## 4 Data Processing and Experimental Results: Sounder Mode
### First Results  
As shown in Figure 2 (red tracks), the sites TS1, TS2 and TS4 were inspected with the Sounder mode. In particular, for each test site, repeated passages of the helicopter have been flown and among the overall acquired dataset, we report in the following some representative reconstruction results.  
Figure 6 reports (right panel) the amplitude of the focused Sounder image relevant to a track flown over the site TS1. The left panel reports the Google Earth ortopho including the observed area. The red line represents the flight track projected on the ground. The focused radar image is arranged in a 2D grid: the vertical axis reports the range coordinate (measured in m); the horizontal axis represents the azimuth coordinate (expressed in latitude). In order to check the accuracy of the focusing procedure in the presence of motion errors, the focused image is compared to the Sounder-to-ground distance (red line) evaluated as the difference between the sensor altitude (provided by the navigation unit mounted onboard the helicopter) and the terrain elevation (provided by the external SRTM Digital Elevation Model). As can be observed in the figure, the red curve is in good agreement with the surface topography. A similar agreement has been observed for all the other datasets, confirming that the instrument, which includes the radar and the navigation unit, is able to accurately reproduce the surface topography variations. Figure 6 also highlights different subsurface returns at apparent depths ranging from a few tens to a few hundred meters. However, their nature is still subject of investigation and interpretation also aided by electromagnetic simulations of surface clutter. Indeed, such simulations account for the true surface tomography provided by the external DEM and can help to avoid interpreting off-nadir surface reflections as nadir subsurface echoes.  
Similar considerations hold also for all the tomographic reconstructions achieved over the different test sites inspected with the Sounder mode. Figures 7 and 8 are two representative images (relevant to two tracks flown over the sites TS2 and TS4, respectively), which clearly highlight how the surface clutter due to surface tomography variations and noise can make very challenging the interpretation of Sounder data in realistic scenarios. By the way, the Sounder results shown in the right panels of Figures 6-8, do not allow the detection of subsurface water-bearing structures, which were expected at a depth of 100 m on the basis of a priori information about the geological setting of the site. However, they show several localized subsurface reflections, which are not related to surface clutter according to the electromagnetic scattering simulations that we carried out. The interpretation of these subsurface reflections is currently under investigation and will be subject of a future work.  
Figure 6: Sounder results relevant to one flight track on TS1. Left panel: Google Earth ortopho including the observed area. Right panel: Corresponding tomographic reconstruction. Mean flight velocity: 45 m/s. Mean flight altitude: 2132 m. Mean terrain height: 930 m.