## 1 Introduction  
Radar imaging [1] represents a powerful tool in several applications, such as security, surveillance, and environmental monitoring, with particular emphasis on disasters and crisis management [2; 3; 4]. In this frame, radar systems mounted onboard aerial platforms, such as airplanes [5; 6; 7; 8], helicopters and drones [9], are gaining increasing interest due to their features allowing to overcome several limitations of radar systems mounted onboard terrestrial vehicles (carts, cars, ground track rails) [10; 11] or spaceborne platforms [12; 13; 14]. More specifically, if compared to the terrestrial imaging radar systems, such as the conventional Ground Penetrating Radar (GPR) [10] or the Ground Based (GB) Synthetic Aperture Radar (SAR) [11], the aerial imaging radar systems are flexible and able to ensure a wider spatial coverage. Furthermore, in critical scenarios, as the ones induced for instance by natural disasters (Earthquakes, volcanoes' eruptions, flooding, rapid landslides), ungentle climatic conditions (cryosphere monitoring) or anthropic actions (ordnance deployment), they allow safe monitoring of areas that would be difficult to reach (if not unreachable) through terrestrial systems. On the other side, in contrast to the satellite platforms [12; 13; 14], the aerial ones allow to timely reach the area of interest, to fly practically along any direction, and to keep very short the so called revisiting time, that is, the time interval elapsing between subsequent observations of the same area. In addition, the aerial radar systems are relatively cheap, thus offering the appealing opportunity of assessing the potentialities of novel technologies and/or measurement modalities, before these are made operative on expensive spaceborne missions. In this regard, the increasing interest registered in the last years toward aerial radar systems operating at low frequencies has been in some extent driven by the ongoing or future spaceborne missions. Indeed, the exploitation of GPR systems for the ongoing planetary exploration missions [15; 16; 17; 18; 19] has certainly taken benefit from the results obtained through the several experimental campaigns carried out in the last years with aerial penetrating radar (usually named Sounder) systems operating in the HF, UHF and VHF bands [20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33]. For the same reason, in anticipation of the forthcoming spaceborne ESA-Biomass mission [34], increasing of the number of aerial P-Band SAR missions is registered in the last years [35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46]. In this context, the Italian Space Agency (ASI) has recently funded the development of an aerial multi-mode pulsed imaging radar system operating in a multi-frequency modality in the UHF and VHF bands. In particular, the system is able to work either as Sounder or as full polarimetric SAR. The development of this system is aimed at making available to the Italian community of researchers and end-users an aerial imaging radar system with several attractive features. First, it grows the relatively small family of available aerial SAR [35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46] and Sounder [23; 24; 26; 59] systems operating in the UHF and VHF bands. Secondly, the system allows to easily collect over the same area radar data characterized by different scattering mechanisms (since it may work either as SAR or as Sounder), at different carrier frequencies and (for the SAR modes) with a diversity in the polarization. The overall system has been originally developed by CO.RI.S.T.A. [47] according to an ASI funding set up in 2010. After, the first version of the system has been upgraded, again by CO.RI.S.T.A., in the frame of a contract signed in 2015 between ASI, CO.RI.S.T.A. and different public Italian Research Institutes and Universities, namely IREA-CNR, Politecnico di Milano and University of Trento, which have been entrusted with the processing of the data acquired by the radar.