an angle to a fixed feed horn, and then rotates the antenna. A variation of a nutated feed makes the feed move in a small circle, rapidly and continuously changing the pointing direction of the beam. In this latter type, neither the feed nor the antenna revolves around the pointing axis of the antenna; only the pointing direction changes, tracing out a narrow cone. The primary difference between the two basic schemes 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. Most early systems used a rotated feed, due to its mechanical simplicity, but later systems often used nutated feeds in order to use the polarization information. In the U.S. Navy Mk. 25 gun fire control radar, spiral scan mode aided target acquisition. Basically conical scan (of the non-revolving nutating feed type), the size of the scan cone cyclically increased and decreased roughly twice a second. The scanned area was several degrees, in all. (Once the target was acquired, the operator switched to conical scan for tracking.) 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. In the SCR-584, the MIT Radiation Laboratory introduced automatic tracking. Automatic guidance for the antenna, and thus any slaved guns or weapons, can be added to a conical scan radar without too much trouble. The control system has to steer the antenna such that a constant amplitude return is received from the target. Unfortunately there are a number of factors