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10 Platform one April/May 2017 | Unmanned Systems Technology Test equipment maker Rohde & Schwarz (R&S) has developed a way to intercept and block transmissions from small UAVs. Instead of using cameras, microphones or radar, the Ardronis system identifies the radio link, finds the source and then disrupts the control of the UAV (writes Nick Flaherty). More than 90% of all commercial UAVs communicate over the commercially available industrial, science and medical bands at 2.4 and 5.8 GHz, while the use of the 433 MHz band is rare. They tend to use proprietary implementations of frequency-hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS), and use a broader spectrum than is required to receive control signals. FHSS alternates the carrier frequency in a pseudo-random hopping sequence, and the transmitter and receiver must be synchronised and use the same hopping algorithm in order to maintain a connection. By contrast, DSSS occupies a fixed and very large bandwidth, although it lowers the spectral power density to such an extent that the control signal is barely above the noise floor, and can only be retrieved by using a precisely matching demodulator. “Both protocols are difficult to intercept,” said YingSin Phuan at R&S. Using an online tool, Ardronis analyses the technical radio parameters such as hop lengths, symbol rate and modulation type, and is able to classify the transmission system, such as HOTT from Graupner, FASST from Futaba, M-Link from Multiplex or DSMX from Spektrum. It can then issue an alert as soon as a remote control unit begins transmitting in a restricted area, even before the UAV takes off. Direction finders and signal analysis are coupled with a detection algorithm that intercepts signals with a signal duration as low as 350 µs and can operate from 1 to 3 km depending on how crowded the spectrum is. Ardronis detects both the UAV via its downlink signals and the remote control unit via its uplink signals, so the system can determine the bearing of the operator. By using multiple direction finders, it is even possible to get a fix on the operator’s precise position. In many cases the system can detect the type of UAV by analysing its radio signature, allowing an assessment of its threat potential. The downlink activities of a UAV can also be analysed to determine whether video is being transmitted. This allows a jamming signal to selectively disrupt the radio link to a particular UAV. That forces the craft into failsafe mode so that it will either land immediately or return to its point of origin. This works when an autonomous UAV is communicating with a base station, but not if the craft is running completely independently. Then, an open interface allows integration of the detector into more complex UAV defence systems that can include other position fixing methods such as radar. Jam sessions on the radio Airborne craft The Ardronis system can identify, intercept and block transmissions to and from specific UAVs