By Marshall Brain
The Stinger missile, officially known as the FIM-92A, was designed to give ground troops a way to deal with low-flying aircraft that may be bombing or strafing, doing surveillance work, servicing ground based enemy troops. The Stinger is a lightweight (about 35 lbs) portable weapon with a reusable launcher. Because it is a shoulder-launched weapon, one person can launch a Stinger missile if necessary. The missile uses a passive infrared seeker to lock on to the heat of an enemy aircraft, making it a "fire-and-forget" weapon.
To fire the weapon, the soldier aims the missile at the target. When the seeker locks on, it makes a distinctive noise. The soldier pulls the trigger, and two things happen: first, a small launch rocket shoots the missile out of the launch tube and well clear of the soldier who is firing it. Then, the launch engine falls away and the main solid rocket engine ignites to propel the Stinger at about 1,500 mph (Mach 2). The missile then flies to the target automatically and explodes.
The Stinger missile can hit targets flying as high as 11,500 feet, and has a range of about 5 miles. They are also extremely accurate. While the missile is flying, the image of the airplane that it is trying to hit may become off-center on its image sensor. When it does, that tells the missile that it is off-course, and the guidance system in the missile has to decide how to get back on course. This is where proportional navigation comes in. The missile looks at the angle of off-centeredness and adjusts its angle of flight proportionally. In other words, it uses a multiplier. If the multiplier is 2, then if the guidance system thinks it is 10 degrees off course, it will change its flight direction by 20 degrees. Then, a tenth of a second later it will look at the angle again, and change again. This allows the missile to actually "anticipate" the path of a moving plane.
