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. It was universal among gun laying radars by the end of WWII.

Conical scanning uses the non-linear nature of the radar's antennas, which are most sensitive in a single direction. By rotating the antenna, the signal returned from the target will grow stronger and weaker depending on how close it is pointed to the target. By comparing the angle of the antenna to the signal strength and looking for a peak, the angle of the target can be determined, and by examining the shape of the returned signal over time, that angle can be measured with very high precision, on the order of 10 to 100 times that of the antenna's natural beam width.

The signal returned by conical scanning can also be compared to a fixed signal using simple electronics. The difference between them represents the error signal, the angle between the target in a selected direction. By taking two such measurements, one "up" and one "right", the resulting signals can be amplified and sent into electric motors that move the antenna in that direction. This causes the antenna to automatically follow the target, dramatically simplifying the operation of the system as a whole, and often eliminating two operators that formerly provided this functionality manually. Systems with fully-automatic tracking of this sort include the American SCR-584 and late-war British aircraft interception radars.

Simple conical scanning systems are subject to angle deception jamming, which causes the error signal to be spoofed and cause the radar to "walk off" the target. This was applied near the end of the war by British systems working against the Würzburg. This led to the replacement of conical scan systems with monopulse radar sets starting in the 1950s. Conical scanning is still used where jamming is not a concern, for example, the Deep Space Network uses it for maintaining communications links to space probes. The spin-stabilized Pioneer 10 and Pioneer 11 probes used onboard conical scanning maneuvers to track Earth in its orbit.