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W. Wagner:
"Sub-surface sensing of the unsaturated soil zone by spaceborne radar instruments";
Keynote Lecture: 78. Jahrestagung der Deutschen Geophysikalischen Gesellschaft gemeinsam mit der Österreichischen Geophysikalischen Gesellschaft, Leoben (invited); 2018-02-12 - 2018-02-15.

Spaceborne radar instruments such as Synthetic Aperture Radars (SARs) and scatterometers send short microwave pulses towards the Earth´s surface and measure the amplitude (and phase) of the backscattered echoes. When impinging on wet soil surfaces, the microwave pulses penetrate only a few millimetres to centimetres into the soil, hence the dominant scattering mechanisms is surface scattering. However, when the soil is dry, the penetration depth can be several decimetres or maybe even larger.
In such a situation, sub-surface scattering mechanisms may also substantially impact the backscattered waves. The existence of sub-surface scattering features in SAR imagery acquired over desert regions is well known since the early days of radar remote sensing. However, there is very little understanding of how these features vary in time and space. A particularly interesting question is if sub-surface scattering also occurs in semi-arid and humid environments during dry periods? Potentially, this would provide a new means to map soil properties over large regions. In this presentation I will give an overview of the state-of-the art of our understanding of sub-surface scattering phenomena as observed by SAR and scatterometer systems, and show results from our work with ASCAT and Sentinel-1, two operational C-band radar systems capable of monitoring the land surface with a high temporal frequency. Our results suggest that sub-surface scattering phenomena are not limited to desert environments, but may also occur in other climate regions under certain, presently only poorly understood circumstances. These results may help to design new experiments and theoretical efforts to gain a better understanding of this weak but nonetheless consistent phenomenon.