A Cost-Benefit Analysis for Gapless Synthetic Aperture Radar Imaging

Abstract

In digital beamforming synthetic aperture radar (SAR), the received echo signal is sampled on a sub-aperture (antenna) level recording data of the individual digital channels. The benefit of using multiple digital channels is to extend the trade-space of SAR, which, by using appropriate operation modes or techniques, yields a significantly improved performance [1]. A remaining restriction that applies to spaceborne platforms hosting both the transmitter and receiver is, that a SAR instrument cannot receive while it is transmitting, as the high transmit power (several kilowatts) would damage the receive hardware designed to detect extremely weak echo signals. Thus, the signal echoes arriving during the time in which the instrument is transmitting will not be recorded. This results in blind ranges along the swath. Note that future SAR constellations consisting of a few transmit-only and several receive-only satellites [2] allow even continuous transmission without blinding the receivers. The width of these blind ranges is proportional to the transmit pulse duration and varies with the incidence angle. For a typical multi-channel SAR operating in stripmap mode to image a swath of 400 km, the width of the gaps is in the order of a few to tens of kilometers, while, in single pol, the SAR image would contain three to five such gaps at distinct ranges. Two main strategies are known: either use appropriate techniques to prevent gaps from appearing in the processed SAR image; or allow gaps in the single SAR acquisition and use successive acquisitions to “fill” the gaps. Thus, following an instrument/mode based or a mission design solution approach, respectively. The first option to circumvent the gaps faces the instrument designers with major challenges. It requires the use of dedicated instrument operation modes that have major implications on the complexity and hardware, both RF and digital. The second option implies multi-beam imaging, which is more or less a conventional stripmap extended by multi-SCORE (SCan-On-REceive) technique. This allows blind ranges in the SAR acquisition and relies on using appropriate mission design for filling the gaps. The paper explores the trade space options for high-resolution wide-swath SAR imaging. Both direct radiating planar array antenna and reflector based digital feed array SAR systems are considered. The paper elaborates on the comparison of multi-beam and gapless imaging from an instrument design, SAR performance, and mission concept point of view. Specifically, the digital on-board processing requirements are detailed. The aim is to raise the awareness for the consequences of each solution strategy. [1] M. Younis, P. Lo ́pez-Dekker, F. Bordoni, P. Laskowski, and G. Krieger, “Exploring the trade-space of MIMO SAR,” in Proc. Int. Geoscience and Remote Sensing Symposium IGARSS’13, Melbourne, Australia, Jul. 2013. [2] G. Krieger, M. Zonno, J. Mittermayer, A. Moreira, S. Huber, and M. Rodriguez Cassola, “MirrorSAR: A fractionated space transponder concept for the implementation of low-cost multistatic SAR missions,” in Proc. European Conference on Synthetic Aperture Radar EUSAR’2018, Aachen, Germany, Jun. 2018.