Thus the system is rather insensitive to antenna pointing errors. Variations in target echo power caused by scintillation are interpreted as target motion. Conical scanning Conical scanning addresses this problem by moving the radar beam slightly off center from the antenna's midline, or boresight, and then rotating it. Given an example antenna that generates a beam of 2 degrees width – fairly typical – a conical scanning radar might move the beam 1.5 degrees to one side of the centerline by offsetting the feed slightly. The resulting pattern, at any one instant in time, covers the midline of the antenna for about 0.5 degrees, and 1.5 degrees to the side. By spinning the feed horn with a motor, the pattern becomes a cone centered on the midline, extending 3 degrees across. The key concept is that a target located at the midline point will generate a constant return no matter where the lobe is currently pointed, whereas if it is to one side it will generate a strong return when the lobe is pointed in that general direction and a weak one when pointing away. Additionally, the portion covering the centerline is near the edge of the radar lobe, where sensitivity is falling off rapidly. An aircraft centered in the beam is in the area where even small motions will result in a noticeable change in return, growing much stronger along the direction the radar needs to move. The antenna control system is arranged to move the antenna in azimuth and elevation such that a constant return is obtained from the aircraft being tracked. Whilst use of the main lobe alone might allow an operator to "hunt" for the strongest return and thus aim the antenna within a degree or so in that "maximum return" area at the center of the lobe, with conical scanning much smaller movements can be detected, and accuracies under 0.1 degree are possible. Construction There are two ways to cause the redirection of the beam from the antenna's midline. The first is referred to as a rotated feed. As its name suggests, a feed horn is set just off the parabolic focal point which causes the energy to focus slightly off the antenna midline. The feed is then rotated around the focal point of the paraboloid to produce the conical rotation. The other system is a nutated feed. A nutated feed offsets the antenna at