Heat-seeking missiles have been in use against both rotorcraft and fixed wing aircraft since the mid-1950s, and countermeasures to deceive their guidance systems have been employed for nearly as long. Typically, countermeasures operate by generating a strong infrared signature which confuses the missile tracking system. At the heart of these systems is some form of powerful infrared emitting source. Often this source requires optical coating(s) to perform wavelength filtering needed for optimum operation. However, both the mechanical configuration of the thin film filter coatings used and the operating extremes to which they are subjected present challenges for the manufacturer.

Infrared Countermeasures

Since their introduction, heat seeking missiles have been a particularly successful and deadly threat for military aircraft. In fact, a report issued in 2009 estimated that over the preceding 25 years, 90% of all US air combat losses had been caused by heat seeking missiles.

The basic operating principles of a heat seeking missile are relatively straightforward, although the actual implementation has become increasingly sophisticated as each new generation of systems seek to overcome the counter-measures developed to thwart them. Simply stated, the heat seeking missile relies on the fact that an aircraft engine is significantly hotter than anything else in the surrounding environment. Therefore, it emits a strong mid-infrared signal which allows it to be easily identified. For helicopters, in particular, the primary sources of IR emission are the engine exhaust duct, any hot engine parts that can be viewed externally (such as turbine blades), parts of the tail boom that are heated by the exhaust plume, and the exhaust plume itself.

The peak of aircraft infrared emission is typically in the 4 μm – 5 μm wavelength range, which also happens to correspond to the window of high atmospheric transmission which occurs in the 3 μm – 5 μm band. As a result, a typical IR guided surface-to-air missile can lock on to an aircraft that is over 3 miles away. Additionally, heat-seeking guidance is passive. This stands in contrast to radar, which sends out a signal that must be returned (and which can therefore be detected by the target). As a result, there is no way for an aircraft to know that it is being targeted by a heat-seeking missile.

Figure 1. A) Evaporative deposition produces porous coatings which can B) absorb moisture, thus changing their spectral response. c) Sputtering produces much denser films which are largely impervious to water absorption.

The simplest and earliest IR counter-measures were flares (hot sources) which could be dropped by an aircraft. The intent is that the missile guidance system will lock on to these, rather than the aircraft engine, and thus be lead away from the real target. However, flares only burn for a short period of time, and an aircraft can only carry a limited number of them. Furthermore, the pilot must be aware of being targeted in order to deploy flares. Especially for helicopters, which often remain relatively stationary in high risk areas for extended periods, a longer duration (always on) counter-measure was required.

As a result, heat-seeking missile counter-measures based on hot sources were first introduced in the 1970s. Most commonly, these rely on a block of silicon carbide material which is heated until it radiates a significant amount of infrared energy, specifically in the 4 μm – 5 μm band utilized by missile tracking systems.

The first heat-seeking missiles utilized a rotating reticle in their imaging systems which would sweep over the image of the target. When the target is off-center, this reticle movement causes a pulsed signal to be produced. When the target is centered on the sensor, the signal becomes constant. This constant signal was required by these early missiles in order to indicate that the target had been acquired, and thus allow a launch.

Typically, the countermeasures for these types of trackers employ a cylindrical mechanical shutter that surrounds the hot source and modulates its output. The resultant stream of infrared pulses is approximately synchronized with the rotation speed of the reticles, thus producing a spurious signal that prevents the tracking system from locking on to the target. Alternately, for a missile already in flight, the IR pulses from the countermeasure confuse the targeting system causing it to veer off course. The countermeasure system can be either placed directly on the rotor-craft structure, or, in some cases, in a towed pod. The latter is employed because sometimes the bright infrared output of the countermeasure actually works to improve the lock of the missile on the target.

Figure 2. The long mean free path of coating material molecules in vacuum deposition techniques makes layer thickness highly dependent upon the orientation of the optical surface relative to the material source.

While subsequent generations of some missile guidance systems have moved past this simple reticle-based technique, advances in the operation of the countermeasure systems have continued to maintain their effectiveness. And, the basic concept of using a bright mid-IR source to jam or confuse the missile tracking remains unchanged.

In some countermeasure systems, the hot silicon carbide block is surrounded by a cylindrical tube, which is optically coated to modify the spectral output of the source in order to further optimize its effectiveness. The advantage of a cylindrical tube is that it makes the angle of incidence on the coating uniform (90°) in all cases. However, producing such a coating presents two significant problems. First, the coating is subjected to a very wide operating temperature range, typically around -55°C to 600°C. It must be able to survive a rapid rise in temperature (to around 400°C) when the source is first powered on. In addition, throughout this entire temperature range, the coating transmission characteristics must remain relatively constant.

The second challenge is that of uniformly coating a cylinder around its entire circumference. The specifics of both these issues, and solutions to them, are examined in the following sections.