The present work addresses the problem of focusing moving target in SAR images. This task is here solved by using an Inverse Synthetic Aperture Radar (ISAR) technique. The ISAR technique performs an autofocus procedure by implementing exhaustive search algorithms, improved by classical convex optimization, of functions based on image contrast or entropy. In this work, we discuss the possibility to perform an autofocus ISAR technique by exploiting the estimation of the target Doppler parameters: namely the Doppler centroid and the Doppler rate, which are related to the target motion parameters. The present algorithm is based on the re-use of efficient autofocus approaches that are classically used in direct SAR imaging. The effectiveness of the proposed method is tested on Cosmo-Skymed Spotlight SAR data of maritime targets. Furthermore, the present Doppler Parameter Estimation Algorithm (DPEA) is compared with a well known ISAR technique, namely the Image Contrast Based Technique (ICBT).

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This paper deals with a feasibility study assessing the reconstruction capabilities of a small Multicopter-Unmanned Aerial Vehicle (M-UAV) based radar system, whose flight positions are determined by using the Carrier-Phase Differential GPS (CDGPS) technique. The paper describes the overall radar imaging system in terms of both hardware devices and data processing strategy for the case of a single flight track. The data processing is cast as the solution of an inverse scattering problem and is able to provide focused images of on surface targets. In particular, the reconstruction is approached through the adjoint of the functional operator linking the unknown contrast function to the scattered field data, which is computed by taking into account the actual flight positions provided by the CDGPS technique. For this inverse problem, we provide an analysis of the reconstruction capabilities by showing the effect of the radar parameters, the flight altitude and the spatial offset between target and flight path on the resolution limits. A measurement campaign is carried out to demonstrate the imaging capabilities in controlled conditions. Experimental results referred to two surveys performed on the same scene but at two different UAV altitudes verify the consistency of these results with the theoretical resolution analysis.Remote Sens. 2020, 12, 774 2 of 22 UAV-based radar imaging receives huge attention in several military and civilian applications, such as surveillance, security, diagnostics, monitoring in civil engineering, cultural heritage and earth observation, with particular emphasis on natural disasters, which should be safely and timely monitored [8]. At the state-of-the-art, radar imaging performed by M-UAVs has been proposed for precision farming [9], forest mapping [10] and glaciology [11]. In addition, M-UAVs have been exploited to perform Synthetic Aperture Radar (SAR), avoiding large platforms when monitoring small areas. In this frame, the first experimentation concerning interferometric P and X band SAR systems onboard UAV platforms has been reported in [12], while a UAV polarimetric SAR imaging system has been proposed in [13]. UAVs have been also exploited in the field of landmine detection as platforms equipped with Ground Penetrating Radar (GPR) systems [14,15].Despite these promising examples, the development of radar systems onboard M-UAV is at an early stage and M-UAV radar imaging still represents a scientific challenge, especially when small and light M-UAV platforms are deployed. Indeed, the full exploitation of smart and flexible M-UAV imaging radar systems requires the development of reconstruction approaches able to deal with non-conventional data acquisition configurations, where data are not collected along a straight linear trajectory due to environmental conditions or presence of obstacles. In this respect, it is worth pointing out that radar imaging, i.e., the possibility to obtain a focused image of the investigated region, strongly depends on the accurate knowledge of platform po...