the same dimension, L,
δx1 ∝ 1
L = vTobs is the synthetic aperture and coincides with the platform flight path during the observation time, where v is the platform velocity and Tobs the observation time. According to the way the scene is illuminated, a SAR system can be classified as stripmap, spotlight or scansar. As the name suggests, in stripmap SAR the radar illuminates a different spot of the surface being imaged at each instant of time, so that a continuous strip is imaged, as shown in Figure 2.2. On the other hand, a spotlight SAR system illuminates a single spot as the radar platform moves, by steering the antenna beam towards the same area, as shown in Figure 2.3. As a consequence, the same area is illuminated for a longer observation period resulting in a finer resolution. As can be easily understood, the swath width depends on the antenna aperture in azimuth and so it is inherently limited. By the way, in order to increase the swath width, the scansar operational mode can be exploited . In this case, the antenna beam is steered to different elevation angles as the radar platform moves along the flight path, i.e., the azimuth direction, as depicted
in Figure 2.4. In this way, by properly setting the elevation offset as the radar moves, a continuous strip can be imaged.