Synthetic Aperture Radar (SAR) technology and applications - the need for a better understanding of the physics behind microwave interaction with terrestrial environments

Synthetic Aperture Radar using active microwave (1 cm to 70 cm wavelength) sensors from satellites in space or from aircraft has been considered a mature technology for imaging terrestrial surfaces since it was developed by the military in the 1960s. There has been and is, near exponential growth in the number of spaceborne SAR sensors and their applications - ranging from land use classification, change detection time series analysis, topographic and surface displacement mapping, soil moisture and snow mapping, monitoring and tracking of ocean features; currents, wind, waves, oil spills, as well as vessels and their wakes, etc. Yet, the detailed interaction of the micro-waves with matter on the ground (as well as in the atmosphere) despite all governed by classical EM is surprisingly little understood. Many of the existing theories behind the applications make very significant adhoc assumptions (in terms of homogeneity and stationarity) about the imaged materials of the terrestrial environments and also limit greatly the complexity of the EM wave interaction by neglecting temporal effects during imaging and excluding higher order or dispersive scattering. Shortcomings of the simplified theories are apparent already for the modern higher resolution SAR sensors operating at different frequencies, pointing to additional or different physical interactions being important (suspected e.g. with foam on ocean surfaces). At the same time full physical numerical simulation (e.g. with FDTD) of unfocused SAR raw data are still far from practically feasible for realistic representations of terrestrial environment; even when making full use of the breath-taking recent advances in parallel computing with GPUs. The lack of a deeper physical understanding of microwave-matter interactions threatens to stymie the full potential of new SAR sensor development (e.g. HRWS, micro SAR satellite constellations) for their future applications.  

After a short introduction to SFU’s Sarlab and the research we are pursuing I will give a high-level explanation, of the SAR principle in the context of the related and better-known coherent imaging of laser holography, and of the concepts of digital raw data focusing and interferometric processing. I will point out some of the limiting assumptions mentioned before and will give a number of application examples that high-light where the physical interactions behind (and the statistical properties of) the received SAR signal are poorly understood to date. In the remainder I will try to identify concrete topic areas where application development is held back currently and where a more physics based approach would be beneficial to the technology. I will elaborate on collaboration possibilities for a number of topics (hopefully considered challenging and interesting to physicists and physics students); most importantly: (i) differences between synthetic aperture vs. “equivalent” real aperture systems related to relaxation-type phenomena at different time scales, (ii) “effective” dispersion descriptions for distributed and volume scattering, and (iii) research on analogue materials simulating micro-wave interaction with real environments at mm-wave (~ factor 15 downscaled), particularly liquids that can mimic water as for mm-wave analogue experiments.