On the Time-Domain Airborne SAR Focusing in the Presence of Strong Azimuth Variations of the Squint Angle

Airborne synthetic aperture radar (SAR) data focusing in the presence of highly variable squint angles is addressed in this work. To do this, a time-domain SAR focusing algorithm has been developed to account for the target-dependent nature of the acquisition squint angle. For comparison purposes, we have considered also a SAR processor based on the use of an azimuth-invariant processing squint angle. Moreover, real data acquired by two different X-band airborne SAR systems have been analyzed. The aims of the work are twofold. First, we show that the adoption of an azimuth-invariant processing squint angle, typically pursued by computationally efficient focusing strategies, may become inappropriate in several airborne missions, which are typically corrupted by motion errors induced by atmospheric turbulence. On the contrary, these problems are circumvented through the developed time-domain focusing strategy, which exploits a target-dependent processing squint angle that is coincident with the acquisition one. Second, we show that the proposed focusing strategy must be properly revised in the case of interferometric processing. In particular, we show that if the (target-dependent) acquisition squint angle is different for the two interferometric channels used, we have to move toward a different focusing paradigm, say, an interferometric focusing solution. This is again based on considering a target-dependent processing squint angle, but for each target, this processing angle should be equal neither to the acquisition squint angle of the "main" acquisition nor to that of the "secondary" one: it must instead lie halfway between them. We prove that this approach is appropriate to obtain high interferometric coherence, and it is suboptimal to separately focus the interferometric channels.