What precision radar system uses continuous rotation of a pencil beam around the aircraft and uses the phase of the return signal modulation to maintain track in both azimuth and elevation?

The system described utilizes a continuous rotation of a pencil beam, which is characteristic of a conical scan radar system. This type of system sends out a series of radar pulses that trace a cone-shaped pattern around the aircraft while continuously rotating. The radar system maintains track by analyzing the phase of the received return signal modulated by the motion of the beam. This allows it to effectively measure both azimuth (the horizontal angle) and elevation (the vertical angle) relative to the aircraft.

In contrast, raster scan typically moves in horizontal and vertical lines, which is not a continuous circular motion. Helical scan involves a helical path that combines vertical and horizontal scanning but is not specific to the continuous rotation of a pencil beam. Lastly, circular scan does not specifically denote the modulation and tracking capabilities found in a conical scanning system but rather implies a simpler circular movement which does not necessarily maintain track effectively in both dimensions. Thus, conical scan is the correct choice due to its specific functionality of maintaining precise tracking in both azimuth and elevation through its continuous rotation and modulation techniques.