JED - January 2013 - (Page 40)
Infrared Systems and Countermeasures – Part 2
IR Guided Missile Threats
By Dave Adamy
The Journal of Electronic Defense | January 2013
nfrared (IR) guided missiles are significant threats to aircraft because the hot aircraft make easily distinguishable thermal targets against the cold sky. These can be air-to-air or ground-to-air missiles, including shoulderfired Man-Portable Air Defense Systems (MANPADS). Open-source literature states that up to 90 percent of aircraft losses are caused by IR missiles. IR missiles passively home on emitted IR energy from a target. As discussed last month, the wavelength of the energy emitted by an object depends on its temperature. The hotter the object, the shorter the wavelength at which its IR emission peaks. IR missile sensor material is chosen for maximum response in the wavelength of peak emission at the temperature of the chosen target of the missile. Early IR missiles operated in the near IR region, requiring very hot targets. Their sensors needed to see the hot internal parts of engines, so the missiles were restricted to attack from the rear of a jet airplane. Later IR missiles use sensors that can operate against cooler targets, such as the engine plume or the aerodynamically-heated, leading edges of wings. Thus, they can attack from any aspect.
Mirrors (Symmetrical) IR Dome IR Sensing Cell
Optical Axis Reticle
Figure 2: The IR seeker focuses received IR energy onto a sensing cell through a reticle.
As shown in Figure 2, the seeker receives radiated IR energy from the target through the IR lens, and focuses it onto an IR sensing cell using multiple shaped mirrors. The IR signals are filtered and passed through a reticle to the IR sensing cell which generates a current proportional to the power of the
THE IR MISSILE
Figure 1 is a diagram of a heat seekθ θ ing missile. On the nose, there is a lens θ θ that is transparent at IR wavelengths. θ Behind the lens is an IR seeker which rack ile T θ generates signals from which the guidMiss θ ance and control circuitry can determine the direction to the target. The guidance and control group controls steering surfaces, such as rollerons, which conFigure 3: IR missiles use proportional guidance to avoid the requirement for a high-g turn as they trol the direction of flight. Then there is approach their targets. a fuse and warhead. Because the missile homes on the target, it will actually hit the target and can, received IR signal. Note that the seeker is oriented along an therefore, use a contact fuse in many cases. Finally, there are a optical axis which is offset from the missile’s thrust axis. As solid state rocket motor and stabilizing tail fins. shown in Figure 3, the missile uses “proportional guidance” so that it will approach the target at a gentle angle. If the missile were aimed directly at the target, it would be required to make IR Dome a “high-g” turn near impact.
Seeker Guidance Control Group Fuze/Warhead Rocket Motor
There are several types of reticles with different characteristics. Figure 4 shows a “Rising Sun” reticle which was used in early IR missiles. This reticle has 50 percent transmittance over half of its surface and the other half has alternating clear and
Figure 1: The amplitude of the signal into the sensing cell varies with the angle between the target and the optical axis of the seeker.
Table of Contents for the Digital Edition of JED - January 2013
The View From Here
From the President
Program Profi le: SEWIP
Index of Advertisers
JED Quick Look
JED - January 2013