Infrared homing
Infrared homing is a passive weapon guidance system which uses the infrared light emission from a target to track and follow it seamlessly. Missiles which use infrared seeking are often referred to as "heatseekers" since infrared is radiated strongly by hot bodies. Many objects such as people, vehicle engines and aircraft generate and emit heat and so are especially visible in the infrared wavelengths of light compared to objects in the background.
Infrared seekers are passive devices, which, unlike radar, provide no indication that they are tracking a target. That makes them suitable for sneak attacks during visual encounters or over longer ranges when they are used with a forward looking infrared or similar cueing system. Heat-seekers are extremely effective: 90% of all United States air combat losses between 1984 and 2009 were caused by infrared-homing missiles. They are, however, subject to a number of simple countermeasures, most notably by dropping flares behind the target to provide false heat sources. That works only if the pilot is aware of the missile and deploys the countermeasures on time. The sophistication of modern seekers has rendered these countermeasures increasingly ineffective.
The first IR devices were experimented with during World War II. During the war, German engineers were working on heat-seeking missiles and proximity fuses but did not have time to complete development before the war ended. Truly practical designs did not become possible until the introduction of conical scanning and miniaturized vacuum tubes during the war. 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. The early examples had significant limitations and achieved very low success rates in combat during the 1960s. A new generation developed in the 1970s and the 1980s made great strides and significantly improved their lethality. The latest examples from the 1990s and on have the ability to attack targets out of their field of view behind them and even to pick out vehicles on the ground.
IR seekers are also the basis for many semi-automatic command to line of sight weapons. In this use, the seeker is mounted on a trainable platform on the launcher and the operator keeps it pointed in the general direction of the target manually, often using a small telescope. The seeker does not track the target, but the missile, often aided by flares to provide a clean signal. The same guidance signals are generated and sent to the missile via thin wires or radio signals, guiding the missile into the center of the operator's telescope. SACLOS systems of this sort have been used both for anti-tank missiles and surface-to-air missiles, as well as other roles.
The infrared sensor package on the tip or head of a heat-seeking missile is known as the seeker head. The NATO brevity code for an air-to-air infrared-guided missile launch is Fox Two.
History
Early research
The ability of certain substances to give off electrons when struck by infrared light had been discovered by the Indian polymath Jagadish Chandra Bose in 1901, who saw the effect in galena, known today as lead sulfide, PbS. There was little use and he allowed his 1904 patent to lapse. 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. It was used for some time by the US Navy as a secure communications system.In 1930 the introduction of the Ag–O–Cs photomultiplier provided the first practical solution to the detection of IR, combining it with a layer of galena as the photocathode. Amplifying the signal emitted by the galena, the photomultiplier produced a useful output that could be used for detection of hot objects at long ranges. This led to developments in a number of nations, notably Britain and Germany where it was seen as a potential solution to the problem of detecting night bombers.
In Britain, research stagnated, with even the main research team at Cavendish Labs expressing their desire to work on other projects, especially after it became clear that radar was going to be a better solution. Frederick Lindemann, Winston Churchill's favorite on the Tizard Committee, remained committed to IR and became increasingly obstructionist to the work of the Committee who were otherwise pressing for radar development. Eventually they dissolved the Committee and reformed, leaving Lindemann off the roster and filling his position with well known radio expert Edward Victor Appleton.
In Germany, radar research was not given nearly the same level of support as in Britain and competed with IR development throughout the 1930s. IR research was led primarily by Edgar Kutzscher at the University of Berlin, working in concert with AEG. By 1940 they had developed the Spanner Anlage 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. These proved largely useless, the pilots complained that the target often only became visible at, when they would have seen it anyway. Only 15 were built and were removed as German airborne radar systems improved though 1942.
AEG had been working with the same systems for use on tanks, and deployed a number of models through the war, with limited production of the FG 1250 beginning in 1943. This work culminated in the Zielgerät 1229 Vampir riflescope which was used with the StG 44 assault rifle for night use.
German seekers
The devices mentioned previously were all detectors, not seekers. They either produce a signal indicating the general direction of the target, or in the case of later devices, an image similar to a television image. Guidance was entirely manual by an operator looking at the image. There were a number of efforts in Germany during the war to produce a true automatic seeker system, both for anti-aircraft use as well as against ships. These devices were still in development when the war ended; although some were ready for use, there had been no work on integrating them with a missile airframe and considerable effort remained before an actual weapon would be ready for use. 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.Aware of the advantages of passive IR homing, the research program started with a number of theoretical studies considering the emissions from the targets. This led to the practical discovery that the vast majority of the IR output from a piston-engine aircraft was between 3 and 4.5 micrometers. The exhaust was also a strong emitter, but cooled rapidly in the air so that it did not present a false tracking target. 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. Finally, they also considered the issue of background sources of IR, including reflections off clouds and similar effects, concluding this was an issue due to the way it changed very strongly across the sky. This research suggested that an IR seeker could home on a three-engine bomber at with an accuracy of about degree, making an IR seeker a very desirable device.
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. A more advanced version allowed the seeker to be directed off-axis by the bombardier in order to lock on to a target to the sides, without flying directly at it. However, this presented the problem that when the bomb was first released it was traveling too slowly for the aerodynamic surfaces to easily control it, and the target sometimes slipped out from the view of the seeker. A stabilized platform was being developed to address this problem. The company also developed a working IR proximity fuse by placing additional detectors pointing radially outward from the missile centerline, which triggered when the signal strength began to decrease, which it did when the missile passed the target. There was work on using a single sensor for both tasks instead of two separate ones.
Other companies also picked up on the work by Eletroacustic and designed their own scanning methods. AEG and Kepka of Vienna used systems with two movable plates that continually scanned horizontally or vertically, and determined the location of the target by timing when the image disappeared or reappeared. The Kepka Madrid system had an instantaneous field of view of about 1.8 degrees and scanned a full 20 degree pattern. Combined with the movement of the entire seeker within the missile, it could track at angles as great as 100 degrees. Rheinmetall-Borsig and another team at AEG produced different variations on the spinning-disk system.
Post-war designs
In the post-war era, as the German developments became better known, a variety of research projects began to develop seekers based on the PbS sensor. These were combined with techniques developed during the war to improve accuracy of otherwise inherently inaccurate radar systems, especially the conical scanning system. One such system developed by the US Army Air Force, known as the "Sun Tracker", was being developed as a possible guidance system for an intercontinental ballistic missile. Testing this system led to the 1948 Lake Mead Boeing B-29 crash.USAAF project MX-798 was awarded to Hughes Aircraft in 1946 for an infrared tracking missile. The design used a simple reticle seeker and an active system to control roll during flight. This was replaced the next year by MX-904, calling for a supersonic version. At this stage the concept was for a defensive weapon fired rearward out of a long tube at the back end of bomber aircraft. In April 1949 the Firebird missile project was cancelled and MX-904 was redirected to be a forward-firing fighter weapon. The first test firings began in 1949, when it was given the designation AAM-A-2 and the name Falcon. IR and semi-active radar homing versions both entered service in 1956, and became known as the AIM-4 Falcon after 1962. 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. However, this relatively low success rate must be appreciated in the context of all these kills representing direct hits, something that was not true of every kill by other American AAMs.
In the same year as MX-798, 1946, William B. McLean began studies of a similar concept at the Naval Ordnance Test Station, today known as Naval Air Weapons Station China Lake. He spent three years simply considering various designs, which led to a considerably less complicated design than the Falcon. When his team had a design they believed would be workable, they began trying to fit it to the newly introduced Zuni 5-inch rocket. They presented it in 1951 and it became an official project the next year. Wally Schirra recalls visiting the lab and watching the seeker follow his cigarette. The missile was given the name Sidewinder after a local snake; the name had a second significance as the sidewinder is a pit viper and hunts by heat, and moves in an undulating pattern not unlike the missile. The Sidewinder entered service in 1957, and was widely used during the Vietnam war. It proved to be a better weapon than the Falcon: B models managed a 14% kill ratio, while the much longer-ranged D models managed 19%. Its performance and lower cost led the Air Force to adopt it as well.
The first heat-seeker built outside the US was the UK's de Havilland Firestreak. Development began as OR.1056 Red Hawk, but this was considered too advanced, and in 1951 an amended concept was released as OR.1117 and given the code name Blue Jay. Designed as an anti-bomber weapon, the Blue Jay was larger, much heavier and flew faster than its US counterparts, but had about the same range. It had a more advanced seeker, using PbTe and cooled to −180 °C by anhydrous ammonia to improve its performance. One distinguishing feature was its faceted nose cone, which was selected after it was found ice would build up on a more conventional hemispherical dome. The first test firing took place in 1955 and it entered service with the Royal Air Force in August 1958.
The French R.510 project began later than Firestreak and entered experimental service in 1957, but was quickly replaced by a radar-homing version, the R.511. Neither was very effective and had short range on the order of 3 km. Both were replaced by the first effective French design, the R.530, in 1962.
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. The K-13 was widely exported, and faced its cousin over Vietnam throughout the war. It proved even less reliable than the AIM-9B it was based on, with the guidance system and fuse suffering continual failure.