Military Drone Detection

Challenges in Detecting, Identifying & Tracking Group-1/2 Drones

Detecting drones can be difficult with small- and medium-sized aircraft posing particular problems.

The United States’ Department of Defence (DOD) uses a classification system for drone size. Classifications are made according to the aircraft’s Maximum Take-Off Weight (MTOW), operating altitude and airspeed.

• Group-1 drones have an MTOW of between 0 and 20 pounds/lbs (nine kilograms/kg), a 1,200 feet/ft (365.8metres/m) altitude and an airspeed below 100 knots (185 kilometers-per-hour).
• Group-2 drones have an airspeed below 250 knots (463 km/h) and fly at altitudes below 3,500ft (1,067m). This classification of drones covers aircraft with an MTOW between 21lbs (9.5kg) and 55lbs (25kg).

The problem with Group-1/2 drones is that they can be difficult to detect, identify and track using conventional radar. As such, specialist radars for drone detection may be called for.

Small Radar Cross Sections

Many of these drones are physically small even when compared to light aircraft. They can also be constructed from materials like plastic or fiberglass which give the drone a lightweight construction. The flipside is that these materials can be bad at reflecting Radio Frequency (RF) signals back to a radar.

These two factors of non-metallic material construction and a small physical size can give a drone a small Radar Cross Section (RCS). RCS indicates how small an object seems to a radar. Much like the human eye, the smaller an object’s RCS, the harder it is for the radar to see it. Many radars have thresholds for the RCSs of the targets they detect, identify and track.

These thresholds serve a practical purpose. If a radar was tuned to detect all targets in the air within range of its antenna the radar’s processor, which determines a target’s characteristics, could become inundated. Skies are filled with flocks of birds, swarms of insects and even snow, hale and rain. All these objects can cause clutter on a radar screen.

The Problem with Clutter

Clutter may inundate a radar screen obscuring the actual target of interest to the radar operator. For example, a bird may have an RCS of 0.01 square meters. A MiG-29K (NATO reporting name Fulcrum-D) combat aircraft has an RCS of between one and 1.5 square meters. If the radar operator is looking for a MiG-29K they may tune their radar to filter out targets with RCSs of under one square meter and above two square meters. Their radar will ignore targets either side of these thresholds, allowing the operator to concentrate on their search for the MiG-29K. Some small civilian hobby drones can have RCSs equivalent to those of birds. As a result, some radars may simply ignore the drones mistaking them for birds.

A Combined Approach to Military Drone Detection

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.