# A Review of Counter-UAS Technologies for Cooperative Defensive Teams of Drones
## 3 Sensing System
### Sensing Technologies Comparison
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sensing Technique** & **Detection Range** & **Classification Capability** &
\begin{tabular}{c} **Global Feature** \\ **Characterization** \\ \end{tabular} \\ \hline RF Scanner & Higher than 150 m & High & Low \\ \hline RF RSS & Higher than 150 m & Low & Low \\ \hline Acoustic & Higher than 150 m & Medium & Low \\ \hline Lidar & Between 50 m and 150 m & Low & Low \\ \hline Radar & Higher than 150 m & Medium & Medium \\ \hline VIS & Higher than 150 m & High & High \\ \hline IR & Lower than 150 m & Low & Low \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Sensing techniques’ relative performances. Table 4 is populated with the characteristics of localization/tracking and robustness against adverse environmental and meteorological conditions. The RF, acoustic, radar and optical VIS sensors have a wide detection range, in particular, the first two are able to work also in NLOS mode. Optical sensors, however, are not a good choice in the presence of adverse weather conditions, and, in particular, VIS sensors are unable to work in the absence of light. As far as the classification process is concerned, optical systems are best expressed in conditions of proximal sensing. RF and acoustic sensors use machine learning-based pattern recognition techniques for the identification of remotely piloted amateur drones. RF systems also allow for the estimation of the position of the pilot in addition to the specific model of drone used. Radars have good classification capabilities based on the micro-Doppler signature. Radar allows the direct estimation of the distance and speed of one or more drones; for this reason, it can be considered as an adequate technology for the extraction of the global features of a drone swarm, and for their localization and for operation of tracking. The use of optical sensors flanked by the ranging capability of the LiDAR allows, in this case, for the extraction of visual features that allow the determination of the geometric characteristics of the swarm, such as the occupied area and the flight configuration. Furthermore, tracking is a task that is typically dealt with via computer vision with a good level of reliability. Table 5 proposes a subdivision into \"main\" and \"complementary\" technologies on the basis of the information developed so far. For each phase of the sensing, \"main\" technologies are indicated, which have a high probability of completing the task successfully. The \"complementary\" technologies were considered those that can improve the result obtained by the \"main\" ones. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sensing Technique** & **Localization** & **Multi-Tracking** & \begin{tabular}{c} **Meteorological** \\ **Conditions** \\ \end{tabular} & \begin{tabular}{c} **Environmental** \\ **Conditions** \\ \end{tabular} \\ \hline RF Scanner & DoA & Possible & - & \begin{tabular}{c} RF Spectrum \\ congestion \\ \end{tabular} \\ \hline RF RSS & DoA & Possible & - & \
\begin{tabular}{c} RF Spectrum \\ congestion \\ \end{tabular} \\ \hline Acoustic & DoA & Yes & Wind & Noise \\ \hline Lidar & DoA/Range & Possible & Fog, rain & Direct Light \\ \hline Radar & DoA/Range/Speed & Yes & - & - \\ \hline Optical VIS & DoA & Yes & Fog, rain & Night \\ \hline Optical IR & DoA & Yes & Fog, rain &
\begin{tabular}{c} Background \\ temperature \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Sensing techniques’ tracking properties and robustness against environmental and meteorological conditions. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Task** & **Main** & **Complementary** \\ \hline Detection & Radar, Acoustic, RF & Optical \\ Classification & Optical, RF, Acoustic & Radar \\ Global Feature & Optical, Radar & Lidar \\ Localization & Radar, Lidar & RF, Acoustic \\ Tracking & Radar, Optical, Acoustic & Lidar, RF \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Main and complementary technologies.