The PALS instrument provides both radar products (including normalized radar cross section in HH, HV, VH, and VV) and radiometer data (including brightness temperature at vertical and horizontal polarization). The UAVSAR is an aircraft-based fully polarimetric L-band radar installed on the NASA Gulfstream-III aircraft. During the campaign, 13 UAVSAR flights were carried out, collecting data from an altitude of 13 km over a swath () between 20 deg and 65 deg of incidence angle. Only data collected between and 45 deg were considered; then, the raw UAVSAR data were normalized to an incidence angle of 40 deg (the one of SMAP) using the histogram method.11 For this study, the data provided by the UAVSAR with a spatial resolution of were used to tune and validate the forward models. The ground measurements of soil moisture were taken together with other surface characteristics close to the time of the airborne acquisitions. In general, multiple-location sampling is required to capture a significant representation of field average moisture. For this purpose, 16 measurements of soil moisture were sampled for every field, with three replicate measurements at each sample point. These 16 sample points were arranged in two transects of eight points with a distance of 200 m, while the points were 75 m apart; the surface covered by the measures was around for each field. Moreover, the measurements were taken 100 m from the field edges. As known, backscattering is also influenced by the soil roughness; this parameter was measured once early in the campaign at two sites in each field. Considering agricultural fields, the roughness is primarily due to land management activities, and since the campaign began after seeding, further effects were not expected. As for vegetation parameters, some static vegetation proprieties (such as plant density) were measured only once, while dynamic proprieties (such as biomass and canopy water content, phenology, etc.) were repeatedly monitored. More details about the acquisition strategy and the SMAPVEX12 data can be found in Ref. 10.