# A Review of Counter-UAS Technologies for Cooperative Defensive Teams of Drones
## 3 Sensing System
### Sensing Technologies
#### 3.1.5 Radar Sensors  
A radar is an active sensor, consisting of a transmitting segment that radiates electromagnetic waves in the frequency range from 3 MHz to 300 GHz, depending on the application. The waves are reflected by the target objects and are received by the receiving segment of the radar. By properly processing the received signal, it is possible, for example, to calculate the time of arrival and the frequency shift due to the Doppler effect to obtain information on the distance and speed of the target. The power of the received signal is directly proportional to the radar cross section (RCS), a parameter that measures how easily an object is detectable and which depends on the size, material, distance and angle of the incident and reflected wave. The RCS of mini-sized drones and their speed have lower values than that of larger drones and aircrafts for which classic radar systems are designed. To increase the detection possibilities of drones, the micro movements of vibrating and rotating structures, such as motors and propellers, can be taken into consideration. In fact, such structures have a characterizing micro-Doppler signature (mDS) [59]), thanks to which, they can be recognized. There is a category of radars, called passive, which are not equipped with a transmitter but use the electromagnetic radiation emitted by external sources, such as those of the towers for broadcasting the DVB-T television signal, normally already present in the communication channel. This category will not be taken into consideration because it requires a priori knowledge of these sources and a static installation of the passive radar.  
The main advantages of the radar are related to the robustness against environmental conditions: the operation is independent of the light conditions and atmospheric conditions. The disadvantage is that to obtain a high detection range, it is necessary to increase the transmission power, the limit of which will depend on the power available on-board the drone. For this reason, it is not possible to use a classic surveillance radar, and the use of the FMCW type (frequency-modulated continuous wave) is preferred, which has, among other things, a more affordable cost. Signal processing can be done on software-defined radio platforms equipped with FPGA technology and RF front-end. The most popular radars in this area are mmWave and UWB radars, with one millimeter and ultra-wideband respectively.  
The literature analysis ([60; 61; 62]) showed that the detection range is typically around 100 m for millimeter-wave radars. In [63], a pair of UWB radars were used to locate a drone up to about 80 m. The classification skills are very good ([64; 65; 66; 67]), thanks to machine learning techniques. In particular, in [67], the authors were able to discern the weight of the payload of a commercial drone through the analysis of the mDS. Using a radar system operating at the 2.4 GHz frequency, the classification allowed recognition of the cases that belonged to the set {no payload, 200 g payload, 500 g payload} with 90% success and a maximum detection distance of 100 m. In addition to the analysis of the scientific literature relating to CUS systems, it is also possible to investigate that relating to the applications of radar for collision avoidance, as the detection problem is common to the two systems. In this regard, by deepening the survey proposed in [68], it is possible to confirm the feasibility of installing these systems on-board small drones, despite the stringent SWaP constraints.