There is a number of very different technologies on the market to detect drone intrusion. The list includes radar (millimetre and centimetre wave), acoustic sensors, infrared detectors, video surveillance systems, Radio Frequency (RF) sensors. Many respected brands represent these technologies, including ELVA, Thales, DroneShield, Drone Detector, Aaronia, Kelvin Hughes, etc. Each of these technologies has its own pros and cons, and their effectiveness depends a lot on the circumstances.
Acoustic detection (aka audio detection) initially came from Navy military labs, and it is a very sophisticated method for surveillance. Unfortunately, it only works in environments where there is no high ambient (background) noise. For example, if located at an airport perimeter, the microphones would be overloaded by the heavy noise of the aircraft engines. If there was a criminal drone pilot, they could mask the drone intrusion by timely overlap with aircraft take-off/landing. Also acoustic drone detection may be useless in urban areas, especially when installed at a noisy open air events like parades, music festivals, etc. Another drawback of acoustic detection is its low-range capabilities, with reliable detection distance of 25 to 100 feet on average. In many cases, it is too close to be able to protect the public as it is too late to react. The so called sound signature of a drone that is advertised as key factor of drone recognition, could be easily mystified by buying another kind of propellers or making more modifications to a drone.
Infrared detection ((aka thermal detection) come to anti-drone market as modification of infrared security cameras with motion sensors. Infrared detection has a longer range than acoustic sensors, and its effective range is about 300 feet or even more. The drawback of the infrared technology is in the fact that a large bird produces much more heat than a plastic drone with electric motors. Also, thermal detection could be just blinded, if a criminal drone pilot would attack his target from the side of the sun, like fighter pilots did during WWII. At night time, infrared detector could be distracted by launching a bright false target like Chinese lantern that uses candle fire to fly, this would be detected over the drone.
Video detection by its principles works very similar to infrared technology. The difference in the spectrum brings practically no advantages. Video detection, again like infrared could be blinded by attacking a target from the side of the sun. Computer algorithms of flight patterns recognition between drones and birds fails against many wild birds like eagles or seagulls, that most of the time glide through the sky. As they stay at a steady level, this confuses the video systems.
Radio Frequency (RF) detection is based on the detection of radio signals between a drone and a pilot control unit. The distance range is quite long, up to 1500 feet or longer. Unfortunately, it fails against so called GPS-mode of drone flight. At this mode, the drone can be programmed to fly by itself to an exact point, using GPS navigation with no radio exchange with the pilot. To fight against this mode, it is common to install GPS/3G-4G/Wi-Fi jammers to get the drone disorientated. Thatís a solution for short-time situations like anti-drone support of presidential motorcade journey, but not suitable for airports or even private residential areas. As this will be distracted by aircraft navigational equipment and personal gadgets on the ground.
Radars became the prime technology for detecting flying vehicles since the tragic day of Dec. 7, 1941 when the brand new US SCR-270, 139MHz radar could identify Japanese aircrafts approaching Pearl Harbour, Oahu at a distance of 130 miles. However, the radar designed to detect traditional aircrafts cannot be modified to detect small drones. These radars operate at a long wavelength, so drones are too small to be visible at this wavelength. A practical example of this would be when a small fish escapes through a large cell fishnet.
Effective anti-drone radar has to be designed from scratch, and to operate in the high-gigahertz spectrum, where millimetre wavelength is smaller than the drone parts. This is simple physics - when the electromagnetic waves touch the drone, they will reflect back to the radar effectively, but only if the wavelength is less than smallest parts of the drone.
It is practically very hard to mislead millimetre-wave radar by any false target or radio noise device. The millimetre-wave radar is insensitive to GPS/3G-4G/Wi-Fi jammers, as well to direct sunlight. It works equally at day or night, in clear weather, sandstorm, smoke or snow conditions. The only challenge for millimetre-wave radar is to distinguish a drone from large birds, this issue could be resolved by enhancement of computer recognition of target footprint.
Millimeter-wave radars are very compact, with the max effective range of 1-2 miles depending on the antenna size. Compared to acoustic, infrared, video or RF sensors, millimetre-wave radar provide the longest operating distance among all existing drone detection technologies.