Conical scanning
Conical scanning is a system used in early radar units to improve their accuracy, as well as making it easier to steer the antenna properly to point at a target.
Conical scanning is similar in concept to the earlier lobe switching concept used on some of the earliest radars, and many examples of lobe switching sets were modified in the field to conical scanning during World War II, notably the German Würzburg radar.
Potential failure modes and susceptibility to deception jamming led to the replacement of conical scan systems with monopulse radar sets.
While this is adequate for locating the target in an early warning role, it is not nearly accurate enough for gun laying, which demands accuracies on the order of 0.1 degrees.
Circuitry designed to monitor any decrease in received signal strength can be used to control a servo motor that steers the antenna to follow the target motion.
There are three difficulties with this method: Conical scanning addresses this problem by moving the radar beam slightly off center from the antenna's midline, or boresight, and then rotating it.
By spinning the feed horn with a motor, the pattern becomes a cone centered on the midline, extending 3 degrees across.
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.
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.
25 gun fire control radar, spiral scan mode aided target acquisition.
Automatic guidance for the antenna, and thus any slaved guns or weapons, can be added to a conical scan radar without too much trouble.
For instance, if the aircraft were to suddenly "brighten" when it was off-axis to the left, the circuitry might interpret this as being off to the right if the change occurs when the lobe is aligned in that direction.
If one knows that the signal is rotated at 25 RPM, as it was in the Würzburg radar, the jammer is built to fade from maximum to zero at the same speed, 25 times a minute.
This system, known as inverse gain jamming, was used operationally by the Royal Air Force against the Würzburg radar during World War II.
Although this denied lobing frequency information to the jammer in the aircraft, it was still possible to simply send out random spikes and thereby confuse the tracking system (or operator).
