Military Technology 06/2021

62 · MT 6/2021 C4ISR Forum are bounced back to the antenna. The radar’s micro-Doppler processing will recognise these numerous minute but important Doppler shifts. A quick look at a hobby website reveals that a typical quadcopter’s rotors can rotate at between 10,350-41,400rpm, which breaks down to between 172-690 revolutions per second. These rates can eclipse those of even the bird world’s most impressive denizens: a hummingbird beats its wings at up to 75 times per second - still not a patch on quadcopter engines. Moreover, a bird’s wings are not necessarily flapping for the en- tirety of its flight. A UAV’s rotors, on the other hand, are always turning, to provide either propulsion, for fixed-wing aircraft, or lift and propulsion for rotorcraft. If the radar starts receiving a sustained avalanche of pulses with microscopic Doppler shifts, its processor can discern that this target is a small UAV. Armed with this information, the radar can begin tracking the target. The Products Micro-Doppler processing is becoming a standard capability for ground-based air surveillance radars tasked with tracking small UAVs. There is a host of radars out there that use this approach to counter the small UAV threat, but several new products have recently graced the mar- ketplace. Blighter showcased its new A800 radar at the DSEI 2020 held in London. Company officials said that this Ku-band (13.4-14GHz/15.7- 17.7GHz) system can detect a DJI PHANTOM UAV at three kilometres. The company’s official literature states that the A800 has an instrumented range of up to 20km, referring to the distance at which the radar can detect, classify and track a target. The A800 can detect targets moving as slowly as 0.37 kilometres per hour. A bonus is that it can also detect and track ground and sea targets: a walking person can be detected and tracked at five kilometres. In the Netherlands, Robin Radar has developed a UAV detection ra- dar which MilTech saw demonstrating its abilities in a real-world setting. The company took its radar to Tartu in Estonia for TangentLink’s EW Live electronic warfare exhibition. Officials from Robin showed delegates the radar’s ability to detect and track UAVs flown during the event. Robin cut its teeth developing bird detection radars - bird strikes having developed into a major danger to aviation in the Netherlands. Robin was able to ex- ploit the software it had developed for its bird detection radars to provide true utility for its counter-UAV systems. Robin has two counter-UAV radars, IRIS and ELVIRA. Both transmit in X-band (8.5-10.68GHz), and both have an instrumented range of around five kilometres. IRIS transmits four watts of power, with ELVIRA trans- mitting twelve watts. The latter provides 10° of elevation coverage, while IRIS increases this to 60°. Company officials told MilTech that their radar can detect and classify targets as small as the Prox Dynamics BLACK HORNET UAV – an aircraft that fits in the palm of a hand and weighs a mere 18 grams. Officials added that they have conducted tests in which the radars have detected and tracked 600 targets. The next steps include further adoption of artificial intelligence approaches, to allow the radar’s use from a mobile platform. Thales has also embraced micro-Doppler processing to detect and track UAVs. In 2017, the company acquired Aveillant, based in Cambridge, whose L-band (1.215-1.4GHz) GAMEKEEPER radar helps protect runway glide paths against UAV incursions. Thales states that the radar has a range of almost seven kilometres, and can detect UAVs at altitudes of up to approximately 900 metres. One radar can be placed at either end of the runway to watch for UAVs entering the runway’s glide path. The GAMEKEEPER radar is helping to keep the skies around Paris Charles de Gaulle international airport safe from UAVs. The recent conflict in the Caucasus is a harbinger of the role UAVs will play in future wars. Likewise, the civilian world will face more danger- ous incidences of UAV use, both accidental and deliberate, in the future. Detecting errant UAVs is the first step in stopping these aircraft becoming a menace – a task in which radar can be a valuable ally. present as small an RCS as possible to hostile air defence radars. Finally, small UAVs might not behave like conventional aircraft or helicopters. The sheer manoeuvrability of small UAVs will eclipse that of larger aircraft, being able, almost literally, to stop and start in mid-air on a sixpence. They can go from the hover to comparatively high speeds in the blink of an eye. These flight patterns can be erratic at best, which can fox a radar. A target with a small RCS moving in a haphazard manner may be dismissed as a bird by the radar. The processor may discount this target and not display it to the operator. In summary, many of today’s ground-based air surveil- lance and civilian air traffic control radars may struggle to detect and track small UAVs for one or more – or all of – the above reasons. The Solutions The inherent limitations of some radars to this vexatious UAV chal- lenge caused radar engineers a period of intense thinking. A way had to be found to accurately detect and track small UAVs. The answer is micro-Doppler processing. This approach exploits a common design fac- tor for these aircraft; they tend to be powered by propeller engines. In the case of fixed-wing small UAVs, this can be a propeller mounted on either the forward or aft fuselage. Conventional rotary-wing small UAVs might have a single main rotor and tail rotor like a helicopter. Quadcopter, hexacopter and octocopter designs, however, have four, six and eight rotors respectively. All radars make use of the Doppler effect, which exploits the differ- ence in frequency between a transmitted RF pulse hitting a moving target and the changed frequency of that pulse when reflected to the radar. If a target is moving towards the radar, the frequency of the reflected pulse will increase. Conversely, if the target is moving away from the radar, the frequency of the reflected pulse will decrease. A useful example is the apparent rise and fall of a police car siren as it approaches and then passes a stationary observer. The Doppler effect is a vital radar attribute, as it shows that a target is in motion and how it is moving relative to the stationary observer. Harnessing the Doppler effect is fine if the radar is tracking convention- al aircraft. However, as noted above, small UAVs cannot be considered conventional aircraft as far as radar is concerned. A low RCS or an erratic flight pattern could cause the radar to ignore the target. Micro-Doppler processing applies the Doppler effect to the spinning rotor blades of the small UAV. Radar pulses hit these blades as they spin around, with the ra- dar observing tiny changes in the frequency of the returned pulses as they Blighter’s A800 radar was launched at the DSEI exhibition in London. Like several other radars designed to detect and track UAVs, this system uses micro-Doppler processing. (Photo: Blighter)