# Portable and Easily-Deployable Air-Launched GPR Scanner
## 1 Introduction  
### Background  
Ground Penetrating Radar (GPR) systems [1] are widely used for underground detection and imaging, thanks to the capability of detecting both metallic and non-metallic objects. Several applications can be cited, such as analysis and inspection of infrastructures [2; 3], archaeological survey [4; 5], glacier thickness analysis [6], material characterization [7; 8] or landmine and Improvised Explosive Devices (IEDs) detection [9; 10; 11; 12; 13]. With respect to other underground detection methodologies such as magnetometry, GPR allows detecting both metallic and non-metallic targets. Concerning the last area of application, air-launched GPR systems based on Unmanned Aerial Vehicles (UAVs) have been developed in the last years [14; 15; 16; 17; 18]. Their main advantages are: (i) safety, as contact with the ground is avoided and thus the risk of accidental IED detonation is minimized; (ii) speed, as UAV-based systems are generally faster than autonomous terrestrial robots; and (iii) capability to inspect difficult-to-access areas. Analyzing the features of the different implemented prototypes [14; 15; 16; 17; 18], it can be concluded that the main challenges are the need for precise positioning and geo-referring of the GPR measurements, the limited UAV flight autonomy, and the choice of proper radiofrequencycomponents. In this regard, the design goal is to achieve a significant bandwidth (to increase range resolution), while using low-frequency bands (to increase the penetration depth). UAV-based GPR systems for landmine and IED detection are based on air-launched GPR, where the distance from the ground to the airborne GPR is in the range of several meters. Air-launched GPR systems have several advantages over ground-coupled GPR [19; 20], especially in the case of landmine and IED detection. However, they come at the expense of fewer penetration capabilities due to the impedance mismatch between the air and the ground that, together with the free-space propagation losses, reduces the dynamic range. Improvement of UAV-based GPR systems requires exhaustive testing of the hardware, such as the GPR antennas or the geo-referring and positioning sensors. Besides, different kinds of targets buried in different scenarios (e.g., sandy and loamy soils, with different degrees of moisture) must be tested for a proper assessment of the detection capabilities of the system. This results in a large number of measurements and tests, which in the case of UAV-based GPR systems might be challenging due to the limited battery life, as well as the need for favorable weather conditions for the flights. In addition, it must be mentioned that flight and operation restrictions may limit the use of GPR systems on-board UAVs in some scenarios of interest (e.g., coastal areas and beaches, aiming to test sandy soils). One possibility for conducting the aforementioned tests is the use of GPR indoor measurement facilities, which usually consist of a planar scanner equipped with motors and encoders that allows accurate positioning, being the scanner placed on top of a scenario filled with the kind of soil and buried objects to be tested. This kind of setups can be used for testing Down-Looking GPR [21; 22] and Forward-Looking GPR systems [23]. However, measurement setups based on planar scanners do not allow testing the uncertainties that UAV-based GPR systems have to face [14], such as positioning and geo-referring uncertainty due to the sensors of the positioning subsystem on-board the UAV, or the need of working with irregular, non-uniform acquisition grids due to the deviation of the UAV from the pre-defined flight path. In the case of indoor GPR testing facilities, positioning uncertainty introduced by sensors on-board the UAV such as Real Time Kinematics (RTK) cannot be tested due to the lack of satellite coverage.