# Bistatic Landmine and IED Detection Combining Vehicle and Drone Mounted GPR Sensors
## 1 Introduction  
The non-invasive detection of hidden or buried objects has attracted an increasing interest due to its practical applicability in several fields such as civil engineering (structural and road inspection), security and defense (landmine detection), and archeology, among others [1]. These techniques are able to detect the concealed objects without physically interacting with them or the surrounding medium. Furthermore, they can even be used to image the inspected area. Electromagnetic induction, thermal imaging, nuclear quadrupole resonance or Ground Penetrating Radar (GPR) are some well-known examples of non-invasive techniques. Among these techniques, GPR has been widely used for subsurface imaging applications [2]. It is based on transmitting an electromagnetic wave and detecting the scattered waves at the air-soil interface and from the buried targets, providing a radar image of the underground. One of its main advantages is that it can detect both metallic and dielectric targets. However, this technique is quite sensitive to the soil heterogeneity, the soil surface roughness and the possible low contrast between the soil and a non-metallic target [3]. As a result, it requires careful configuration and advanced signal processing techniques to overcome these issues and improve the detectability of the system. GPR systems can be classified using different criteria. According to the distance between the antennas and the soil, they can be classified as ground-coupled or air-launched systems. The former usually allow a better penetration into the soil and are less affected by the reflections at the air-soilinterface (provided the antennas are well-matched to the soil impedance). However, they need to be in contact with the soil, which also slows down the scanning speed and should be avoided when searching for dangerous targets such as landmines and Improvised Explosive Devices (IEDs). The latter avoid the interaction with the soil, but the strong obscuring clutter (due to impedance mismatch at the rough air-soil interface) greatly compromises the detection of buried targets. GPR systems can also be classified as Forward-Looking GPR (FLGPR) [4] and Down-Looking GPR (DLGPR) [5]. In vehicle mounted FLGPR systems, the antennas look ahead of a vehicle, with an angle of incidence that helps to maximize TM (Transverse Magnetic) waves penetration into the soil and/or to minimize reflections from the air-soil interface backscattered to the receiver. However, they have lower resolution (being difficult to distinguish whether the targets are over or under the surface) and sensitivity (since much of a flat-topped target's reflections are in the forward opposite direction from the transmitter). Concerning DLGPR systems, the antennas are perpendicular to the soil surface, which yields higher resolution at the expense of stronger clutter. In landmine detection, the scanning system must keep a safety distance from the inspected area in order to avoid the threat of explosion, which thus strongly favors FLGPR. To address this issue, a GPR system on board an Unmanned Aerial Vehicle (UAV) or a drone has been recently presented for subsurface imaging [6; 7]. This system provides high resolution subsurface images since it allows the coherent combination of measurements using a Synthetic Aperture Radar (SAR) algorithm. However, the strong clutter at the air-soil interface clearly degrades the detection capabilities, especially when the contrast between the target and the soil is low. It must also be noticed that, although bistatic SAR systems have gained an increasing interest in the last years [8], most GPR systems (both DLGPR and FLGPR) adopt a monostatic or a quasi-monostatic configuration. It would be desirable to combine the advantages of FLGPR and DLGPR systems in order to obtain both good penetration into the soil and high resolution. This article is devoted to analyzing this novel GPR configuration.