There are essentially two generations of MANPADs. The first generation, which includes the SA-7, has uncooled seekers with lead sulphide detectors operating in the 1-3µm near-infrared band, which can only see hot engine parts. The engagement geometry is highly constrained: the missile has to be launched from the rear of the aircraft or the seeker will not see the hot tailpipe.
First-generation IR-guided missiles can be defeated in two ways: flares and jammers. The uncooled sensor cannot tell the difference between the target and a flare and can be spoofed by a waveform jammer that generates a fluctuating infrared signal - "think of a fan blade in front of a hot brick", says David Schmieder, senior research engineer at Georgia Tech Research Institute. The jammer has to have a radiant intensity greater than that of the engine.
The second generation, which includes Raytheon's Stinger, has cooled seekers with indium antimonide detectors operating in the 3-5µm mid-infrared band, which can see the exhaust plume and skin emissions. This provides an all-aspect lock-on capability, allowing the missile to be fired from the front or the side.
The SA-7 Grail and its derivatives are the most prolific of MANPADs. The original SA-7a entered service in 1966 with its successor, the improved SA-7b, arriving in 1972. The missile is limited in range, speed and altitude capability. The seeker has no protection against decoys or jammers and is easily confused by solar or ground heat. Indeed, Schmieder's theory about the Kenyan incident is that the SA-7 locked on to sun glare from the 757's wing, but the wing was too small a target for the missiles, which had to hit the aircraft to explode (the SA-7 has a contact fuze as opposed to the Stinger's proximity fuze). "Glare from the wing would have provided a very attractive target," he says. "But the probability of hitting anything as thin as a wing would be very small. The missile would pass within a few feet."
In fact, the SA-7, although simple to operate, does not have a great success rate. Later versions, the SA-14 Gremlin, SA-16 Gimlet and SA-18 Grouse, have much better seeker and kinematic capabilities. In addition to the large numbers of SA-7s built in Russia, improved versions of the missile have been produced in China (H-5), Egypt (Sakr Eye), Pakistan (Anza) and Yugoslavia. The SA-14, -16 and -18 are more lethal and difficult to defeat, but whether any are in the hands of terrorists is unknown.
About 400 Stingers supplied to the Mujahedin in the mid-1980s remain unaccounted for, but are believed to be nearing the end of their useful life. Later Stingers, and both the SA-16 and SA-18, have dual-colour seekers that minimise their vulnerability to flares and jammers. Although such advanced weapons may not be in the hands of al-Qaeda and its associates, they could pose a longer-term threat to civil aviation, requiring more sophisticated countermeasures and pushing up the cost of defending airliners.