The best-known passive IR application is the seeker head of a heat-seeking missile — from shoulder-launched MANPADS to modern air-to-air missiles. The seeker has to do one thing very well: find the target's heat, hold on to it and turn it into steering commands (Passive homing) — while the opponent tries to deceive it with flares (Flares) and DIRCM (Directional Infrared countermeasures). How a seeker does that determines its generation.
Types/generations of seeker heads (MANPADS)
IR seekers have evolved through four generations, each generation an answer to the countermeasures against the previous one. The development is easy to follow in MANPADS:
| Gen. | Technique | Characteristics | Examples |
|---|---|---|---|
| 1 | Uncooled/cooled PbS, spin-scan reticle, MWIR/NIR | tail-chase only (rear aspect, hot exhaust); very susceptible to flares and the sun | SA-7, Redeye |
| 2 | Cooled, conical-scan | more accurate, partly all-aspect, somewhat better flare rejection | SA-14, early Stinger |
| 3 | Two-colour (two bands, e.g. IR + UV), rosette scan | all-aspect (including head-on), good flare discrimination based on spectrum and kinematics | Stinger (FIM-92), SA-18 Igla |
| 4 | Imaging IR (focal plane array, FPA) | "sees" an image instead of a point source; image processing, target recognition, excellent CCM | SA-24 Igla-S, AIM-9X, IRIS-T, ASRAAM |
The common thread: each generation makes the seeker harder to deceive. Gen 1 falls for almost any flare; a gen-4 imaging seeker sees the silhouette of an aircraft and recognises a flare as a separate point source falling away (Advanced flares). That is why the defence against modern seekers is shifting from flares to DIRCM (Directional Infrared countermeasures).
Seeker scan patterns
A non-imaging seeker has only one or a few detectors. To know where in its field of view the target is (and not just that there is heat), it "scans" the scene — and the way it does so determines its accuracy and jam resistance:
- Spin-scan: a rotating reticle (a kind of coded "windmill") modulates the target signal in amplitude (AM). The pointing error follows from that modulation. Drawback: a target right on the axis produces little signal (the seeker is "blind" there), and a continuous IR source (flare/jammer) can easily imitate the modulation.
- Conical-scan (con-scan): the line of sight nutates in a small cone around the axis. This gives a frequency-like (FM) error measurement that is more accurate around the centre and harder to jam.
- Rosette-scan: the detector traces a rosette-shaped path over the field of view and so samples almost the whole scene — a quasi-image with which the target can be distinguished from the background and from flares.
- Staring FPA (imaging): a 2D detector matrix looks at the whole field of view continuously; no mechanical scan is needed any more. Image processing delivers the pointing error and target recognition — the gen-4 approach.
FOV and FOR
FOV is what you see at this moment, as if looking through a straw. FOR is everything you can see by turning your head.
In practiceA MANPADS seeker has a narrow FOV: the gunner has to point the tube roughly at the aircraft and wait for the lock tone before firing. In a modern combat aircraft the seeker of an IR missile can be slaved to the pilot's helmet sight: it can then look far off to the side, giving a large FOR.
Two concepts you come across with every seeker (and every EO/IR sensor) — and that are often confused:
- FOV (Field Of View): the angle the seeker sees at this moment — the "cone" of the detector. A narrow FOV gives high sensitivity and resolution, but sees only a small patch of sky.
- FOR (Field Of Regard): the total area the seeker can cover by moving its FOV around (see gimbal, Gimbal). The FOR is much larger than the FOV.
Mnemonic: FOV = what you are looking at now; FOR = everything you can look at by turning your eyes/head. The seeker moves its narrow FOV through the large FOR to search for a target and, once found, to keep tracking it while missile and target manoeuvre.
Gimbal
How does the seeker move its FOV through the FOR? With a gimbal: a cardan suspension (usually two axes — pitch and yaw) on which the optics/detector can rotate. The gimbal does three things at once:
- Searching: steering the narrow FOV through the FOR until the target is found.
- Tracking: keeping the line of sight on the target while the missile rolls and steers — the gimbal decouples the looking from the missile's body motion (otherwise the image would swing in all directions).
- Providing the pointing error: the angle between the line of sight and the missile axis feeds the guidance law (proportional navigation, Guidance methods).
The gimbal has mechanical end stops: the gimbal limit (look-angle limit). If the target manoeuvres outside that angle — for example a hard turn close past the missile — the seeker can no longer follow it and the lock breaks. A high gimbal limit (large FOR) and a fast gimbal make a missile much more agile against evading targets.
Some modern seekers are strapdown: no moving gimbal, but a fixed wide FPA with the "looking" done electronically in software. Fewer moving parts, but it requires a large detector and heavy image processing — comparable to the step from mechanical antenna to AESA in radar (AESA — Active Electronically Scanned Array).