R.D. Gall – Post-graduate Student, Peter The Great St. Petersburg Polytechnic University
S.B. Makarov – Dr.Sc.(Eng.), Professor, Director of Institute of Physics, Nanotechnology and Telecommunications of Peter The Great St. Petersburg Polytechnic University
Detection of small-sized unmanned aerial vehicles (UAVs), for example, in areas of airports, stadiums, entertainment sites with large crowds of people is the first step in the fight against the illegal use of UAVs. The second stage is the rapid jamming of the UAV control signals to minimize the risk of the protected area invasion. As a rule, these two stages are combined.
More than 80% of the remote control systems of the multi-rotor UAV, on the physical level, use signals with frequency hopping and a simplex communication protocol. According to this protocol, control signals are transmitted only in the direction of the remote control - the UAV. All UAVs use the same protocol, but the individual sequence of frequency hops (the frequencies in the time-frequency matrix are assumed known) with the same repetition period τ. Commands from the remote control to the UAV are transmitted at the same center frequency (considered known) and in packet mode with pseudo-random frequency hopping (FHSS): each packet is transmitted at its own frequency from the time-frequency matrix and has a frequency shift keying (2-FSK).
The aim of the work is to develop a method to detect the control signals of the UAV group, classify them (determine the law of frequency hopping) and jam the control signals while the UAV is in the jamming zone.
The method is based on the division of the process of jamming into two time stages: until the moment of determining the frequency hops sequence of each detected control line and after the moment of determining.
At the first stage, the time intervals of the detector and the jammer operation replace each other, and the structural interference will coincide in time only with some part of the packet. At the second stage, after determining the sequence of frequency hops, it is supposed to use a synchronous structural interference. Such a structural interference will coincide in time with the entire packet.
To identify the frequency hops of the UAV control signals, we use a comb of bandpass filters in the entire band of signal (the bandwidth of each bandpass filter is equal to the bandwidth of each frequency channel). Let’s consider the filtered signal at the output of each filter. Then one of the five hypotheses is correct.
The first hypothesis corresponds to the case only noise was present in the frequency channel. In this case, the control interval will be carried out again at the next interval.
The second hypothesis corresponds to the case when a mixture of signal and noise was present in the frequency channel. But neither the beginning nor the end of the control signal packet were in the control interval. In this case, at the next interval, a structural interference will be radiated.
If the third, the fourth or the fifth hypotheses is valid, the beginning of the packet, the end of the packet or the end of one packet and the beginning of the other were in the control sample. In these cases, the time moment of packet beginning or end should be estimated (depending on the hypothesis). At the same time, we use the generalized maximum likelihood criterion and maximum likelihood estimate to distinguish between hypotheses and simultaneously evaluate these parameters. If the third hypothesis is correct, the structural interference is radiated during the time Th – (τк – ), where is the time estimation of the packet beginning. If the fourth hypothesis is correct, the control interval will be made again at the next interval. If the fifth hypothesis is correct, the interference is emitted in the corresponding frequency channel during the time (τк – ), where is the time estimation of the packet beginning.
By simulation modeling, it was shown that the developed method provides a higher degree of structural interference than the asyn-chronous method with an energy detector, and this makes it possible to obtain a time advantage for suppressing the UAV control signal.
With a signal-to-noise ratio greater than 3−10 dB, the developed method provides the UAV control signal jamming at the end of ten received packets, which allows the control signal jamming over a double period of the received packets frequency hops sequence. For example, for the Futaba FASST UAV control system, the conditions for jamming the control signal will be fulfilled after approximately 576 ms, during which the UAV will have time to fly about 20 meters.
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