Polarimetric X-and C-band measurements by the University of Sheffield ground-based synthetic aperture radar (GB-SAR) indoor system provide three-dimensional images of the scattering processes in wheat canopies, at resolutions of around a wavelength (3-6 cm). The scattering shows a pronounced layered structure, with strong returns from the soil and the flag leaves, and in some cases a second leaf layer. Differential attenuation at horizontal (H) and vertical (V) polarization, due to the predominantly vertical structure of the wheat stems, gives rise to marked effects. At both C and X bands, direct return from the canopy exceeds the soil return at large incidence angles for VV polarization, but is comparable to or less than the soil return in all other cases. At HV, the apparent ground return is probably due to a double-bounce mechanism, and volume scattering is never the dominant term. Direct sensing of the crop canopy is most effective at X band, VV, and large incidence angles, under which conditions the return is dominated by the flag leaf layer. Field measurements with the outdoor GB-SAR system suggest, however, that for sensitivity to biomass and reduced susceptibility to disturbances by rainfall, a two-channel C-band system operating at a medium range of incidence angles is preferred.Index Terms-Agriculture, backscatter modeling, synthetic aperture radar (SAR).

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Differential interferometric synthetic aperture radar (DInSAR) has traditionally been used for the detection and accurate monitoring of surface movement in a scene and has found applications in fields such as mining subsidence and earthquake deformation. In these studies, the phase is understood to directly relate to the radial component of the physical deformation of the surface. In this paper, however, we use a novel combination of microwave and optical laboratory measurements to demonstrate the presence of persistent and coherent phase changes in a temporal sequence of DInSAR images, related solely to moisture change in a soil. This is confirmation of recent reports suggesting that, in some circumstances, the DInSAR signal may include a significant soil moisture signal. Laboratory measurements were used to obtain a set of high-resolution C-band DInSAR images of a sandy soil sample of an area of 2.0 m × 1.8 m and a depth of 0.2 m, with the fractional volumetric water content varying between 0.1 and 0.4. To independently monitor the soil surface for physical movement, a time-lapsed set of high-resolution digital optical images was continuously acquired. Although the soil underwent a large moisture change, the soil surface was static to within ±0.1 mm over the majority of the experiment. The DInSAR sequence displayed dynamic and complex variations of the phase, although a linear relationship with moisture change was evident when the mean phase change was considered. The work raises the possibility that DInSAR could be used for the monitoring of soil moisture change in a scene, a parameter of significant economic and environmental importance.Index Terms-Differential interferometric synthetic aperture radar (DInSAR), ground-based SAR (GB-SAR), interferometry, soil moisture.

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