# A Review of Counter-UAS Technologies for Cooperative Defensive Teams of Drones
## 3 Sensing System
### Sensing Technologies
#### 3.1.2 Radio Frequency Sensors  
Radio frequency (RF) sensors capture the electromagnetic signals radiated by a malicious drone or by the remote pilot's radio control, if present. It is, therefore, a passive method that does not require the transmission of electromagnetic waves and, therefore, has no restrictions on use (e.g., in an urban environment). Most commercial drones use an uplink radio channel for remote control commands and a downlink channel for telemetry and video signal. In the case of autonomous drones, there may be only direct downlink transmission to the ground control station (GCS) or communication between the nodes of the network in a swarm. The detection systems based on this technology make use of a RF receiver between 400 MHz and 6 GHz and an array of antennas for the possible exploitation of MIMO techniques. The receiver can be implemented through software-defined radio (SDR) due to the reconfigurability and flexibility characteristics of the radio frequency front-end and associated baseband processing. RF detection can be performed with techniques based on a known protocol or recognition of the spectral pattern. In this case, we refer to drones that communicate with the remote pilot through communication standards such as IEEE 802.11 (Wi-Fi), a case that covers a large part of commercial drones. In this way, it is possible to retrieve the MAC address of the device and trace the specific drone model.  
In addition to the recognition of the spectrum and the communication protocol, it is possible to recognize the RF fingerprint of the radio controller and then carry out the classification of the drone through machine learning techniques, as described in [40]. These techniques are not very effective if a known pattern is not used, if the communication scheme has been customized or if the MAC address database is not updated ([41]).  
For this reason, techniques based on the localization of the RF signal have been developed. So, the DoA estimation is carried out in two different ways: based on received signal strength (RSS) or spectral analysis. In the first case, the results are less accurate than in the second. For example, in [42], an architecture based on an array of four antennas and an SDR platform for processing was proposed, in which an angular precision between 1.9\({}^{\circ}\) and 6\({}^{\circ}\) was achieved over a coverage range between \(-60^{\circ}\) and 60\({}^{\circ}\). In [43], an experiment was presented in which, thanks to the use of commercial SDR platforms (FPGA-based), it was possible to localize small drones with a maximum range of 75 m. Although previous publications have been ground-based, they described techniques that could also be used on-board. In [15], a UAV-based system was described, in which a tracker drone can track an intruder drone by measuring RSS. The coordinated use of multiple drones for locating the RF source was also considered in [44; 45]. In the latter, the air-to-air communication channel was simulated and compared with the ground-to-air one in an urban context. The research aimed to analyze the differences in terms of location accuracy vs. SNR. The results showed a clear advantage for the air-to-air solution.