Finding the Frequency
The difficulties radars can experience in detecting, identifying and tracking drones, chronicled in Drone Detection Radar, highlights the need for a different approach. Radio links connecting the drone to its pilot can be particularly useful for finding and tracking Group-1/2 unmanned aircraft systems (UAS).
The International Telecommunications Union (ITU) is the United Nations organization tasked with regulating global use of the radio spectrum. The ITU has earmarked two Radio Frequency (RF) bands; 2.4 gigahertz/GHz to 2.483GHz and 5.725GHz to 5.825GHz for use by drone operators to provide a radio link between the pilot and aircraft.
RF Signal Detection
This RF link provides a means by which a drone, and its pilot, can be detected and tracked. A radio receiver and antenna can detect signals moving between the drone and the pilot. A single antenna will allow the bearing of the drone in relation to the radio receiver’s antenna to be determined.
By using two antennas a set distance apart the drone’s location can be pinpointed via its radio transmissions. Theoretically, a radio receiver with a five-meter (16ft) high antenna should detect radio transmissions from a Group-1 drone flying at 1,200ft at a range of 41.2 nautical miles (76.3km).
Electronic Support Measure (ESM)
Using the RF link connecting a drone to its pilot is not the only way by which a drone can be detected and tracked. Perhaps the drone is equipped with a radar mapping the ground below or detecting surface targets? This radar will transmit specific frequencies. Once again, it may be possible to detect, locate and track these signals using an Electronic Support Measure (ESM).
An ESM is an enhanced version of the radio receiver and antenna ensemble discussed above. The asset of using the ESM is that the system may be capable of matching the signal it has detected with the signal’s likely source. For example, RF traffic on frequencies of 2.4GHz and 5.8GHz means there is a strong likelihood that a drone is the source of these signals. Likewise, it may be possible to detect transmissions from a drone’s radar payload as a means of geolocating the aircraft. In fact, any RF emission made by the drone can potentially be exploited to find it.
Combined C-UAS with RF Detectors
Crucially, there is no single technology which is infallible for detecting, identifying, locating and tracking a drone. Instead, passive RF sensing should be combined with other sensors like acoustics, specialist radar and optronics to ensure the highest likelihood that an errant drone does not escape unnoticed.