## 5 Experimental Micro-Doppler Signatures
### Bird Characteristics  
In this section, the setup for collecting data on birds is explained. Bird flights were collected with a 10-GHz radar at Shaver's Creek, a bird sanctuary near Penn State University. Most of the birds in this ariary are disabled, but this did not significantly affect their flight patterns. Through the help of the trainers at Shaver's Creek, birds flew from perch to perch, as the radar system recorded the induced Doppler shifts. The enclosure was wooden with wire meshing in between columns. Since the radar system only tracks radial movement, the enclosure did not play a role in the spectrogram quality. The reflections from the wall behind the bird did not affect its micro-Doppler signature. However, this was not verified by a bird flight outside of the enclosure. Figure 18 shows the bird enclosure and radar system. Four different birds were used in this experiment: a bald eagle, golden eagle, broad-winged hawk, and American kestrel. These are shown in Figure 19 (obtained from [[https://www.allaboutbirds.org/guide/](https://www.allaboutbirds.org/guide/)]([https://www.allaboutbirds.org/guide/](https://www.allaboutbirds.org/guide/)), accessed on 11 March 2022). The physical characteristics of each bird are provided in Table 6.  
Figure 20 shows the bald eagle spectrogram. Within this spectrogram, a large velocity bandwidth is initially observable at around 300 ms. This represents the downstroke of the bird, quickly followed by an upward flap. The back half of the spectrogram shows the soaring of the bird, as it mechanically controls the rest of the flight. Figure 21 shows the golden eagle spectrogram. The golden eagle spectrogram is quite similar to the bald eagle, with strong flaps present at 200 and 500 ms. Unlike the UAVs, the flapping is not consistently periodic because flapping requires large amounts of energy. If the birds had a larger environment to fly in, the micro-Doppler signatures may look quite different because soaring would be more viable. Figure 22 shows the spectrogram for the broad-winged hawk. The broad-winged hawk is the second smallest bird. Smaller birds require a higher periodicity in flaps compared to eagles, but their flaps do not need to be as strong because they are supporting a smaller payload. The micro-Doppler signature is quite cluttered because the body of the bird undulates. This makes the peaks of the wing flaps less pronounced. The maximum velocity achieved by the broad-winged hawk was around 5 m/s. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Bird Feature** & \
\begin{tabular}{c} **American** \\ **Kestrel** \\ \end{tabular} &
\begin{tabular}{c} **Broad-Winged** \\ **Hawk** \\ \end{tabular} & **Golden Eagle** & **Bald Eagle** \\ \hline \multirow{3}{*}{Weight} & 68–163 gm & 340–454 gm & 2.95–6.12 kg & 3.00–6.30 kg \\  & (0.15–0.36 lb) & (0.75–1 lb) & (6.5–13.5 lb) & (6.6–13.9 lb) \\ \multirow{2}{*}{Length} & 21.0–31.8 cm & 34.3–44.5 cm & 69.9–83.8 cm & 71.1–96.5 cm \\  & (8.25–12.5 in) & (13.5–17.5 in) & (27.5–33 in) & (28–38 in) \\ \multirow{2}{*}{Wingspan} & 50.8–61.0 cm & 81.3–101.6 cm & 1.83–2.13 m & 1.83–2.29 m \\  & (20–24 in) & (32–40 in) & (6–7 ft) & (6–7.5 ft) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Bird characteristics. Figure 19: Bird types. (**a**) Small Bird 1: American kestrel; (**b**) Small Bird 2: Broad-winged hawk; (**c**) Big Bird 1: Golden eagle; (**d**) Big Bird 2: Bald eagle. While Figures 20-23 depict the spectrograms of each bird flying almost horizontally from one end of its enclosure to the other, Figure 24 shows a comparison between a bald eagle flying down vs. flying up. Both flight types (flying down and flying up) were collected independently and separately, as the bird flew from one end of its enclosure to the other. The micro-Doppler bandwidth is larger when flying up, which makes sense because the wings need to flap harder to generate more thrust (similar to the quadcopter). Through bird data collection, the biggest challenge was getting the birds to fly. On hot days, the birds did not want to cooperate.