Threats to The

By BRUCE VAN VOORST

Of all the weapons of modern warfare, none is more venerable than radar. The seemingly magical technology that enables planes, ships and artillery units to spot the enemy from afar has made the difference between defeat and victory in many a battle. In a Nova TV episode called Echoes of War, which was shown on the Public Broadcasting System last week, radar was hailed as the military's unsung hero of World War II. As physicist I.I. Rabi once recalled, "Maybe we could have won it without the atomic bomb . . . but without radar we could have lost it."

Yet for all its past glory, radar is facing its most perilous assault ever. All the major military powers are working on stealth technologies designed to defeat radar. The U.S. Air Force's new B-2 Stealth bomber, for example, is supposedly almost invisible to radar because its sleek shape and special composite construction tend to absorb rather than reflect electronic signals. The same techniques will soon be used to introduce stealth missiles, ships, satellites and tanks. Moreover, military designers have developed missiles and other weapons that can zero in on electronic signals and thus destroy the ships and planes carrying radar. Faced with these trends, some Pentagon experts have raised a disturbing question: Is radar becoming obsolete?

The issue is of utmost importance to the U.S. armed forces. Virtually all American warplanes use radar, and many costly weapons systems, from the Navy's Aegis system to the Army's Patriot missile, are heavily reliant on the technology. By one estimate, about a quarter of U.S. military investment is radar related. If heavy use of radar becomes questionable, the Pentagon will have to rethink its whole strategy and allocation of resources.

The development of radar (short for radio detection and ranging) applications in the U.S. stemmed from the accidental discovery in 1922 that a ship moving between a radio transmitter and receiver interfered with the signals. The technology came into its own in World War II, when it progressed rapidly from a crude early-warning system barely able to locate ships and aircraft to a sophisticated electronic eye that can spot the periscope of a submerged submarine. Radar works because electronic signals bounce off objects, just as a voice is reflected by walls or buildings. Radar transmits radio waves and "listens" for an echo. The direction of the echo and the elapsed time from transmission determine an object's location. Unlike relatively slow sound waves, radio signals travel at the speed of light and can circle the globe 7 1/2 times a second. Therefore, radar can almost instantly spot targets at great distances. Because it can see through clouds and at night, radar is superior to all other sensors, including optical, infrared, acoustic and magnetic.

But radar has a serious drawback: its signal is a blazing electronic beacon that can make the transmitter as much the hunted as the hunter. "Like a flashlight in a dark forest, radar can spotlight certain trees," says Theodore Postol, an electronics expert at the Massachusetts Institute of Technology. "But everybody in the woods can see it."

Many countries, including the Soviet Union, have an array of antiradiation missiles that can home in on radar-equipped ships or planes and destroy them. It was just such a missile, fired by an Iraqi warplane, that nearly sank the U.S.S. Stark in the Persian Gulf two years ago, killing 37 sailors. A study by the General Accounting Office points out that the Pentagon "has 15 radar ((systems)), costing over $10 billion, which are vulnerable to ARMs." Says Thomas Amlie, a Defense Department official: "Navy's Aegis wouldn't last a half hour in a real war." No one is more aware of radar's vulnerability than the troops. "Turn radar on," goes the saying on the front lines, "and you're dead."

But radar designers are working to overcome its inherent weaknesses. Many systems try to avoid detection by transmitting intermittently or changing the frequency of their signals thousands of times a second. Another approach is "bistatic" radar, in which the transmitter is separated from the receiving "ears." For example, a land-based missile launcher may rely on a radar receiver, but the transmitter may be some distance away. Thus even if an enemy plane detects and destroys the transmitter, the receiver and the missile launcher are not knocked out.

Radar experts also think they can get the better of stealth technology. They maintain that no plane can be completely invisible, since radar always sees something, if only the atmospheric turbulence created by the aircraft. But the radar designers admit that the signals bouncing back from stealth planes will be very weak and hard to distinguish from background noise. The solution, they say, is to improve the computer software used to process the signals. One advanced system, developed by the French and known as "multistatic" radar, uses several different receivers to collect weak signals. Such a system may be able to spot stealth aircraft.

In short, it is too soon to write radar's epitaph. Military history teaches that every technological breakthrough provokes a technical response. Observes Congressman Dave McCurdy, an Oklahoma Democrat who sits on the House Armed Services Committee: "There's a lot of room for radar to evolve, particularly in software and signal processing. Military technology is like a continuing chess game, with no final move."