# Portable and Easily-Deployable Air-Launched GPR Scanner
## 3 Results
### Detection of Metallic and Plastic Targets Buried in Loamy Soil  
The first application focuses on the detection and imaging capabilities of landmines and IEDs. The payload consists of the same hardware described in [14] for the airborne-based GPR prototype. Taking advantage of the fact that the selected UWB radar module has two receiving ports, three antennas were mounted in the payload (see Figure 1) following an Rx-Tx-Rx configuration so the number of sweeps with the sliding system to cover the scanning area can be decreased with respect to a single Tx-Rx configuration (which requires more sweeps to avoid aliasing). UWB linear-polarized Vivaldi-like antennas, working from 600 MHz to 6000 MHz [26], were used. These antennas exhibit small displacement of the phase center with the frequency, so no phase center compensation techniques are required. Linear sweep is performed by displacing the payload along the plastic arm on top of the frame of the setup (\(y\)-axis in the path shown in red color in Figure 5a). After performing a round-trip sweep, the plastic arm with the payload are manually moved to another position (displacement along the \(x\)-axis in the rotated path shown in Figure 5a) to perform the next sweep. The average spacing between consecutive sweeps was 5 cm. Figure 5 shows the positions acquired after performing a full scan, and Figure 6 shows the roll, pitch, and yaw angles given by the IMU. In the case of the \(z\) coordinate, the overall variation is within a range of 6 cm. The fluctuations due to the direction of the sweep (forward and reverse) can be noticed in the height, as well as in the roll and the pitch. The scenario for the test was a grass field, which is a loamy soil with estimated relative permittivity between 5 and 7 and moisture around 30-50%. Due to the high propagation losses for this kind of soil, an effective working frequency from 600 MHz to 3 GHz is considered for GPR data processing of the results that have been presented in this section. This maximum working frequency ensures that, given the aforementioned cross-line spacing of 5 cm, the Nyquist sampling theorem is fulfilled when applying SAR processing. The Nyquist sampling theorem requires the inspected area to be sampled with points separated a distance equal to or smaller than \(\lambda_{min}/2\), where \(\lambda_{min}=c/f_{max}\) is the minimum wavelength. For a maximum working frequency of 3 GHz it results in \(\lambda_{min}=10\) cm, and thus \(\lambda_{min}/2=5\) cm. The used Vivaldi antennas exhibit a gain ranging from 5 dB to 12 dB within the 600 MHz to 3 GHz frequency band [26]. As first target, an 81 mm metallic mortar grenade of 30 cm length was buried at a depth of 15 cm (Figure 7a). SAR imaging results without applying the equalization of the frequency response of the system are depicted in Figure 7. As aforementioned, free-space was considered as the constitutive parameter of the imaging domain, so the reflection on the buried target is delayed proportionally to  
Figure 5: Example of measured Real Time Kinematics (RTK) positions for a single 2D acquisition grid with the portable setup. XY plane before (blue) and after (red) coordinate system transformation (**a**). Height above ground for each measurement position (**b**). Figure 6: Example of measured values of roll (**a**), pitch (**b**), and yaw (**c**) angles provided by the IMU on-board the controller for a single 2D acquisition grid with the portable setup. The solid black line represents the average value. the wave-speed in the soil. Provided that \(\varepsilon_{\rm r}\in[5.0,7.0]\), the following estimation of the true depth of the target can be calculated from the position of the echo (Figure 7b-d):  
\[36\ \rm cm/([7.0,5.0])^{1/2}=[13.6,16.1]\ \rm cm.\]  
Taking into account the considered bandwidth (BW = 2400 MHz), the theoretical range or vertical resolution would be \(\Delta\rm R=c/(2 BW)=6.25\ \rm cm\). However, it must be mentioned that no frequency response equalization is applied.