Contents lists available at ScienceDirect Remote Sensing of Environment journal homepage: www.elsevier.com/locate/rse Assessment of a micro-UAV system for microwave tomography radar imaging Giovanni Ludeno a , Ilaria Catapano a , Alfredo Renga b , Amedeo Rodi Vetrella b , Giancarmine Fasano b , Francesco Soldovieri a, ⁎ a Institute for Electromagnetic Sensing of the Environment (IREA) - National Research Council of Italy (CNR) Napoli, Italy b Department of Industrial Engineering (DII) University of Naples “Federico II” Napoli, Italy ARTICLE INFO Keywords: UAV system Radar survey Imaging Data processing Microwave tomography ABSTRACT Micro-Unmanned Aerial Vehicles (UAVs) are flexible observation platforms suitable to cover inaccessible areas on demand. Accordingly, huge attention is deserved towards development of miniaturized sensing technologies compliant with UAV payload constrains and capable of providing high-resolution images of the region under test. As a contribution to this topic, the paper presents a prototype UAV system for radar imaging made by a commercial micro-UAV equipped with a miniaturized low power radar, whose imaging capabilities are en- hanced by a properly designed data processing strategy. Such a strategy involves a processing step performed in time domain, which accounts for a procedure devoted to compensate flight altitude variation and a Singular Value Decomposition (SVD) based noise filtering approach. After, the focused images of the surveyed scenario are obtained by using a microwave tomographic approach, which integrates data about UAV position and faces the imaging as a linear inverse scattering problem. A feasibility experiment, carried out to test the operational mode of the assembled system, is presented. The obtained results corroborate that the integration of GPS and flight altitude information into the microwave tomography approach allows valuable radar imaging capabilities in terms of target localization accuracy. 1. Introduction Unmanned Aerial Vehicles (UAVs), also referred to as drones, re- present a valuable alternative to conventional observation platforms, i.e. satellite, manned aircraft and ground-based systems. Indeed, they allow the acquisition of high-resolution remote sensing data with high operational flexibility, relatively low cost hardware, manageable op- erative conditions and great versatility. Beyond military purposes, UAV based sensing technologies are gaining huge attention for civil appli- cations such as border protection, coastal surveillance, hurricane and polar ice cap monitoring, forest fire and/or natural disaster detection, aerial photography, crop dusting, package delivery, pipeline and power line monitoring (Whitehead and Hugenholtz, 2014). The use of UAV in the field of photogrammetry is now well assessed (Colomina and Molina, 2014; Toth and Jóźków, 2016) in applications such as coastal area monitoring (Gonçalves and Henriques, 2015; Anastasios et al., 2016), structural geology (Bemis et al., 2014), gla- ciology (Bhardwaj et al., 2016) and cultural heritage surveys (Remondino, 2011). In addition, thermal cameras on UAV (possibly combined with daylight cameras) represent a good solution for monitoring territory and structures (Yahyanejad and Rinner, 2015). Hyperspectral sensors can be deployed on UAV for vegetation mon- itoring (Aasen et al., 2015), whereas laser scanner on UAV has been operatively deployed for power lines surveys (Matikainen et al., 2016). With respect to the above-mentioned sensing technologies, the de- ployment of radar systems suitable for UAV installation represents an interesting possibility. Radar systems are, indeed, able to operate in all- weather and day/night conditions and, thanks to the ability of micro- waves to penetrate materials, they make possible to detect and localize not only surface objects but also buried/hidden targets. The use of radars on aerial platforms (such as airplanes and heli- copters) is now becoming an assessed solution and several systems and methodologies are present in literature (Hellsten, 1992, Papa et al., 2014; Catapano et al., 2014a, 2014b; Eisenburger et al., 2008). Here, we focus on the possibility to deploy an ultra-wide band (UWB) radar system on a micro-UAV. In this way, it is possible to pave the way to the pervasive use of this kind of systems in several appli- cations, ranging from the support to search and rescue operations during crisis events, to the surveys in cultural heritage and agriculture. In particular, the joint use of miniaturized radar systems, which are https://doi.org/10.1016/j.rse.2018.04.040 Received 29 November 2017; Received in revised form 19 April 2018; Accepted 21 April 2018 ⁎ Corresponding author. E-mail address: soldovieri.f@irea.cnr.it (F. Soldovieri). Remote Sensing of Environment 212 (2018) 90–102 0034-4257/ © 2018 Elsevier Inc. All rights reserved. T