Technology Explained

Infra-red Trackers

The infra-red tracker is the technology at the core of infra-red missile warning systems, heatseeking missiles and IRS&T sets. Its function in the simplest of terms is to scan a field of view (FOV) for any sources of infra-red radiation of a particular wavelength or band of wavelengths and to measure with some accuracy the angular coordinates of these sources. These sources will be exhaust plumes and airframes which radiate heat and thus infrared radiation (for more detailed discussion of the infrared signature of an aircraft, please see TE March 1982, TE May 1987).

There are numerous ways of building infra-red trackers, the most important of these are the reticle, rosette scanning and focal plane array techniques, which are common to heatseeking weapon guidance, infra-red warning systems and IRS&T sets.

In spite of their differences in construction and performance, all infra-red trackers will have some features in common. All will employ either an array of or a single detector element. A detector is a small piece of a semiconductor material which is photosensitive, ie its electrical properties will change when illuminated by light of a given wavelength. Numerous materials have been used for detectors over the last four decades, with PbS very popular in early days, later yielding to lnSb. The favoured materials today are Mercury Cadmium Telluride (HgCdTe) which is unfortunately very tricky to fabricate detectors from but provides superior sensitivity to longer wavelengths and Platinum Silicide which is less sensitive but far easier to process.

The performance of any detector material is usually judged by its sensitivity over some range of wavelengths (colours). Sensitivity is a measure of the amount of electrical response the detector will provide to some given level of incident radiation, ie the more sensitive the detector material the fainter the radiation it can detect. Sensitivity will vary with wavelength as a result of quantum physical effects in the detector and more mundane considerations such as detector surface finish. The quantum physical aspect is given by the photoelectric effect where photons of impinging light or infra-red excite electrons (carriers for the physics oriented reader) within the material, if the photon is energetic enough (ie its wavelength short enough) the electron will be dislodged from its place in the detector’s crystalline lattice and becomes detectable as a minute electrical current. The minimum photon energy required to dislodge a carrier determines the longest wavelength detectable by the detector material and hence the lowest possible temperature target which may be detected. This is important as the infra-red signature of a rocket exhaust plume, engine tailpipe, fuselage rear end and airframe all differ in peak wavelength, the hotter the target the shorter the brightest wavelength and the broader the range of wavelengths emitted (readers are advised to look up blackbody radiation in any decent physics textbook). A 2 micron band detector will thus see only rocket exhaust plumes, afterburner plumes and tailpipe cavities, a 4 micron band detector all of the above and hotter parts of the airframe from all aspects, while an 8 micron band detector all of the above and the whole airframe or even its wake. As it turns out the longer wavelength detectors must be cooled for operation, as room temperature itself will excite enough electrons in the detector to completely swamp any photoelectric current from a sought target.

Ideally the designer of an infra-red tracker would seek a detector matched to the wavelength of the sought target while requiring minimum cooling. Cooling a detector is another can of worms and many techniques have been used in practice. Missiles will employ gas or thermoelectric cooling, whereas IRS&T equipment may also employ closed cycle refrigeration to reduce turnaround time. Gas cooling involves the rapid expansion of a compressed gas usually provided from a bottle installed in the launcher or launch system, as instances the Nitrogen cooled USN AIM-9D PbS detector or Argon cooled USN/USAF AIM-9L Sidewinder InSb detector. Thermoelectric cooling is achieved via the use of a Peltier heat pump, passing an electrical current through such a device cools one end of it and heats the other, the USAF AIM-9E/J’s PbS detector was cooled in this fashion The choice of detector material cooled to an appropriate temperature will determine the sensitivity and longest detectable wavelength seen by the infra-red tracker. Most detectors are sensitive to visible light due to the nature of the photoelectric effect and this can cause problems in the real world. The sun is a bright source of visible and -infra-red light (6000K blackbody radiator) and any highly reflective object such as a cloud will also appear as such to a tracker, this is the reason why inferior heatseeking missiles chase the sun or clouds. The solution to this problem is simple in concept although often tricky in implementation; the detector will view the outside world through an optical filter which will suppress all visible and infrared light of those wavelengths which are not desired. Filters are usually implemented by depositing very thin layers of glass with alternating refractive indices and thicknesses of the order of the W’. 3-Velengths of light involved, upon the surface of the glass window used by the detector. Clever choice of thicknesses will cause an effect termed interference; undesirable wavelengths are reflected and the remainder passed. The gold hue apparent on the nose windows of infra-red missiles is such an interference filter.

A cooled detector with its optical filter is the core of an infra-red tracker. What remains is some mechanism to focus the incoming light on to the detector and point the detector’s field of view within the tracker’s field of view. This mechanism is the tracker’s optics, which are often stabilised in pitch and yaw to provide a jitter free output.

The oldest and perhaps dominant optical arrangement today is the rotating reticle scheme, first put to use by the Germans on A-4 ballistic missiles. The rotating reticle scheme (see TE March 1982 for illustration) uses a rapidly spinning reticle (or mirror) which has a pattern of transparent spokes etched on half of its surface, the other being semi-transparent, the detector views the outside world through the whirling pattern of the reticle.

The pattern chops all incident light and the resulting output from the detector is a train of pulses synchronised to the reticle. The pattern on the reticle is designed such that the size of the pulse train is proportional to the angular displacement of the target relative to the axis of rotation of the reticle. The remaining angular coordinate is derived from the relative timing (phase) of the pulse train relative to the reticle rotation.

The rotating reticle tracker is penalised by the need for precision fabrication of the reticle which must be exact mechanically while also often doubling up as an infra-red filter, alternate construction employs a concave mirror with the reticle pattern etched on its surface. The strength of this technique lies in the simplicity of the analogue electronics required to decode the target position. Its practical limitations lie in its narrow field of view and high susceptibility to amplitude modulation (AM) jamming, the simple technique of flashing an infra-red source at a rate comparable to that of the reticle rotation can cause the electronics to lock on to the jammer’s pulse train and drive the tracker away from the target (as with AM jamming of conical scan radar). A newer and mechanically/optically simpler arrangement Is that of the rosette scanning tracker. A rosette scanning tracker will usually employ a single fixed detector and moving optics which have a small instantaneous field of view. The optics then scan a much larger field of view in a rosette pattern, which has the appearance of a flower with multiple petals. The strength of this technique lies in the simplicity of the scanning mechanism and a of hardware in the optical path; the penalty is in The complexity of the electronics required cf. a rotating reticle tracker.

Jam resistance of these trackers has not been discussed at length in the open literature, no doubt for fear of compromising weapons such as the FIM-92 Stinger SAM.

The newest family of optical trackers are imaging or Focal Plane Array (FPA) trackers, which will become the dominant tracker in the next decade or so. A FPA tracker will employ a planar array or field of detector elements which stare at the whole field of view simultaneously. By reading the outputs of the detectors with electronics embedded in the same chip/carrier, an image of the viewed scene can be recreated for viewing by an operator (eg FUR applications) or analysis by an image pro cessing computer to detect and track targets. The construction of the necessary optics is simple with no moving parts – a lens/ window/filter and a support to position the chip carrier within the required focal plane of the optics, this arrangement is thus cheap to produce, align/calibrate and has a high tolerance for acceleration thus being very suitable for missile guidance. Four technologies are being used or evaluated for use in imaging/ array trackers; Mercury Cadmium Telluride (HgCdTe), Platinum Silicide, Indium Antimonide (lnSb) and Iridium Silicide.

HgCdTe is a well established material with excellent sensitivity extending down to the 8-12 micron band (cool targets, ie FUR applications, satellite tracking, detecting stealth vehicles) but it is difficult to fabricate arrays from because of a very large variation in sensitivity from detector to detector. Such an array will introduce clutter (noise) into the image it views and this will understandably make it more difficult for the image processing algorithm to sift targets from the background. As a result additional signal processing is usually required to compensate for array nonuniformity. At the time of writing Rockwell International were offering 128 x 128 element arrays in HgCdTe. The alternative to HgCdTe in 5. 5 micron band applications (warm targets such as airframes from all aspects, vehicles, exhaust plumes) is lnSb which is easier to fabricate and offers good uniformity. 128 x 128 and 256 x 256 element arrays are offered by Amber Engineering of Goleta, California, who are developing arrays for both civilian and military applications.

Another alternative is the use of Platinum Silicide which is unfortunately about fifty times less sensitive than HgCdTe and is spectrally limited to the 2. 5-4 micron band (ie hot targets such as aircraft/airframes, vehicles, missile exhaust plumes); its strength lies in high uniformity of array sensitivity and ease of fabrication and thus low cost, commercial Silicon fabrication techniques are used as for eg MOS memory chips. This infers another major advantage, the ability to embed within the same slab of Silicon all the necessary electronics to scan and read out the image from the array, thus radically cutting the cost and complexity of the optical system as a whole.

Platinum Silicide arrays of 512 x 512 elements are now offered by Hughes, the highly sought (by the military) 1024 x 1024 array size is expected to become available in the next two years as the technology matures. Iridium Silicide offers like HgCdTe operation in the 8-12 micron band but is very immature as a production technology today.

The choice of detector array having been made for the application, the designer of the tracker will have to select a tracking algorithm and suitable image processing computer or logic for the task. In established applications such as missile guidance (eg AGM-65A/B/D Maverick) tracking can be done with a hardwired electronic contrast lock which places ‘gates’ about an operator specified area of contrast in the viewed scene or image. The position of the ‘gates’ in time relative to the video signal (ie scanned line by line picture) from the array readout electronics then determines the angular position of the target. A newer technique is to save the image in a dedicated area of computer memory and then have a powerful processor sift through the image and accomplish the above in software; this strategy offers flexibility in algorithm performance and the ability to autonomously search for and recognise targets.

As with rosette scanning trackers little has been written in the open literature about suitable techniques for jamming imaging trackers. At a guess, possible techniques could involve seduction of the tracing gate with flares (ie gate stealing) or causing the target to scintillate with phased IR Jammers at its extremities. (eg wingtips), this latter technique would cause jitter in a centroid tracking algoritim. Its effectiveness would be very limited, only during the terminal phase of a fighter or missile attack with the purpose of forcing a near miss just outside proximity fuse range or spoiling the gun aim. In general the passive nature of the IR tracker makes it all the more to jam as the jammer has no means of directly assessing its effect on the threat as with microwave jammers. While all designs will exhibit generic and specific vulnerabilities to jamming, good intelligence and other sensors may be required to identify the IR tracker equipped threat in order to select an appropriate jamming technique.

An alternative to jamming is the use of a suitable laser to blind the tracker by saturating its detector(s), this is a brute force method not unlike barrage jamming which can be very effective against trackers with a slow auto gain control response. By the time the tracker recovers the target will have drifted out of its field of view and the threat has been defeated. In the context of defeating infra-red trackers, Stealth or more formally Low

Observables technology is a potent alternative. A LO design will conceal its engine hot en? from trackers by reducing the viewing

Aperture (1e range of viewing angles). This is achieved by exhaust placement (cf Northrop B-2A upper surface exhausts), shape and the use of louvres and shielding structure. Exhaust plumes are then cooled to a temperature at which the brightest infrared colours are absorbed by atmospheric carbon dioxide and water vapour over fairly short distances. The cumulative effect of these measures is that an infra-red tracker can detect a LO design only from a very narrow range of angles, usually from astern/above, at relatively short range under clear weather conditions only. Tracking is only possible when these conditions are maintained, ie manoeuvre by the LO target or use of cloud will defeat the tracker.

As it is most apparent, there is a broad range of tracker types available and each will have its appropriate offensive and defensive application. Offensive applications involve both weapon guidance and fire control systems.

Infra-red Search and Track Systems

The IRS&T set has been in use, in its various guises, since the 1960s when USAF air defence fighters such as the F-1018 Voodoo and F-106A Delta Dart carried nose mounted nitrogen coole _ d lead selenide cross array IRS&T sets to detect high flying

Russian as the Bear and Bison. The strength of

the IRS&T Is In its ability to detect and track a target passively and thus undetectably. This is considered to be a major asset in air defence operations as it vastly reduces the warning time to attack on a target. In a situation where a large bomber target may carry powerful onboard jamming equipment the use of an Air Intercept (Al) radar by an interceptor will not only provide the bomber with timely warning of impending attack, it will also provide the bomber with an opportunity to defend itself by jamming.

This strategy was adopted by the US Navy which equipped its F-4B air defence fighters with chin-mounted AAA-4 IRS&T sensors, used in part to cue early AIM-9B Sidewinder seekers, these were deleted from later versions as the aircraft’s role shifted to air superiority/strike. The new F-14A became the Navy’s principal air defence fighter and it also carried an IRS&T set although these have been largely replaced by AXX-1 Television Camera System TV telescopes.

The Russians have certainly never been shy about copying Western designs and they too deployed IRS&T sets from the 1960s onward, fitting these to Foxbat and Flogger aircraft of the IA-PVO-Strany. While this equipment has not been publicised in likely to be similar to late sixties US designs, providing a 2 micron band detection capability in clear weather conditions.

The subject of greater concern is the current generation of Soviet IRS&T equipment which is fitted with a boresighted laser rangefinder and in effect provides the same fire control functions as an air intercept radar, under appropriate atmospheric. The fundamental failing of the passive IRS&T is its 1nab1hty to measure range to the target, although some estimate could be made given the intensity of the target’s infra-red signature. The ability of a Fulcrum, Flanker or Foxhound to carry out a full GCI (intercept) and missile or gun attack with electroopt1ca_l systems alone renders the hundreds of pounds of microwave Jammers aboard Western strike aircraft quite ineffective.