Micro-Doppler signatures
Drone rotors spin at 5,000–10,000 rpm. They create high-frequency Doppler sidebands around the main return — a 'JEM' (jet-engine modulation) signature. Birds flap their wings 2–10 times per second, producing a very different micro-Doppler pattern. With enough pulse coherence (typically 128–256 pulses), the classifier can distinguish quadcopter, fixed-wing drone and bird with 95%+ accuracy.
Sensor fusion
Pure radar is rarely enough. Modern C-UAS systems fuse radar with RF detection (sniffing the drone's control link), electro-optical cameras with computer vision and sometimes acoustic sensors. Radar provides the cue ('something at 247° azimuth, 1.4 km'), the camera confirms drone vs bird and the RF subsystem identifies the make and model.
Defeat mechanisms
Once classified, the system can jam the control link, spoof the GPS or — in military contexts — engage with kinetic effectors. Stadiums and airports prefer non-kinetic defeat to avoid the drone falling on a crowd. Systems like AUDS, DroneShield and Anduril Sentry are deployed at major airports.
Swarms
The next problem is swarms. A single radar can track dozens of drones, but engaging 50 simultaneously exceeds most jammers' bandwidth. High-energy lasers and high-power microwaves are emerging as the only practical defeat against true swarms.