Why drones are hard
Small RCS (0.01 m² is typical for a DJI Phantom). Slow speed (10–20 m/s — radar clutter filters often reject anything that slow). Low altitude (under terrain masking and bird flocks). Mostly plastic, with only the motors and battery returning significant energy.
The new generation: 3D Ku-band staring radar
Companies like Echodyne, Robin Radar and DeTect built X- and Ku-band electronically scanned arrays that stare continuously at the protected volume instead of spinning. Coupled with machine-learning classifiers, they tell a quadcopter from a crow with ~95% accuracy.
Multi-sensor fusion
Drone detection is rarely just radar. RF sensors listen for the 2.4 / 5.8 GHz control link. Acoustic arrays pick up rotor whine. EO/IR cameras visually confirm. A modern counter-drone system fuses all four; radar provides the early track, the others provide the classification.
Then what?
Detection is half the problem. Mitigation — jamming the control link, GPS spoofing, net guns, kinetic interceptors — is the harder half, and legally restricted in most countries to military and approved law-enforcement use.