# Portable and Easily-Deployable Air-Launched GPR Scanner
## 3 Results
### Testing of Antennas Exhibiting Large Displacement of the Phase Center  
The last example is devoted to show how the proposed portable setup can be used to test different kinds of antennas, aiming to improve the detection capabilities of the GPR system. The Vivaldi antennas used for the measurements shown in Section 3.1 and Section 3.2[26] have a minimum working frequency within the range 550-650 MHz, where the radiation pattern becomes less directive (antenna gain is around 4 dB). Thus, two Log-Periodic Antennas (LPA) working in the 400-1200 MHz band, whose gain is around 7 dB in the lower part of the working frequency band [27], were tested. These antennas were mounted in the payload as shown in Figure 13. It can be observed in the same figure how the GNSS antenna was placed on top of a 1 m plastic mast to minimize interferences between GNSS and radar signals. Both Vivaldi and LPA antennas were measured at a spherical range in an anechoic chamber (Figure 14a,b), so the full characterization of the antenna parameters in their working frequency band is available. As mentioned before, Vivaldi antennas exhibit a quite flat response in the frequency domain (Figure 14c,e). However, the LPA antennas have a large displacement of the phase center, as can be observed in Figure 14d. If no phase center compensation is applied, that is, if it is assumed that the path traveled by the waves for each individual frequency is the same, the resulting SAR images would be blurred. Figure 12: Measurements in sandy soil. Picture of the portable measurement setup in the selected scenario (**a**). Picture of the 18 cm diameter metallic disk, buried 18–20 cm deep (**b**). Picture of the 18 cm diameter plastic disk, buried 16–18 cm deep (**c**). Synthetic Aperture Radar (SAR) images, normalized reflectivity: cut \(z=\) -28 cm (**d**), cut \(y=\) 22 cm (**e**), and cut \(x=\) 18 cm (**f**). Different techniques for phase center compensation can be applied. For example, a model to correct the dispersion introduced by a sinuous antenna is proposed in [28]. In this contribution, a methodology based on the measurements taken with the portable setup was conducted, avoiding the need for accurate characterization of the antennas at a spherical range in an anechoic chamber. The method is based on placing flat metallic targets above the ground (Figure 15a), so the echo of these targets can be used to detect the path length from the transmitter to the receiver at different frequency bands. As the range or vertical resolution depends on the frequency bandwidth, and taking into account that the LPA bandwidth is around 800 MHz, the LPA frequency band was divided into three sub-bands of 250 MHz each, that provide 60 cm range resolution, enough to locate the placement of the reflection at the metallic target. The position of the peak of the reflectivity for each 250 MHz sub-band is shown in Figure 15b. Figure 13: Picture of the portable air-launched GPR measurement with the Log-Periodic Antennas (LPA) antennas. Figure 14: Measurement setup of the Vivaldi antenna (**a**) and the LPA (**b**) at a spherical range in the anechoic chamber using a UWB probe antenna. Position of the phase center for the Vivaldi antenna (**c**) and the LPA (**d**). The amplitude of the measured field for the Vivaldi antenna (**e**) and the LPA (**f**). The blue line corresponds to the E-plane and the red line corresponds to the H-plane. Figure 15: Phase center compensation for the LPA. Picture of the measurement setup with the metallic reference objects (**a**). Reflectivity along \(z\)-axis (range) for a position located on top of a flat metallic object (**b**). Sub-bands of 250 MHz (\(\Delta\)R = 60 cm) are considered. Next, the position of the peak of the reflectivity for each sub-band (\(d_{offset}(f)\)) was recorded. If this distance is multiplied by the wavenumber, a phase of value \(k_{0}(f)d_{offset}(f)\)) is obtained. This phase term partially compensates the phaseshift introduced by the displacement of the LPA phase center with frequency.