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Passive seekers will pick up an emanation (light or heat) generated by the target and steer themselves toward it. Active seekers will send out radar pulses and steer toward anything solid enough to bounce them back. A hybrid is the semi-active seeker. In this system, a ground transmitter will illuminate the target with powerful radio waves. These, rejected from the target, will be picked up by the oncoming missile and will guide it in for the kill.
Television Steering. For air-launched missiles directed against invisible targets many miles from the mother plane, the problem is more difficult. They must gather their own guiding information. They may seek the target, attracted like deadly moths by the heat given off by a city or industrial area. More likely they will watch the ground ahead with radar or television eyes. The picture will pass over a microwave beam to the mother airplane. Sitting in his cockpit, the bombardier can watch a screen and see what the missile itself sees. If the missile is off its course (as determined by a map), he gives it radio hints that point it toward the target. When the doomed city comes in sight (at 30 miles per minute), he turns the missile downward. Then another radio signal or an automatic fuse explodes its atom bomb.
This baleful guidance system is not so futuristic as it sounds. Television-guided aircraft have already flown over U.S. cities. The remote-control pilots several hundred miles away saw rivers, bridges, buildings. "We picked out the city hall," said one pilot. "We could have flown that plane right into the mayor's office."
The television or radar-picture guidance system is good only for some 500 miles. At greater distances, the microwave beam between missile and mother plane will hit the curving earth. But less range than 500 miles is plenty for many vital missions. Keeping safely out of reach of enemy radar, the bomber could launch its attack. Presumably both plane and missile would keep radio silence until the missile has only minutes to fly. This would leave time for the guiding system to correct the course of the missile. It would not leave time for enemy interceptors to reach and attack the bomber.
Stars & Magnetism. Surface-to-surface missiles will have a wide choice of guidance systems. If the target is visible (from the ground or a high-flying airplane), the missile will be steered to it by radio command guidance. Usually the target will not be visible, but its position will be known on a map. Then the missile will follow a radio beam or steer automatically toward a selected point in a pattern of radio waves marked out in space. When it reaches that point it will curve downward. If the target's position is not known accurately, the missile will search for it with television or radar eyes, reporting its findings to a screen at a faraway control point. An operator watching the screen will steer the missile to the burst point.
Beyond the range of line-of-sight radio (a few hundred miles) the guidance problem gets tougher. The missile is on its own, and it must steer itself by some "frame of reference" that reaches all the way to the target. Several systems are in development, two of them familiar in principle: magnetic (compass) navigation and navigation by the stars.