Since its perfection in the 1930s, radar has remained the principle means par excellence of detecting aircraft in all weathers, day and night.
Radar uses an elegantly simple principle. Radio waves are transmitted from an antenna, collide with an object in range and are bounced back to that antenna. This process lets a radar obtain a myriad of data about a target. Those Radio Frequency (RF) transmissions disclose to the radar operator information about the target’s speed, altitude, vector and aircraft type.
Military radars are primarily used for the detection, location, identification and tracking of air targets. These air targets include conventional aircraft, and also missiles, rockets and even artillery shells. However, the revolution in uninhabited aviation, which gathered pace from the late 1990s, imposes some challenges on radar.
Changing Military Priorities
Many conventional air search radars are not designed to detect targets as small as drones, and this shortcoming is not accidental. During the Second World War, and during the Cold War that followed, the military priority was to detect, locate, identify and track combat aircraft.
As the Cold War unfolded advances in missile technology saw these targets added to radar’s repertoire. Subsequent advances in radar technology enabled the detecting of progressively smaller targets like incoming artillery shells.
United States-led involvement in counter-insurgency operations in Afghanistan and Iraq led to improvements in the abilities of radars to detect and track incoming rockets. During conflicts in these countries rocket fire emerged as a favorite insurgent tactic used against US and allied forces.
The Drone Revolution
Military use of drones is not new and pilotless aircraft were employed during the Second World War. However, Israel’s experience pioneering drones as a military asset precipitated a revolution. From the 1990s onwards, militaries began to adopt drones en masse. ‘Dull, dangerous and dirty’ was the mantra for the missions that drones would undertake. Using drones kept personnel out of harms’ way and freed them to perform other missions where there was no substitute for the human factor.
Drones come in assorted shapes and sizes, but it is the smallest aircraft which can create the biggest problems for radar engineers. Known as Group-1 drones in US Department of Defence parlance, these aircraft have a maximum take-off weight of up to 20 pounds (nine kilograms), a ceiling of 1,200 feet (366 meters) and top speeds of 100 knots (185 kilometers-per-hour). To compound matters, drones are often constructed from lightweight materials like plastic or carbon fiber.
The small size of some tactical drones and the materials used for their construction, are challenging for conventional radars. Firstly, non-metallic materials are bad conductors of electricity, hence bad conductors of electromagnetic RF signals. Bad electrical conduction means that less RF energy is reflected to the radar, making the drone harder to detect. The small physical size of the drone causes a similar challenge.
Radar Cross Section (RCS)
Radar Cross Section (RCS) is a measurement of how ‘visible’ a target is to a radar. A general rule of thumb is that the larger the object, the easier it is for a radar to detect. That said, materials like those discussed above when used in aircraft construction can help to reduce a large aircraft’s RCS. A large airborne target like a Boeing B-52H Stratofortress strategic bomber can have an RCS of around 100 square meters. A bird’s RCS can be as small as 0.01 square meters.
Micro-Doppler Shift
Some of the smallest drones have comparable RCSs to birds meaning that they are ignored by some radars. Radars can often filter out certain targets to avoid clutter. Aircraft share the skies with birds, clouds and even swarms of insects. A radar operator may tune their system to ignore these targets to avoid their screens being cluttered by targets they are not interested in to avoid obscuring the targets they want to see. However, in tuning their radar to avoid birds, the radar operator may miss a potentially threatening drone.
The Growing C-UAS Radar Industry
These engineering challenges have triggered a growing drone detection radar industry. Several companies and research institutions offer specialist radars designed to detect drones. A raft of techniques are used which depend on sophisticated radar target detection and processing algorithms. Some of these algorithms will use Artificial Intelligence (AI). AI techniques embedded in the radar’s software will have been ‘trained’ to recognise ornithological flight characteristics and to ignore targets exhibiting these. Drones tend to follow straight, predictable patterns of flight, unlike birds, greatly helping detection.
Another technique involves exploiting micro-Doppler Shift. Doppler Shift occurs when the transmitted frequency of a radar signal rises or lowers after it collides with a target moving to or from the radar. The fast-moving blades of the drone’s rotors cause very small changes in the returned frequency of the signal. These frequencies rise and fall as the blades move towards and away from the radar.
As drones often have at least four rotors, the radar’s processor will detect several micro-Doppler Shifts caused by all these rotors. This multitude of infinitesimally small shifts lets the radar determine that the target is a drone and not a bird given that the Doppler Shift caused by flapping bird wings will be very different in their characteristics. Moreover, drone rotors are continually spinning whereas bird wings are not always flapping.
The widespread use of drones by the militaries of both Russia and Ukraine in the ongoing civil war in that latter country shows that the mass employment of drones on the battlefield is here to stay. As a result, the technological development of counter-drone radar technology will continue to advance.
