# A Review of Counter-UAS Technologies for Cooperative Defensive Teams of Drones
## 3 Sensing System
### Sensing Technologies
#### 3.1.4 LiDAR Sensors  
LiDAR uses electromagnetic radiation at the optical and infrared wavelengths. It is an active sensor that emits electromagnetic waves and receives reflected waves, similarly to the operation of the radar, only at much higher frequencies, between 200 THz and 400 THz. Thanks to the calculation of the time difference between the emitted and received ray (time of flight), it is possible to process a 3D map of the surrounding environment and, thus, obtain the position, direction and speed of the objects in the scene. The speed can also be calculated from the doppler shift due to moving objects. These sensors are widely used in the automotive sector for safety systems related to autonomous driving (adaptive cruise control, lane-keeping, emergency braking). They can be used for simultaneous localization and mapping (SLAM), which allows robots to orient themselves in an unknown space and GPS-denied environment. The compact size allows it to be used on-board UAVs, both as a payload, for example for aerial mapping applications, and as avionics for collision avoidance systems.  
There are different types of LiDAR sensors: those that measure only the range, called 1D, to those that measure the angle of arrival in the azimuth and elevation, as in an optical camera, and in addition, they obtain information on the range. It should be noted that the maximum operating range depends on the reflectivity value of the material and the color of the object hit by the light radiation. Given the wavelengths, the LiDAR (especially in the 1D case) can have a reduced operation in conditions of fog, clouds or rain, but offers the advantage of being able to be used also in conditions of low ambient light (at night, for example). The processing of the data acquired by this type of sensor requires a relatively low-medium processing effort compared to other types of sensors.  
Most publications in the context of counter-UAS systems are ground-based. In [54], a LADAR (laser detection and ranging) is described, based on LASER, with a peak power of 700 kW, which allows for the increase of the operating range up to 2 km. In [55], an interesting experimental test campaign was carried out with a 3D LiDAR system mounted on a land vehicle to determine the probability of detection for mini drones. The results showed how, with sensors with a maximum operating range of 100 m, it is possible to have a high detection success rate for targets within 30 m. In [56], sensor fusion techniques were applied between a 3D LiDAR sensor and an RGB camera for detection, localization and tracking applications, with a maximum range of 50 m.  
In addition to the CUS systems, publications in the field of collision avoidance systems (CAS) for UAVs that use sensing techniques based on LiDAR were also considered. The problem of obstacle detection is very similar to that of the detection/identification and localization of malicious UAVs. In [57], machine learning and data fusion techniques were used for the combined use of 3D LiDAR and optical cameras, obtaining an obstacle detection range of about 30 m. In [58], a 2D LiDAR sensor was used, obtaining a detection range of about 8 m compared to the sensor's maximum range of 25 m. The analysis of the state of the art shows how LiDAR-based technology is widely used for the detection of targets concerning CUS systems and obstacles concerning CAS. The ability to separate objects from the background and range measurement are interesting features for this category of sensors and can be used, for example, in the extrapolation of geometric features related to a possible scenario involving a hostile swarm.