Inside Unmanned Systems provides actionable business intelligence to decision-makers and influencers operating within the global UAS community. Features include analysis of key technologies, policy/regulatory developments and new product design.
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51 unmanned systems inside February/March 2017 example, can have unintended consequences such as destroying or disrupting non-targets in the vicinity. One of the critical shortcom- ings of protection systems is the integrated workf low between the detection system and the immediate interaction between the detec- tion and countermeasures systems to attain a high level of success. Figure 1 gives a workf low for consideration and forms the framework for this article on the challenge to address detecting possible threats from drones. Situational Awareness: Detection, Identifi cation and Direction Finding While no universal standard exists for the re- mote control of drones, the majority of drone technologies are radio controlled and emit an uplink (controller to device) and downlink te- lemetry or video signal back to the user. The RF link establishment between the controller and the drone can take on the or- der of 20 to 30 seconds prior to the ability of a drone to take f light. This gives a detection system based on the RF spectrum a substan- tial advantage in assessing the threat versus an electro-optical, acoustical, or radar system. While the vast majority operate in the unli- censed ISM bands at 2.4 GHz or 5.8 GHz, oth- er frequency bands are also used including 433 MHz and 4.3 GHz. Some of the older frequen- cies used by radio controlled devices include 27 MHz, 35 MHz, 40.68 MHz and 72 MHz, which could also offer an extended range of control to several kilometers. The radio controlled technologies used in- clude Frequency Hopping Spread Spectrum (FHSS); Direct Sequence Spread Spectrum (DSSS); Bluetooth®; and Wireless LAN to name a few. While drones using WLAN and Bluetooth technologies would have a very limited range in the 100's meters, the FHSS systems (e.g. HOTT, FASST, M-Link, DSMX) operating 2.4 GHz can have ranges up to 3 km. Figure 2 shows an example of a drone using FHSS technology in the 2.4 GHz ISM band. In the upper display, the RF spectrum shows the signal persistency, which is the presence of a signal over a period of time that is channel- ized into four frequency channels. The lower display shows the spectrogram, or waterfall, of the same signal over time. A detailed analysis of this signal will show this signal is hopping be- tween the different RF spectrum channels with a specific burst duration, channel sequence or hopping pattern, and at a specific hopping rate (~ 100 hops/s). From this display, the clean spec- trum and lack of other signals makes it easy to recognize the pattern of this FHSS drone. Figure 2: Example of FHSS technology used for a 2.4 GHz drone. Figure 3: A drone signal in the presence of normal traffi c from Wireless L AN.