Conical scanning
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Conical scanning is a system used in early radar units to improve their accuracy, as well as making it easier to properly steer the antenna 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 lobe switching sets were modified in the field to conical scanning during World War II, notably the German Würzburg radar. With simple electronics, antenna guidance can be made entirely automatic, although potential failure modes led to the replacement of conical scan systems with the somewhat similar monopulse radar sets.
A basic radar antenna commonly has a beam width of a few degrees. While this is fine for locating the target in the early warning role, it is not nearly accurate enough for gun laying, which demands accuracies on the order of 0.1 degrees. It is possible to improve the beam width through the use of larger antennas, but this is often impractical. Additionally, the beam itself typically has a fairly constant return strength if the target is located anywhere near the middle. Once the aircraft is close to centered, the operator has little ability to "fine tune" the aim.
Conical scanning addresses this problem by "moving" the radar beam slightly off center from the antenna's midline, 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 to the sides in our example.
There are two processes 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 columnate slightly off the antenna midline. The feed is then rotated around the focal point of the paraboloid to produce the conical rotation. The other process is a nutated feed. A nutated feed offsets the parabolic antenna dish at an angle to produce the desired angular variance. The dish is then rotated around a fixed feed horn to produce the conical rotation. The primary difference between the two is in polarization. As the feed horn in the rotated process spins, the polarization changes with the rotation and will thus be 90 degrees off in polarization when the feed is 90 degrees off its initial axis. As the feed horn is fixed in nutated feeds, no polarization changes occur.
The key concept is that a target located at the midline point will generate a return no matter where the lobe is currently pointed, whereas if it is to one side it will only generate a return when the lobe is pointed in that general direction. To the operator, this appears as a bright dot if centered, and a dimmer one if it is not. Since the speed that the lobe "crosses" the target is faster if the target is further from the midline, the brightness of the dot on the oscilloscope display is a direct indication of how close the target is to the center. 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. While the lobe itself might allow the 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 noticed, and accuracies under 0.1 degree were possible.
Since the lobe is being rotated around the midline of the antenna, conical scanning is only really appropriate for antennas with a circular cross section. This was the case for the Würzburg, which operated in the microwave region. Most other forces used much longer-wavelength radars that would require paraboloid antennas of truly enormous size, and instead used a "bedspring" arrangement of many small dipole antennas arranged in front of a passive reflector. To arrange conical scanning on such a system would require all of the dipoles to be moved, an impractical solution. For this reason the US Army simply abandoned their early gun laying radar, the SCR-268. This was not particularly annoying, given that they were in the process of introducing their own microwave radar in the aftermath of the Tizard Mission, the SCR-584.
Automatic guidance for the antenna, and thus any slaved guns or weapons, can be added to a conical scan radar without too much trouble. In effect the electronics simply has to note when the maximum return is received, note the direction the lobe was pointed at that time (even mechanically), and then move the antenna in that direction. However finding the maximum point is often difficult in electronics, notably when the target returns a "maximum" over a wide range of angles as it is swept by the lobes "maximum return" area. Generally it is easier to find a minimum point, often by setting some threshold below which the signal is considered to be zero. Attached to a conical scan radar, the minimum would be triggered when the lobe was opposite the target in the scan pattern. By moving the antenna towards the maximum, or away from the minimum, the radar can keep itself pointed at a single target. This is commonly known as "locking on", or "lock-on".
Unfortunately there are a number of factors that can dramatically change the reflected signal. For instance, changes in the target aircraft's direction can present different portions of the fuselage to the antenna, and dramatically change the amount of signal being returned. In these cases, a conical scan radar might interpret this change in strength as a change in position. 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. This problem can be solved by using two similar signals that can otherwise be distinguished, often by using different polarizations, leading to the monopulse radar, so-named because it always compares signal strength from a single pulse against itself, thereby eliminating problems with all but impossibly fast changes in signal strength.
Conical scan radars can be easily jammed. If the target knows the general operating parameters of the radar, it is possible to send out a false signal timed to grow and fade in the same pattern as the radar lobe, but inverted in strength. That is, the false signal is at its strongest when the radar signal is the weakest (the lobe is on the "far side" of the antenna compared to the aircraft), and weakest when the signal is the strongest (pointed at the aircraft). When added together with the "real" signal at the radar receiver, the resulting signal is "always strong", so the operators have no real idea where in the lobe pattern the target is located.
Actually accomplishing this in hardware is not as difficult as it may sound. 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 second. Then all that is needed is to sync the signals up, which is accomplished by looking for the low point in the signal (which is generally easier to find) and triggering the pattern at that point. This system, known as inverse gain jamming, was used operationally by the Royal Air Force against the Würzburg radar during WWII.
It is possible to arrange a radar so the lobes are not being moved in the broadcaster, only the receiver. To do this, one adds a second antenna with the rotating lobe for reception only, a system known as CORSO, for Conical Scan on Receive Only (compare to LORO, a similar system used against lobe switching radars). Although this denied lobbing 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). This technique, called SSW for Swept Square Wave, doesn't protect the aircraft with the same sort of effectiveness as inverse gain, but is better than nothing and often fairly effective.
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