# Small-UAV Radar Imaging System Performance with GPS and CDGPS Based Motion Compensation
## 3 Experimental Results
### First Test Case  
The first experimental test was performed on 5 July 2019, during a hot sunny day with weak wind state [21]. Three targets were considered: one cylindrical wood trunk (here referred as target 1) placed at 0.5 m above the ground, whose size are: 0.6 m length and 0.14 m of diameter; two metallic trihedral corner reflectors, having size 0.40 m \(\times\) 0.40 m \(\times\) 0.57 m and referred as target 2 and target 3. These latter were used as on-ground targets and target 3 was covered with a cardboard box. The targets were positioned along a straight line, with a relative distance of 10 m (see Figure 6) [21]. The main radar system parameters adopted for data collection are reported in Table 1[21]. The UAV was manually piloted (in GPS mode) and two flights at different altitudes, herein indicated as Track 1 and Track 2, were carried out. Both tracks were performed on the same scenario by positioning the UAV more or less at the same starting point \((x,y)\). The first flight had a duration of 22.3 s and covered a 36.5 m long path at the mean flight altitude of about 4.5 m; along this track, data were gathered in 319 not evenly spaced measurement points. Track 2 had a duration of 28.6 s and covered a 33 m long path at an average flight altitude about 10 m; along this track data were gathered in 409 not evenly spaced measurement points. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Specification** \\ \hline Carrier Frequency & 3.95 (GHz) \\ Frequency Band & 1.7 (GHz) \\ Pulse Repetition Frequency & 14.28 (Hz) \\ Sampling Time & 61.1 (ps) \\ Start Fast Time & 5.1 (ns) \\ End Fast Time & 122.3 (ns) \\ Integration Index & 12 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Operative Radar System Parameters: First Test Case. Figures 7 and 8 depict the raw radargrams (Figures 7a and 8a), the estimated east-north UAV trajectory and the corresponding along-track direction (Figures 7b and 8b), the estimated UAV altitudes and the corresponding average value (Figures 7c and 8c). Specifically, Figures 7b and 8b depict a zoom of the estimated east-north trajectories obtained by means of GPS (blue color) and CDGPS (red color) for both the tracks, respectively. Moreover, these Figures show the corresponding zoom of the along-track directions (dashed blue line--GPS, dashed red line--CDGPS). Similarly, Figures 7c and 8c show, as blue and red solid lines, the estimated altitudes and the corresponding averages (dashed blue and red lines). The dashed lines in Figures 7 and 8 depict the straight line obtained by means of the MoCo. In Figure 7a, three diffraction hyperbolas corresponding to targets 1, 2, and 3 are clearly visible and their apexes are placed at 5 s, 12.5 s, and 17.5 s along the slow time axis. In Figure 7a, the presence of horizontal constant signals accounts for the antennas coupling, while the signal appearing at fast time values higher than 60 ns are clutter due to lateral objects. Despite these undesired signals, the UAV radar system is capable of detecting the three targets as well as to recognize that the last encountered corner reflector (target 3) is hidden by a weakly scattering object, as testified by the presence of a small apex above the last hyperbola. Figure 6: First test case: radar imaging scenario. Figure 7: Test 1-Track 1: (**a**) raw data; (**b**) east–north UAV positions estimated by GPS (solid blue line) and CDGPS (solid red line), along-track direction defined by GPS (dashed blue line) and CDGPS (dashed red line); (**c**) UAV Altitude estimated by GPS (solid blue line) and CDGPS (solid red line), average altitude defined by GPS (dashed blue line) and CDGPS (dashed red line). For what concerns Track 2, unfortunately, the hyperbolas related to the target 1 (the wood trunk) is not clearly visible in the raw radargram (see Figure 8a). This effect is due to the smaller intensity of the field backscattered by the trunk being higher flight altitude (the radar transmits the same power whatever the flight altitude is).