# Small-UAV Radar Imaging System Performance with GPS and CDGPS Based Motion Compensation
## 3 Experimental Results
### Second Test Case  
The second test was carried out on the 21 February, 2020, during a cold day with a moderate wind state. In this second case, four targets were placed along a straight line (see Figure 13). Target 1 was a couple of chipboard shelves, having size 0.38 m \(\times\) 0.60 m, placed at 0.70 m above the ground and with a relative distance of 5 m from the target 2. The target 2 was the same wood trunk adopted in the previous experiment, placed at 0.5 m above the ground and with a distance of 10 m far from target 3. Target 3 was a void inside small box of plasterboard, having size of 0.53 m \(\times\) 0.53 m \(\times\) 0.1 m. Finally, as target 4 was used a tuff brick of dimensions 0.27 m \(\times\) 0.41 m \(\times\) 0.14 m, placed on ground and far 5 m form target 3. The upper side of target 3 and target 4 are 0.53 m and 0.27 m from the ground, respectively. Two flights were performed, again with manual UAV piloting in GPS mode. It is worth pointing out that due to the wind effect, Target 3 was repositioned before carrying out the second flight and, in order to assure its stability, which was compromised by foliage presence on the ground, it was located 9 m far from Target 2 and 6 m far from Target 4. The radar operative parameters adopted for this second test case are reported in Table 4.  
The first flight covered 27 m in 41.9 s with an average flight altitude of about 4.2 m; along this track, 420 not evenly points were collected. Track 2 had a duration of 37.9 s and covered 32.20 m with an average altitude of 5.6 m; along this track, 380 unevenly radar scans were collected. Figures 14 and 15 show the raw radargrams acquired along the two flights and the UAV positions estimated by GPS and CDCPS. Within the east-north plane, positioning differences appear as smoothly varying offsets of several meters (Track 1) or a few meters (Track 2). As concerns the altitude difference, it shows some significant variations during the time interval corresponding to Track 1, while it assumes smaller values in Track 2. As stated above, CDCPS can be assumed as a reference for standalone GPS performance. The diffraction hyperbolas corresponding to the four targets are clearly visible in Figure 14a and their apexes along the slow time axis are at 9 s, 19 s, 31 s and 37 s. In Figure 15a, the hyperbolas corresponding to Target 1, 2, and 4 can be easily identified at 9 s, 15 s, and 33 s; while the response of \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Specification** \\ \hline Carrier Frequency & 3.95 (GHz) \\ Frequency Band & 1.7 (GHz) \\ Pulse Repetition Frequency & 10 (Hz) \\ Sampling Time & 61.8 (ps) \\ Start Fast Time & 5.1 (ns) \\ End Fast Time & 87.2 (ns) \\ Integration Index & 12 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Operative Radar System Parameters: Second Test. Figure 13: Second Test Case: radar imaging scenario. Target 3 is less visible. This may be due to less intensity of the backscattered signal caused by the flight altitude, which is higher with respect to the Track 1. In this second test case, the flight trajectory estimated by GPS and CDGPS have a similar path into the North-East plane, even if there is a bias that is more significant for Track 1 than for Track 2 (see Figures 14b and 15b); while the altitudes exhibit different profiles, even if their average values are quite similar. The tomographic images referred to Track 1 and Track 2 are depicted in Figures 16 and 17, respectively. These images have been obtained by adopting the signal processing parameters indicated in Tables 5 and 6, respectively. Respect to the previous test case, here, we want to remark that the Subdomain Apertures have the same size, i.e., 4 m. This parameter has been chosen again by considering the target hyperbola extent in the processed radargrams.