# Portable and Easily-Deployable Air-Launched GPR Scanner
## 3 Results
### Detection of Metallic and Plastic Targets Buried in Loamy Soil
Thus, lower frequencies will contribute more to the SAR image than higher frequencies [13; 21], so the effective range resolution is worse than 6.25 cm, as can be observed in Figure 7c,d. Figure 8 represents the XY cut of the SAR image at the position where the buried target is found with the selected measurement positions (red dots) on top, aiming to provide a comparison between the size of the acquisition grid and the imaging domain. For this example, around 5000 measurement positions were selected based on the criteria defined in [14]. Regarding the definition of the imaging domain, as explained in [14], the maximum axis-aligned rectangle in the horizontal plane inside the bounding box enclosing the observation domain (i.e., the measurements) is computed and this rectangle is shrunk by a scale factor of \(sf_{\rm t}\) and \(sf_{\rm ct}\) in the track and across-track directions respectively (to avoid edge effects in the SAR image). These scale factors were set to \(sf_{\rm t}=0.95\) and \(sf_{\rm ct}=0.85\) in this example. The imaging domain was discretized using a 2 cm step along the \(x,y\) and \(z\) axes, resulting in around 120,000 points. If equalization of the frequency response is applied in the SAR processing [13; 21], the range resolution is improved, as observed in Figure 9 (\(\Delta\rm R\) is within the range of 5-7 cm). However, this equalization also reduces the amplitude of the echo of the target, as higher frequencies suffer from more free space losses than lower frequencies. If Figures 7 and 9 are compared, there is a 5 dB loss in the amplitude of the echo of the buried mortar grenade. Figure 7: Picture of the mortar grenade (**a**). Reflectivity normalized amplitude, in dB: cut \(z=-36\ \rm cm\) (**b**), cut \(y=40\ \rm cm\) (**c**), cut \(x=-2\ \rm cm\) (**d**). Figure 8: Representation of the selected measurement positions (red dots) superimposed to the reflectivity image in the XY plane for the mortar grenade. Figure 9: Picture of the mortar grenade (**a**). Reflectivity normalized amplitude, in dB, after equalizing the frequency response of the GPR system: cut \(z\) = -36 cm (**b**), cut \(y\) = 40 cm (**c**), cut \(x\) = -2 cm (**d**). One of the issues when digging a hole to bury a target is how the fact of altering the soil affects the results. For example, in figure 24c of [21], the reflection at the interface between the dry sand (the one used to fill in the hole) and the wet, compact sand can be observed. Aiming to test the impact of the contrast between the material used to fill in the hole and the surrounding, unaltered soil, a measurement with no target buried in the hole was conducted. Results are presented in Figure 10, where it can be observed that the reflectivity level at the same position where the mortar grenade was buried is now around \(-20\) dB, that is, around \(10\) dB smaller than the reflectivity of the mortar grenade (Figure 7). Next, a \(16\) cm diameter plastic anti-personnel landmine was buried also at a depth of \(15\) cm (Figure 11a). After performing a full acquisition with the portable setup, recovered SAR images are shown in Figure 11b-d. It can be observed that the echo due to the plastic landmine is clearly visible, being \(3\) dB weaker than the echo of the metallic mortar grenade (Figure 7). Despite the fact that the mortar grenade is metallic, it has a cylindrical shape whereas the plastic landmine has flat faces that contribute to enhance the specular reflection (i.e., strong backscattering). Figure 10: Picture of the scenario under test. Dashed red line indicates the placement of the filled hole with no objects in it (**a**). Reflectivity normalized amplitude, in dB: cut \(z=-36\) cm (**b**), cut \(y=40\) cm (**c**), cut \(x=-2\) cm (**d**).