An Active Radar operates by transmitting a radio signal of known frequency and power in a focused direction and then detecting the reflected signal that is bounced back from the target. Doppler radars are the most common type of radar used in C-UAS technologies. Doppler radars differentiate the return signal based on a frequency shift from the original transmitted frequency,. which allows the radar to dismiss the detection of stationary objects.

Radars can be two-dimensional (2D) or three-dimensional (3D); 2D radars provide direction and distance to the target, while 3D radars also provide the target’s altitude. Two-dimensional radars typically use a single antenna that rotates to cover the desired field of view. These radars may provide the UAS’s distance from the radar (or the target’s range) and its bearing or azimuth (degrees from true North in the horizontal plane). Three-dimensional radars use phased array stationary antenna panels with multiple internal antennas (array) on a single panel. This type of radar can change the direction of the RF signal it emits by manipulating the phase of the signal emitted by each of the internal antenna elements–much like focusing a lens on a camera.

Reference- DHS Counter-UAS Technology Guide