This paper presents a novel system to obtain images from the underground based on ground penetrating radar (GPR). The proposed system is composed by a radar module mounted on board an unmanned aerial vehicle (UAV), which allows the safe inspection of difficult-to-access areas without being in direct contact with the soil. Therefore, it can be used to detect dangerous buried objects, such as landmines. The radar measurements are coherently combined using a synthetic aperture radar (SAR) algorithm, which requires cm-level accuracy positioning system. In addition, a clutter removal technique is applied to mitigate the reflection at the air-soil interface (which is caused by impedance mismatching). Besides the aforementioned advantages, the system can detect both metallic and dielectric targets (due to the use of a radar instead of a metal detector) and it allows to obtain high-resolution underground images (due to the SAR processing). The algorithms and the UAV payload are validated with measurements in both controlled and real scenarios, showing the feasibility of the proposed system.

Advances in unmanned aerial vehicle (UAV) technology have fostered its use in a wide range of areas, such as agriculture and forestry, surveillance and security, and infrastructure inspection. One of the advantages of UAVs is their ability to conduct remote inspection and sensing by placing different kinds of sensors on board them. In this sense, UAV-based ground-penetrating radar (GPR) systems are of particular interest as they bring together the advantages of UAVs and GPR, resulting in contactless subsurface sensing and imaging systems capable of performing a fast scanning of difficult-to-access scenarios. This contribution reviews the advances on UAV-based GPR systems, describing their architecture and subsystems. In particular, an analysis of different UAV-based GPR systems is presented, focusing on the technical solutions adopted in each case and the detection capabilities that have been achieved. Attention will be also given to the methodologies implemented to obtain 3D high-resolution images of the underground. Finally, the main challenges faced by these systems concerning further improvements of the scanning throughput and the detection accuracy will be discussed.

This paper presents a compact, low-cost unmanned aerial system for antenna measurement. The proposed system overcomes existing limitations in terms of unmanned aerial vehicle positioning and data geo-referring accuracy using a real-time kinematic positioning system to achieve centimeter-level accuracy. Amplitude-only measurements acquired using a low-cost power sensor are processed by means of the phaseless sources reconstruction method. This is an iterative phase retrieval technique that allows recovering an equivalent currents distribution, which characterizes the antenna under test (AUT). From these equivalent currents, near-field to far-field transformation is applied to calculate the AUT radiation pattern. This contribution also analyzes probe antenna characterization and the impact of positioning and geo-referring accuracy on the radiation pattern. Two application examples of antenna measurement at S- and C-bands using the implemented system are presented.

This work presents an enhanced autonomous airborne Synthetic Aperture Radar (SAR) imaging system able to provide full 3D radar images from the subsurface. The proposed prototype and methodology allow the safe detection of both metallic and non-metallic buried targets even in difficult-to-access scenarios without interacting with the ground. Thus, they are particularly suitable for detecting dangerous targets, such as landmines and Improvised Explosive Devices (IEDs). The prototype is mainly composed by an Ultra-Wide-Band (UWB) radar module working from Ultra-High-Frequency (UHF) band and a high accuracy dual-band Real Time Kinematic (RTK) positioning system mounted on board an Unmanned Aerial Vehicle (UAV). The UAV autonomously flies over the region of interest, gathering radar measurements. These measurements are accurately geo-referred so as to enable their coherent combination to obtain a well-focused SAR image. Improvements in the processing chain are also presented in order to deal with some issues associated to UAV-based measurements (such as non-uniform acquisition grids) as well as to enhance the resolution and the signal to clutter ratio of the image. Both the prototype and the methodology were validated with measurements, showing their capability to provide high-resolution 3D SAR images.

An improved Ground Penetrating Radar (GPR) system on board an Unmanned Aerial Vehicle (UAV) is presented in this contribution. The system has been designed for the detection and imaging of buried targets and, in particular, landmines and Improvised Explosive Devices (IEDs). Resting on the hardware and architecture of a previous aerial platform, in the proposed system the scanning area is increased and the detection capabilities are improved. These improvements are achieved by employing two receiving antennas and new processing techniques that increase the Signal-to-Clutter Ratio of the GPR images. Besides, parameters affecting the GPR image resolution, such as the flight speed and the amount of measurements that can be processed together using Synthetic Aperture Radar (SAR) techniques, are also studied. The developed system exhibits several advantages: safety and faster scanning speeds, together with the capability to detect both metallic and non-metallic targets, as shown in the examples presented in this contribution.

The usage of Unmanned Aerial Vehicles (UAV)-based Ground Penetrating Radar (GPR) systems has gained interest over the last years thanks to advantages over ground-based systems such as contactless inspection and capability to reach difficult-to-access areas. The former is of paramount importance concerning the detection of buried threats such as Improvised Explosive Devices (IEDs) and landmines. Current state-of-the-art UAV-based GPR systems are able to provide centimeter-level resolution thanks to the use of GPR-Synthetic Aperture Radar (SAR) processing techniques. One of the challenges to keep improving these systems is the scanning throughput, that is, the area that can be scanned in a given time. This contribution presents an array-based GPR-SAR system for subsurface imaging, aiming at maximizing the scanning throughput without jeopardizing the imaging capabilities of the system. First, the antenna array is mounted on a portable setup to evaluate its performance and imaging capabilities. Next, the antenna array is integrated into the UAV platform, and the UAV-based GPR-SAR system with the array is tested in realistic scenarios with different kinds of buried targets. Results show that the scanning throughput is significantly improved and, furthermore, the coherent combination of all transmitting-receiving channels of the array provides enhanced detection capabilities.