# Portable and Easily-Deployable Air-Launched GPR Scanner
## 2 Working Principle of the Portable Setup  
### Hardware Description  
The portable setup for air-coupled GPR testing is depicted in Figure 1. It consists of a plastic frame that supports a sliding arm that can be manually displaced along a distance of 150 cm. The sliding arm supports a plastic box (where the payload is placed) that can be also displaced along the arm by means of two ropes (a video illustrating this procedure can be watched at [[https://bit.ly/2](https://bit.ly/2) WlbJxz]([https://bit.ly/2](https://bit.ly/2) WlbJxz)).  
The dimensions of the sliding system and the plastic box limit the scanning area to 100 cm \(\times\) 100 cm approximately, depending on the configuration of the payload. The height above the ground can be adjusted by means of screws located on the legs of the setup, allowing a maximum height of 130 cm. These screws allow adjusting each leg individually, making its deployment easier in the case of irregular terrains.  
The payload mounted in the plastic box contains almost the same hardware to be integrated into the UAV [14]. As it can be observed in Figure 2, the main elements are: (1) the radar module, consisting of an M-sequence Ultra-Wide-Band (UWB) radar ranging from 100 MHz to 6 GHz [21]; (2) two UWB antennas (or three, depending on the number of the used receiving channels); (3) a dual-band Global Navigation Satellite System (GNSS) antenna connected to an RTK beacon [24]; (4) a micro-computer, that controls the communication between the different subsystems (positioning subsystem, radar subsystem) and a base station (a computer). The micro-computer used in this setup is the same as the one on-board the UAV [14]. The payload also includes, attached to the micro-computer, the common positioning sensors on-board UAVs, i.e., Inertial Measurement Units (IMUs), barometer and GNSS receiver. In the payload, the UAV control functionality is not required to be programmed in the micro-computer.  
Figure 3 shows a scheme of the subsystems and devices of the air-launched GPR. Communication between the payload and the base station (payload control) is made using ad-hoc WiFi, as in the case of the UAV system [14]. The router that creates the WiFi also acts as the Internet gateway. RTK corrections are taken from a GNSS base station and sent to the payload via a mobile communications network. All the elements are powered using a LiPo battery and the required voltage converters. Finally, a radiofrequency (RF) attenuator was connected between the output port of the radar module and the transmitting antenna. The reason is that the signal transmitted by the radar can interfere with GNSS signals, worsening the Signal-to-Noise ratio below the threshold required by the RTK system to provide cm-level accuracy positioning. Figure 3 shows a variable attenuator that can be adjusted from 0 dB to 11 dB. In practice, small, lightweight attenuators can be used to reduce the weight of the payload.  
Figure 1: Picture of the portable setup for air-launched Ground Penetrating Radar (GPR) imaging.  
Deployment of the setup takes around 10-15 min, requiring another 5 min to initialize all the hardware and software. Performing a full \(100\) cm \(\times\) 100 cm area acquisition usually requires 10 min. It must be pointed out that the hardware and software of the portable setup can be operated by a  
Figure 3: Scheme of the subsystems and devices of the air-launched GPR.  
Figure 2: Picture of the payload.  
single person, as the payload (radar and positioning sensors) is configured to work in continuous acquisition mode.