Counter-drone systems, also called C-UAS (counter-unmanned aircraft systems), detect hostile drones and stop them. Armies, airports and border forces use them. A hostile drone may carry a bomb, spy on a camp or carry drugs across a border. At an airport, even a harmless drone near the runway can stop flights.
A drone is hard to detect. It is small, it flies low and slowly, and an ordinary radar can mistake it for a bird. So a counter-drone system uses several sensors together and does two jobs, in order. First it detects. A radar sees the drone's echo. A radio sensor hears the signal between the drone and its operator. A camera, including a thermal imaging camera, confirms what it is, and computer vision can tell a drone from a bird. Defence electronics supplies the radar and the sensors.
Then it stops the drone. A jammer sends a strong radio signal that cuts the link between the drone and its operator, so the drone lands or flies home. Other systems fire a net or a laser, or send an interceptor drone to hit it. A jammer also blocks other radio signals nearby, so it must be used with care near an airport or a town.
Here is a typical case. Radar at a border post in Punjab picks up a small object flying low at 40 km/h. The radio sensor hears a control signal, and the camera shows a drone carrying a package. The system alerts the guards and jams the signal. The drone falls inside the border. Border forces in India have reported drones carrying drugs and weapons across the Punjab border, which is one reason for such systems.
The same sensors are used in security screening systems for sites, and the makers also build defence test equipment.