Lasers zero in on radar

Press, 10 February 1982, Page 12

Lasers zero in on radar

Froni

the "Economist."

London

Radar has been the main sensor for spotting and identifying military targets since World War 11. Now the Vought company in the United States is developipng a new kind of “radar” that uses lasers . instead of elec< tromagnetic radiation. 1. Strictly speaking, radar is electromagnetic and works by projecting a thick, lobeshaped beam. This produces' good mesurements of a target's range. But the thick beam means radar has two problems. It has trouble pinpointing the angular position of small targets and also determining ■ their sizes and shapes.

The radar beam can be “sharpened up” electronically — and signal-process-ing techniques have been developed that allow moving

targets to be distinguished ’from stationary ones. Nonetheless.the resolution is not good enough to tell ■ the' difference between, say, a locomotive and a truck. An addded drawback of electromagnetic radar is that its resolution gets worse as its. antenna gets smaller. This makes it hard for radars in small devices, such as missiles, to get the very high resolution they need to attack targets that may be “hidden”' among' buildings or other objects. '

The attraction of lasers’ is that, in contrast to conventional radar’s biggish beam, they project pencil-thin beams of light. These beams can distinguish between objects only a few feet apart, at distances of several miles. Till now, the small size of laser beams has also carried a penalty. It has been impractical to design a means of scanning, with the accuracy and speed needed to cover a lage area, with thousands of small pencil-beam pulses. Vought, a newcomer to high-technology sensors, be-

lieves it may have solved the problem. It is developing a “laser radar” for use in guiding its new small hypervelocity anti-tank missile. In generating the laser, energy is reflected from one, surface to. another within a cavity (see diagram). Vought has . developed a “magic mirror,” made of a transparent .material coated with vanadium dioxide, for use within (' this cavity. . The material, when bombarded by an electron beam, becomes a mirror at the point Where the beam strikes it. By using a device called a thermochromic cathode-ray tube (CRT), the rejecting spot may be moved rapidly, and precisely, thus altering the geometry of the cavity and moving the laser beam across the target. The computer can then be programmed to make the laser scan an area systematically. There are still problems in doing this rapidly enough to cover a suffi-

ciently large area to locate the target and to cover it completely enough to provide the precise definition needed. ? '

Vought’s solution is simple, as such things go. The computer is given some basic instructions, similar to those a. human follows unconsciously when, for examaple, looking for a person or car in a forest. The computer is given “pictures” of what to look for (typically tanks). When the laser radar starts scanning, it looks at only every eighth possible spot within its field of view.

If it begins to “see” a “picture” (really a set of spots) that satisfies some of the criteria for calling it a tank, the computer redirects the laser beam back to the “high-interest” area and asks it to scan every spot within it. The results can be startling: a tank, painted out in great detail, including gun, turret and even the “bustle” on the rear of the turret. Laser beams are attenu-

ated (weakened) in the atmosphere by clouds and other material that conventional radar can see through. So their range is much shorter and their performance more erratic. Also, there are some questions — even within the short ranges now being considered — whether lasers can produce better and more reliable pictures than an in-fra-red scanner, the other main contender for guiding “fire-and-forget” anti-tank missiles. What is no longer in doubt is that “laser radars’” can be made to work. They can produce pictures that can distinguish tanks from other vehicles — or almost anything else.