Editing Infrared homing (section)

===Spin-scan===
Most early seekers used so-called ''spin-scan'', ''chopper'' or ''reticle'' seekers. These consisted of a transparent plate with a sequence of opaque segments painted on them that was placed in front of the IR detector. The plate spins at a fixed rate, which causes the image of the target to be periodically interrupted, or ''chopped''.{{sfn|Deuerle|2003|pp=2401-2403}}

====Hamburg system====
The ''Hamburg'' system developed during the war is the simplest system, and easiest to understand. Its chopper was painted black on one half with the other half left transparent.{{sfn|Kutzscher|1957|p=212}}

For this description we consider the disk spinning clockwise as seen from the sensor; we will call the point in the rotation when the line between the dark and light halves is horizontal and the transparent side is on the top to be the 12 o'clock position. A photocell is positioned behind the disk at the 12 o'clock position.{{sfn|Kutzscher|1957|p=212}}

A target is located just above the missile. The sensor begins to see the target when the disk is at 9 o'clock, as the transparent portion of the chopper is aligned vertically at the target at 12 o'clock becomes visible. The sensor continues to see the target until the chopper reaches 3 o'clock.{{sfn|Kutzscher|1957|p=212}}

A [[signal generator]] produces an AC waveform that had the same frequency as the rotational rate of the disk. It is timed so the waveform reaches its maximum possible positive voltage point at the 12 o'clock position. Thus, during the period the target is visible to the sensor, the AC waveform is in the positive voltage period, varying from zero to its maximum and back to zero.{{sfn|Kutzscher|1957|p=212}}

When the target disappears, the sensor triggers a switch that inverts the output of the AC signal. For instance, when the disk reaches the 3 o'clock position and the target disappears, the switch is triggered. This is the same instant that the original AC waveform begins the negative voltage portion of its waveform, so the switch inverts this back to positive. When the disk reaches the 9 o'clock position the cell switches again, no longer inverting the signal, which is now entering its positive phase again. The resulting output from this cell is a series of half-sine waves, always positive. This signal is then smoothed out to produce a DC output, which is sent to the control system and commands the missile to turn up.{{sfn|Kutzscher|1957|p=212}}

A second cell placed at the 3 o'clock position completes the system. In this case, the switching takes place not at the 9 and 3 o'clock positions, but 12 and 6 o'clock. Considering the same target, in this case, the waveform has just reached its maximum positive point at 12 o'clock when it is switched negative. Following this process around the rotation causes a series of chopped-off positive and negative sine waves. When this is passed through the same smoothing system, the output is zero. This means the missile does not have to correct left or right. If the target were to move to the right, for instance, the signal would be increasingly positive from the smoother, indicating increasing corrections to the right. In practice a second photocell is not required, instead, both signals can be extracted from a single photocell with the use of electrical delays or a second reference signal 90 degrees out of phase with the first.{{sfn|Kutzscher|1957|p=212}}

This system produces a signal that is sensitive to the angle around the clock face, the ''bearing'', but not the angle between the target and the missile centerline, the ''angle off'' (or ''angle error''). This was not required for anti-ship missiles where the target is moving very slowly relative to the missile and the missile quickly aligns itself to the target. It was not appropriate for air-to-air use where the velocities were greater and smoother control motion was desired. In this case, the system was changed only slightly so the modulating disk was patterned in a [[cardioid]] which blanked out the signal for more or less time depending on how far from the centerline it was. Other systems used a second scanning disk with radial slits to provide the same result but from a second output circuit.{{sfn|Kutzscher|1957|p=214}}

====Later concepts====
AEG developed a much more advanced system during the war, and this formed the basis of most post-war experiments. In this case, the disk was pattered with a series of opaque regions, often in a series of radial stripes forming a pizza-slice pattern. Like the ''Hamburg'', an AC signal was generated that matched the rotational frequency of the disk. However, in this case the signal does not turn on and off with angle, but is constantly being triggered very rapidly. This creates a series of pulses that are smoothed out to produce a second AC signal at the same frequency as the test signal, but whose phase is controlled by the actual position of the target relative to the disk. By comparing the phase of the two signals, both the vertical and horizontal correction can be determined from a single signal. A great improvement was made as part of the Sidewinder program, feeding the output to the pilot's headset where it creates a sort of growling sound known as the ''missile tone'' that indicates that the target is visible to the seeker.{{sfn|Chang|1994|pp=13-14}}

In early systems this signal was fed directly to the control surfaces, causing rapid flicking motions to bring the missile back into alignment, a control system known as "bang-bang". Bang-bang controls are extremely inefficient aerodynamically, especially as the target approaches the centerline and the controls continually flick back and forth with no real effect. This leads to the desire to either smooth out these outputs, or to measure the angle-off and feed that into the controls as well. This can be accomplished with the same disk and some work on the physical arrangement of the optics. Since the physical distance between the radial bars is larger at the outer position of the disk, the image of the target on the photocell is also larger, and thus has greater output. By arranging the optics so the signal is increasingly cut off closer to the center of the disk, the resulting output signal varies in amplitude with the angle-off. However, it will also vary in amplitude as the missile approaches the target, so this is not a complete system by itself and some form of [[automatic gain control]] is often desired.{{sfn|Chang|1994|pp=13-14}}

Spin-scan systems can eliminate the signal from extended sources like sunlight reflecting from clouds or hot desert sand. To do this, the reticle is modified by making one half of the plate be covered not with stripes but a 50% transmission color. The output from such a system is a sine wave for half of the rotation and a constant signal for the other half. The fixed output varies with the overall illumination of the sky. An extended target that spans several segments, like a cloud, will cause a fixed signal as well, and any signal that approximates the fixed signal is filtered out.{{sfn|Chang|1994|pp=13-14}}{{sfn|Deuerle|2003|pp=2401-2403}}

A significant problem with the spin-scan system is that the signal when the target is near the center drops to zero. This is because even its small image covers several segments as they narrow at the center, producing a signal similar enough to an extended source that it is filtered out. This makes such seekers extremely sensitive to flares, which move away from the aircraft and thus produce an ever-increasing signal while the aircraft is providing little or none. Additionally, as the missile approaches the target, smaller changes in relative angle are enough to move it out of this ''center null'' area and start causing control inputs again. With a bang-bang controller, such designs tend to begin to overreact during the last moments of the approach, causing large miss distances and demanding large warheads.{{sfn|Deuerle|2003|pp=2401-2403}}

====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}}

====Crossed array seekers====
The ''crossed array seeker'' simulates the action of a reticle in a con-scan system through the physical layout of the detectors themselves. Classical photocells are normally round, but improvements in construction techniques and especially solid-state fabrication allows them to be built in any shape. In the crossed-array system (typically) four rectangular detectors are arranged in a cross-like shape (+). Scanning is carried out identically to the con-scan, which causes the image of the target to scan across each of the detectors in turn.{{sfn|Deuerle|2003|p=2407}}

For a target centered in the FOV, the image circles around the detectors and crosses them at the same relative point. This causes the signal from each one to be identical pulses at a certain point in time. However, if the target is not centered, the image's path will be offset, as before. In this case the distance between the separated detectors causes the delay between the signal's reappearance to vary, longer for images further from the centerline, and shorter when closer. Circuits connected to the mirrors produce this estimated signal as a control, as in the case of the con-scan. Comparing the detector signal to the control signal produces the required corrections.{{sfn|Deuerle|2003|p=2407}}

The advantage to this design is that it allows for greatly improved flare rejection. Because the detectors are thin from side to side, they effectively have an extremely narrow field of view, independent of the telescope mirror arrangement. At launch, the location of the target is encoded into the seeker's memory, and the seeker determines when it expects to see that signal crossing the detectors. From then on any signals arriving outside the brief periods determined by the control signal can be rejected. Since flares tend to stop in the air almost immediately after release, they quickly disappear from the scanner's gates.{{sfn|Deuerle|2003|p=2407}} The only way to spoof such a system is to continually release flares so some are always close to the aircraft, or to use a towed flare.