# A Review of Counter-UAS Technologies for Cooperative Defensive Teams of Drones
## 4 Neutralization Systems
### Electronic Neutralizers
#### 4.1.1 Radio Frequency Jamming  
Radio frequency (RF) jamming techniques allow users to disturb, lower the quality of or interrupt communications between the malicious drone and the respective remote-control station. They consist of generating an interfering signal in order to lower the SINR (signal-to-interference-plus-noise ratio) input into the receiver of the m-drone in order to make it difficult, if not impossible, to receive the information sent by remote control. Obviously, it is also possible to jam on the remote control in order to disturb any feedback data sent by the m-drone. A jammed drone can have different reactions depending on how it is designed [5]: it can make a landing in its current position, it can perform a return-to-home procedure, it can fall to the ground without control or it can fly in a random direction with no control. RF jamming can be applied to other signals in addition to the remote control one. For example, in [70], jamming was applied to a video link used for the first-person view (FPV) function, showing the possibility of disabling this function and preventing the operator from maneuvering the drone in the absence of LOS conditions. Jamming can also be used to improve the robustness of a wireless communication in the presence of an eavesdropper. This is the case of cooperative jamming, in which a relay node transmits a jamming signal at the same time as the legitimate source transmits its message in order to disturb any eavesdropper [71]. By reversing the perspective, jamming can also be used to increase the probability of interception of a communication. In [72], for example, a legitimate drone,used to follow the flight of two suspicious UAVs in order to prevent any threats, transmits jamming signals to the receiving UAV in order to force the decrease of the exchanged data rate and increase the likelihood of interception of information exchanged between suspicious drones. There are several radio frequency jamming techniques. In this paper, we refer to the simplified taxonomy used in [10], but a more detailed one is available in [73]. The first technique, the noise jamming (also known as barrage jamming), is the simplest to implement and consists of applying a noisy signal to a portion or to the entire spectral band occupied by the signal which has to be jammed in order to reduce the channel capacity and increase the number of errors in the received data. Noise jamming can also be used in the presence of m-drones equipped with a synthetic aperture radar (SAR) able to offer autonomy thanks to SLAM techniques. In this case, the interference generated by the jamming signal may be sufficient to mask the echoes related to the SAR, making the latter unusable, as highlighted in [74]. The second technique is the tone jamming: in this case, one or more tones (i.e., narrow band signals) are employed for the purpose of generating interference. The effectiveness depends on the positioning of the tones and the transmitted power. The third technique, named sweep jamming, consists of transmitting a narrow band signal that sweeps the spectrum of frequencies of interest over time. At each instant of time, only a portion of the spectrum is covered, but in a certain period (the amount of time necessary to make a complete sweep) the whole band of interest is affected. The fourth, and final, technique is smart jamming, also known as protocol-aware jamming. It is applicable when the characteristics of the target signal are known a priori. For example, if the communication system under jamming uses frequency hopping spread spectrum (FHSS) and the hopping pattern is known, then the neutralizer can perform the same frequency hops as the target and reduce the bandwidth required by the interfering signal.