Editing Infrared homing (section)

====Conical scan====
A great improvement on the basic spin-scan concept is the ''conical scanner'' or ''con-scan''. In this arrangement, a fixed reticle is placed in front of the detector and both are positioned at the focus point of a small [[Cassegrain reflector]] telescope. The secondary mirror of the telescope is pointed slightly off-axis, and spins. This causes the image of the target to be spun around the [[reticle]], instead of the reticle itself spinning.{{sfn|Deuerle|2003|pp=2404-2405}}

Consider an example system where the seeker's mirror is tilted at 5 degrees, and the missile is tracking a target that is currently centered in front of the missile. As the mirror spins, it causes the image of the target to be reflected in the opposite direction, so in this case the image is moving in a circle 5 degrees away from the reticle's centerline. That means that even a centered target is creating a varying signal as it passes over the markings on the reticle. At this same instant, a spin-scan system would be producing a constant output in its center null. Flares will still be seen by the con-scan seeker and cause confusion, but they will no longer overwhelm the target signal as it does in the case of spin-scan when the flare leaves the null point.{{sfn|Deuerle|2003|pp=2404-2405}}

Extracting the bearing of the target proceeds in the same fashion as the spin-scan system, comparing the output signal to a reference signal generated by the motors spinning the mirror. However, extracting the angle-off is somewhat more complex. In the spin-scan system it is the length of time between pulses that encodes the angle, by increasing or decreasing the output signal strength. This does not occur in the con-scan system, where the image is roughly centered on the reticle at all times. Instead, it is the way that the pulses change over the time of one scan cycle that reveals the angle.{{sfn|Deuerle|2003|p=2405}}

Consider a target located 10 degrees to the left of the centerline. When the mirror is pointed to the left, the target appears to be close to the center of the mirror, and thus projects an image 5 degrees to the left of the centerline of the reticle. When it has rotated to point straight up, the relative angle of the target is zero, so the image appears 5 degrees down from the centerline, and when it is pointed to the right, 15 degrees to the left.{{sfn|Deuerle|2003|p=2405}}

Since angle-off on the reticle causes the length of the output pulse to change, the result of this signal being sent into the mixer is [[frequency modulated]] (FM), rising and falling over the spin cycle. This information is then extracted in the control system for guidance. One major advantage to the con-scan system is that the FM signal is proportional to the angle-off, which provides a simple solution for smoothly moving the control surfaces, resulting in far more efficient aerodynamics. This also greatly improves accuracy; a spin-scan missile approaching the target will be subject to continual signals as the target moves in and out of the centerline, causing the bang-bang controls to direct the missile in wild corrections, whereas the FM signal of the con-scan eliminates this effect and improves [[circular error probable]] (CEP) to as little as one meter.{{sfn|Deuerle|2003|pp=2404-2405}}

Most con-scan systems attempt to keep the target image as close to the edge of the reticle as possible, as this causes the greatest change in the output signal as the target moves. However, this also often causes the target to move off the reticle entirely when the mirror is pointed away from the target. To address this, the center of the reticle is painted with a 50% transmission pattern, so when the image crosses it the output becomes fixed. But because the mirror moves, this period is brief, and the normal interrupted scanning starts as the mirror begins to point toward the target again. The seeker can tell when the image is in this region because it occurs directly opposite the point when the image falls off the seeker entirely and the signal disappears. By examining the signal when it is known to be crossing this point, an AM signal identical to the spin-scan seeker is produced. Thus, for the cost of additional electronics and timers, the con-scan system can maintain tracking even when the target is off-axis, another major advantage over the limited field of view of spin-scan systems.{{sfn|Deuerle|2003|p=2405}}