## 6 Conclusions and Further Developments  
In this work, we have presented a recently developed aerial imaging radar system operating in the UHF and VHF bands as Sounder and as full polarimetric Synthetic Aperture Radar (SAR). More specifically, three operational modes are possible: Sounder, SAR-Low and SAR-High, each working at a different frequency. To obtain a first evaluation of the potentialities of the system, a helicopter-borne campaign has been conducted in May 2018 over an arid region located in southeastern Morocco, around the city of Erfoud at the northern edge of the Sahara Desert. Several tracks, covering four different test sites (denoted as TS1, TS2, TS3 and TS4), have been flown by exploiting all the three available radar acquisition modes. From the huge dataset collected during the campaign, we have processed a small subset and presented first results. In particular, to show the potentialities of the Sounder system, we have presented some representative results relevant to the TS1, TS2 and TS4 sites. To focus the data, we have applied a tomographic reconstruction approach capable to deal with the actually flown flight tracks by exploiting the information provided by the INS-GPS system mounted onboard the helicopter. To check the accuracy of the obtained results, the focused images have been compared to the Sounder-to-ground distance evaluated by exploiting the INS-GPS system and the external SRTM DEM. A good agreement between expected and obtained results has been achieved. To show the potentialities of the SAR system, we have shown some representative results relevant to the SAR-Low mode data acquired over the TS2 and TS4 sites. More specifically, to focus the data, we have applied a Back Projection approach operating in time domain and capable of exploiting the information provided by the INS-GPS system mounted onboard the helicopter and the external SRTM DEM of the observed area. To show the potentialities of the system related to its full-polarimetric capability, we have considered one radar acquisition carried out over the TS2 site and shown the correlation coefficients between the polarimetric channels as well as the obtained Pauli decomposition. It has been shown that these polarimetric products well match the scattering behavior expected for the observed area (which can substantially be considered as a rough bare soil in the presence of a limited number of well confined man-made targets). To show the interferometric potentialities of the system, we have considered three repeat pass acquisitions carried out over the TS4 site and shown the obtained interferograms along with the corresponding coherence maps. In particular, it has been shown that the obtained interferometric coherence is quite good, but for the near range areas and some well confined azimuth strips, due to the spatial decorrelation effects induced by the large spatial baselines generated by the severe track deviations of the helicopter during the radar acquisitions. By the way, in the high coherence regions, which represent the large part of the observed area, interferometric fringes are well visible. Summing up, first results relevant to both the Sounder and the SAR modes are promising. Of course, further activities aimed at assessing the full capabilities of the system are planned for the next months. In particular, full exploitation of the available SAR polarimetric channels in order to obtain added value products such as soil moisture as well as surface roughness maps [80] is matter of current investigation. Beside, acquisition of massive repeat pass interferometric data-sets is planned for the very near future, in order to fully exploit the capabilities of the UHF and VHF bands for the retrieval of the terrain topography below dense forests [81] or the subsurface structure over areas covered by snow/ice [42] through advanced tomographic SAR processing techniques.