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The paper deals with development and validation of a VHF band airborne radar designed for subsurface exploration. The radar system is a pulsed one and transmits a chirp pulse at the central frequency of 165 MHz with 40 MHz bandwidth. A preliminary system validation has been performed by means of flight trials over Morocco desert in May 2018. An ad-hoc signal processing technique has been developed to produce focused images of the scene under investigation starting from raw and motion uncompensated data. In particular, the data processing technique is based on a microwave tomographic approach that has the novel feature to compensate the distortion effects produced by the non-rectilinear motion of the aircraft based on navigation data. The analysis and interpretation of the data is supported by comparison with electromagnetic simulations of the scene.

This letter deals with the design, realization, and validation of a multimode/multifrequency airborne radar designed for both surface and subsurface prospections. The system operates in the frequency band from very high frequency (VHF) to ultrahigh frequency (UHF) and works in two different modes: 1) a nadir-looking sounder in the VHF band (carrier frequency of 163 MHz); and 2) a side-looking imager (i.e., synthetic aperture radar) in the UHF band with two channels at 450 and 860 MHz, respectively. The system validation has been carried out for the “sounder” mode due to helicopter-borne surveys carried out over an area in the Campania region, Southern Italy. The surveys have provided a first proof of system capability in obtaining useful information about the surface and shallower subsurface layers over a large scale and in a relatively short time. In particular, the data collected by the sounder have been processed by means of a microwave tomographic reconstruction approach, and features consistent with tunnels buried at a depth of 15 m have been identified.