This technique (that is less and less used, due to the progresses in digitizer technology) for digitising signals at a frequency lower than the pulse bandwidth. Of course, there is drawbacks: The majors one is that this is obtained to the cost of a limitation in range swath. A second drawback is that it requires a sharp edged low-pass filter in front of digitizers. And for some reason linked to causality principle, such a filter have a long propagation time. Furthermore, it also present undulations of the phase and amplitude along the frequency band passed through (the undulations are stronger near the edges of the band). These undulations are directly alterating the range profiles since -as we shall see- they are simply the frequency spectrum of the digitized signal.
Though not critical for computing a single SAR image, this is very annoying in the case we need to compare images obtained by several radar (or radars subsystems) since the low-pass filters are different and the relative phase between images presents undulations with range. These undulations are very problematic for many applications (detailled in the following chapters of this website) such as interferometry and polarisation analysis.
TO BE CONTINUED...