Science / Medicine : NEW HORIZONS IN RADAR : The U.S. and Australia are developing a new technology to upgrade aging early warning networks.

Prompted by technical advances and the need to upgrade their aging early warning networks, military services in the United States and Australia have embarked on ambitious programs to build the first unclassified over-the-horizon radars designed for everyday operation. Using the earth’s ionosphere as a giant mirror, the radars can “see” over the horizon, keeping tabs on ships and low-flying airplanes as far as 2,500 miles away.

The U.S. Air Force has begun testing a system in Maine, is finishing another near the California-Oregon border and has further plans for systems in Alaska and in the Midwest.

The U.S. Navy, meanwhile, just tested what it calls a relocatable over-the-horizon radar, designed to be moved as needed during a war to track and assess enemy air and sea forces. It is now being set up on the Aleutian Island of Amchitka, for operation next year.

And the Australian military plans to break ground next year for a network of three over-the-horizon radars for monitoring northern approaches to the continent.

The radars, which can cost a half-billion dollars each, are uniquely suited to diverse military needs, from warning of a bomber attack to monitoring the launch of test missiles deep within the borders of a foreign country. There have even been reports that over-the-horizon radar has promise in detecting stealth aircraft.

But promising civilian applications have also been explored in tracking suspected drug-running aircraft and long-range monitoring of weather and sea conditions. Both take advantage of the vast swath--6 million square miles--that can be covered by a single over-the-horizon radar within about 30 seconds.

Many of the principles behind the new radars were first proven by a pair of experimental Navy radars built in California in the 1960s. The Wide Area Research Facility in the Central Valley and the Sea Echo radar on San Clemente Island are still used for research into new technologies and the ionosphere.

That work is critical because it is the ionosphere--layers of charged particles blanketing the earth at altitudes between 50 and 300 miles--that makes the new radars work. Conventional radar waves, which are emitted in frequencies above 200 million cycles per second (200 megahertz), penetrate the ionosphere and continue out into space.

Over-the-horizon radars, on the other hand, use frequencies between 5 and 28 megahertz, which are bent and reflected by the ionosphere back toward the Earth’s surface hundreds of miles away from the radar. From there, a small portion--perhaps 1%--of the radio waves are reflected back toward the ionosphere and then down to the radar, carrying with them the coded imprint of any craft that happens to be within the radar’s sweep. Thus the systems are known as over-the-horizon backscatter radars (OTH-B for short).

Depending on the frequency used and the conditions in the ionosphere, the radar’s beam might reach the Earth’s surface anywhere from 600 to 2,500 miles from the transmitter, If an aircraft happens to be flying within that beam, it too will reflect some radio waves back toward the radar via the ionosphere, regardless of how low the aircraft may be flying. A conventional radar would not see an aircraft sneaking in at a low altitude, because of the Earth’s curvature, until it was much closer--perhaps less than 50 miles away.

In a dimly lit control room in Bangor, Me., operators of the OTH-B would see the aircraft as a small yellow square of light on the screen of a television-type monitor. Powerful computers automatically compare the aircraft’s flight path with those of scheduled civilian and military flights. If no match can be found, operators notify the nearest Air Force regional operations control center.

There, based on information from satellites and other sources, officials decide whether to “scramble”--send out aircraft for a closer look. Since the radar went into initial, limited operation late in 1987, Soviet aircraft have been intercepted 36 times, according to the 24th Air Division, which is responsible for operating the radar. Most of the aircraft are bombers on training flights. Occasionally they fly within 115 miles of Cape Hatteras, N.C., an Air Force spokesman said.

Although the idea behind them is simple, OTH-Bs are among the most sophisticated of radars. When an aircraft flies into their beam, the amount of radio energy the aircraft reflects back to the antennas is minuscule compared to the signal bounced back from the Earth’s surface below the craft, typically that of the sea. That surface echo, called clutter, may be 10,000 times stronger than the signal of interest, so finding the signal is a lot like finding the proverbial needle in a haystack.

Making matters worse is the ionosphere itself, which changes radically from year to year as sunspots come and go, from month to month as seasons change, and even from minute to minute at dawn and dusk. Each of these tends to affect one or more of the ionosphere’s four layers of charged particles, requiring operators to make adjustments to compensate. Especially troublesome are the sudden ionospheric disturbances caused by solar flares or other events on the sun, which can blind or cripple an OTH-B radar for periods ranging from a few minutes to several hours, according to Jurgen Buchau, a researcher at the Geophysical Laboratory at Hanscom Air Force Base in Massachusetts. (Since the episodes are unpredictable and short-lived, Buchau noted, they are not considered useful to an enemy plotting a surprise attack.)

To cope with those problems, OTH-B radars have enormous antennas and many computers (the Air Force models have 28 apiece) to analyze signals and compare radar tracks with known flight paths. Each of the Air Force units consists of three 4,000-foot transmit antennas and three mile-long receive antennas. To avoid interference, receive and transmit antennas are about 100 miles apart; the West Coast receive site is in Tulelake, Calif., in the northern part of the state, while the transmit site is in Christmas Valley, Ore.

When finished sometime in 1991, the West Coast units will cover a 180-degree arc from Mexico to Alaska, out as far as the Hawaiian Islands. The radar, like its counterpart on the East Coast, is being built for the Air Force by General Electric’s Government Electronic Systems Division in Syracuse, N.Y.

To spot targets in a sea of clutter, OTH-B radars make use of the Doppler effect: When a signal from a radar is bounced off a target moving toward the radar, the frequency of the signal that comes back is higher. How much higher depends on how fast the target is moving. OTH-B radars use that frequency shift to distinguish a target from the surface below it.

Air Force officials say the radars have no trouble spotting ordinary bomber aircraft. A more delicate question now is whether they can spot aircraft built with so-called stealth technologies, known as “low-observable” techniques in military jargon. Such aircraft have shapes and exteriors designed to minimize the reflections of waves from conventional radars, which use relatively high frequencies and look up at their targets, rather than down from the ionosphere.

If OTH-B radars can perform well against stealth aircraft, as several radar experts have speculated, the Air Force’s B-2 stealth bomber would seem less able to complete its mission of penetrating Soviet airspace. The Soviets already have at least three OTH-B radars, according to open Pentagon reports.

These days, Air Force officials take care when discussing the issue. The head of the Air Force’s OTH-B program, Col. John O. Lenz, said: “With this kind of radar, because of the frequencies you’re using and the angle at which you look at the target, we present a little bit tougher problem for the designer of offensive low-observable craft. It’s simply mathematics.” No tests using stealth aircraft have been done and none are planned, Lenz added.

Still, the Department of Defense has become more open since the early days of OTH-B research about three decades ago, when the technology was in its preliminary stage and was not discussed publicly. Since then there have been scattered, unconfirmed reports of various forms of over-the-horizon radar being employed by the United States and Great Britain for spying.

Such military uses have been the mainstay of work on OTH-B radars, but their civilian potential has not been ignored. In 1973, the Navy disclosed that it had used an experimental radar on the Maryland coast to monitor the size and direction of ocean waves over vast areas of the North Atlantic. The wave information let scientists quickly infer the wind patterns above the waves with great accuracy, the Navy said.

The procedure was never put into regular use in the United States, but it has long been used in Australia. Meteorologists there have for years had access to data from an experimental OTH-B radar called Jindalee (the name is aboriginal for bare bones).

And the U. S. radars have not been entirely limited to military uses. Officials from the Drug Enforcement Administration and the Air Force confirmed that the Maine OTH-B radar has been used to track suspected drug-running aircraft over the Caribbean. And that application is likely to become common as the military begins fulfilling its role in government efforts to stem the influx of illegal drugs. Officials of the North American Aerospace Defense Command, which is directing and coordinating surveillance of airborne drug traffickers, recently identified the proposed set of four OTH-B radars as the cornerstone of its future plans against aerial smuggling.