Infrared homing

Anti-aircraft IR systems began in earnest in the late 1940s, but the electronics and the entire field of rocketry were so new that they required considerable development before the first examples entered service in the mid-1950s.

[3] The ability of certain substances to give off electrons when struck by infrared light had been discovered by the famous Indian polymath Jagadish Chandra Bose in 1901, who saw the effect in galena, known today as lead sulfide, PbS.

[4] In 1917, Theodore Case, as part of his work on what became the Movietone sound system, discovered that a mix of thallium and sulfur was much more sensitive, but was highly unstable electrically and proved to be of little use as a practical detector.

Eventually they dissolved the Committee and reformed, leaving Lindemann off the roster,[7] and filling his position with well known radio expert Edward Victor Appleton.

[5] By 1940 they had successfully developed one solution; the Spanner Anlage (roughly "Peeping Tom system") consisting of a detector photomultiplier placed in front of the pilot, and a large searchlight fitted with a filter to limit the output to the IR range.

This provided enough light to see the target at short range, and Spanner Anlage was fitted to a small number of Messerschmitt Bf 110 and Dornier Do 17 night fighters.

Nevertheless, a summer 1944 report to the German Air Ministry stated that these devices were far better developed than competing systems based on radar or acoustic methods.

[13] Aware of the advantages of passive IR homing, the research program started with a number of theoretical studies considering the emissions from the targets.

[14] Studies were also made on atmospheric attenuation, which demonstrated that air is generally more transparent to IR than visible light, although the presence of water vapour and carbon dioxide produced several sharp drops in transitivity.

Kutzscher's team developed a system with the Eletroacustic Company of Kiel known as Hamburg, which was being readied for installation in the Blohm & Voss BV 143 glide bomb to produce an automated fire-and-forget anti-shipping missile.

The company also developed a working IR proximity fuse by placing additional detectors pointing radially outward from the missile centerline.

The Falcon was a complex system offering limited performance, especially due to its lack of a proximity fuse, and managed only a 9% kill ratio in 54 firings during Operation Rolling Thunder in the Vietnam War.

[27] The Soviets introduced their first infrared homing missile, the Vympel K-13 in 1961, after reverse engineering a Sidewinder that stuck in the wing of a Chinese MiG-17 in 1958 during the Second Taiwan Strait Crisis.

A small number of the resulting L models were rushed to the UK just prior to their engagement in the Falklands War, where they achieved an 82% kill ratio, and the misses were generally due to the target aircraft flying out of range.

[23] The Argentine aircraft, equipped with Sidewinder B and R.550 Magic, could only fire from the rear aspect, which the British pilots simply avoided by always flying directly at them.

[citation needed] Based on the same general principles as the original Sidewinder, in 1955 Convair began studies on a small man-portable missile (MANPADS) that would emerge as the FIM-43 Redeye.

[30] The three main materials used in the infrared sensor are lead(II) sulfide (PbS), indium antimonide (InSb) and mercury cadmium telluride (HgCdTe).

This led to new seekers sensitive to both the exhaust as well as the longer 8 to 13 micrometer wavelength range, which is less absorbed by the atmosphere and thus allows dimmer sources like the fuselage itself to be detected.

Modern all-aspect missiles like the AIM-9M Sidewinder and Stinger use compressed gas like argon to cool their sensors in order to lock onto the target at longer ranges and all aspects.

[citation needed] This situation leads to the use of a number of designs that use a relatively wide FOV to allow easy tracking, and then process the received signal in some way to gain additional accuracy for guidance.

Some of the earliest German seekers used a linear-scan solution, where vertical and horizontal slits were moved back and forth in front of the detector, or in the case of Madrid, two metal vanes were tilted to block off more or less of the signal.

[38] 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 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.

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.

[42] 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.

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.

Objects within the seeker's FOV produce completely separate signals as it scans around the sky; the system might see the target, flares, the sun and the ground at different times.

Early jammers like the AN/ALQ-144 used a heated block of silicon carbide as an IR source, and surround it with a spinning set of lenses that send the image as a series of spots sweeping around the sky.

Their implementation is further complicated by placing filters in front of the imager to remove any off-frequency signals, requiring the laser to tune itself to the frequency of the seeker or sweep through a range.

Newer missiles are smarter and use the gimballed seeker head combined with what is known as proportional guidance in order to avoid oscillation and to fly an efficient intercept path.