Science >> Science Articles

New Age of Precision Brought to Navigation by Modern Gyroscopes

William J. Broad - January 3, 1984

THE quiet revolution in the navigation of planes, submarines, spacecraft and missiles is picking up speed as internal guidance systems become smaller, cheaper, more versatile and more accurate, according to industry experts and scientists working in the field.

For a quarter-century these ''black boxes'' have guided all sorts of airborne objects from here to there with increasing precision. They have sensed, remembered and computed every movement, however slight, constantly checking actual motion against flight plans. The stakes have been high. In the Apollo program, even tiny errors in navigation would have sent the spacecraft far off course in its quarter-million-mile trek between the earth and the moon.

The heart of a guidance system used to be, and sometimes still is, a spinning gyroscope. But new technologies such as lasers and powerful computer chips are transforming the science of guidance. The Pentagon's new antisatelLight warhead, which itself rotates at 20 revolutions per second for stability, is guided to its target by a tiny internal ring of laser light that easily sorts out and defines the welter of motions as the weapon spins through space.

Technical advances have not eliminated failure, as was forcibly demonstrated in December when one of the three identical guidance systems on the space shuttle Columbia broke down. And though human error is the prime suspect in the wandering off course of the South Korean jetliner shot down over the Soviet Union last September, a guidance system failure has not been ruled out.

But experts say ruggedness and precision are increasing as electronic circuits replace mechanical parts. ''I don't think the possibilities in terms of accuracy have been completely developed at all,'' said Dr. Charles Stark Draper, a pioneer in guidance and founder of a laboratory in Cambridge, Mass., that bears his name and today employs 2,000 people. ''A lot of people have introduced a lot of new ideas. The kind of performance you're going to get will depend on the effort.''

Not just a technical issue, the evolution of guidance systems is sometimes seen in a political light. Last November the city of Cambridge voted on whether to ban research related to nuclear weapons, the primary issue being the Draper Laboratory, which designs guidance systems for many American missiles.

Circumstances far less controversial than those of the nuclear era prompted the initial quest for precise guidance. In the late 1920's, airplane pilots sought a way of learning their whereabouts in three dimensions while flying at night or in a cloud bank. At the time there were no radio direction finders.

The solution was to rely on inertia - that little tug a person feels whenever a car accelerates, for example, or turns a corner. Newton's first law of motion says a mass once set in motion tends to remain in motion, unless acted on by an external force. In concert with this law, a gyroscope tends to keep its initial plane of rotation once set spinning. Its stability allows sensitive measuring devices around it to record changes in direction.

In 1948 Dr. Draper, working for the Air Force, combined gyros with simple computers and devices known as accelerometers, which sense changes in speed, to form the first true inertial guidance system. An important advance was that the gyro was ''floated'' in a viscous liquid so quick accelerations would not throw it out of whack. The prototype weighed two tons, but the system was later scaled down. On its first flight, it guided an aircraft 500 miles to within a mile of its target destination.

Two decades later, Dr. Draper's laboratory, an offshoot of the Massachusetts Institute of Technology, made a system that guided the Apollo capsule to a splashdown in the Pacific a quarter of a mile from a recovery ship. ''It got to the point that they told the ships not to go to the exact spot so they wouldn't get hit,'' Dr. Draper said in an interview.

Today the cutting edge of guidance development is occupied by the laser gyro. Rather than relying on the forces of inertia, it measures changes in counterrotating beams of laser light that flash around in a tight circle. If the laser gyro itself turns a bit, one beam of light will travel slightly farther around the ring in a given instant of time, the other slightly less far. Differences in the time it takes the laser beams to travel around the ring add up to a precise measurement of the gyro's motion.

The advantages of laser gyros are numerous, according to scientists at Honeywell Inc., which makes ring laser gyros currently used on the Boeing 737, 757, and 767 jetliners. A conventional mechanical gyro works in dramatically different ways at different temperatures and takes some time to reach a stable speed. Laser beams, on the other hand, always travel at the speed of light.

About the size of a hard-cover book, a ring laser gyro also does away with the complicated system of mechanical gimbals that suspend conventional gyros. Laser gyros can be strapped down to any handy surface in a plane, missile or spacecraft.

''They're cheaper, smaller and weigh less than the old gimbal systems,'' said John Gautraud, a vice president of the Northrop Corporation, which makes guidance systems for the military. Without moving parts, they are simply more reliable.

At Litton Industries, one of the world's largest producers of inertial navigation systems, Joseph F. Caligiuri, a vice president, said the laser gyro might eventually be ''transcended by newly emerging technologies, such as fiber optics, an even more advanced application of light energy to inertial navigation.''

The current king of accuracy, however, is not the laser gyro or a fiber optic device but an esoteric creation for the military known as the electrically suspended gyro. At its heart is a hollow beryllium sphere, which has reference marks on its surface and is suspended in a magnetic cradle. Nothing touches it. Even air is removed from the housing in order to reduce friction. As the sphere spins, a beam of light is bounced off its reference marks and thus measures changes of orientation.

This type of incredibly precise gyro is the navigational brains behind the new generation of American missiles - the land-based MX and sea-based Trident. Older American missiles use mechanical gyros suspended on gimbals, pivots that allow the gyros to remain level whatever maneuvers the missile might perform. The extent of the new accuracy is secret, but published accounts say the MX can send warheads to within a few hundred feet of targets.

And such precision will probably increase. According to Dr. Kosta Tsipis, a physicist at the Massachusetts Institute of Technology and expert on military systems, the accuracy of missiles historically has doubled every seven years and will continue to do so because of advances in guidance systems both for missiles and ejected warheads. ''The improvement may slow somewhat as room for improvement narrows,'' he said, ''but there's little reason to expect the pattern will be fundamentally different in coming years.''

For instance, strides will continue to be made in the realm of electronics, according to military experts. Increased logic, memory and computational ability in very small spaces allow a missile to obtain finer readings from guidance instruments, to process them in a more complex manner and to send more sophisticated directions to control systems. Moreover, tiny chips allow greater use of redundant circuits to perform backup calculations and to take over if other systems fail.

All this alarms some critics of the nation's weapons programs. Last year, a local ordinance was proposed in Cambridge that would have made the pursuit of nuclear weapons research within the city limits a crime. The Cambridge voters rejected the proposal, but the prime target was the Draper Laboratory, which hired a team of public relations consultants to fight the measure. The laboratory has $140 million in Defense Department contracts and has designed guidance systems for such missiles as the Polaris, Poseidon, Trident, Thor, Titan, Minuteman and MX.

Robert A. Duffy, president of the Draper Laboratory, said the accuracy and application of all guidance systems would increase, both in military and commercial areas, despite the criticism. ''There's no question,'' he said in an interview. ''You can see these systems appearing on the more expensive executive aircraft. My own feeling is that we've not yet begun to see where it's all going to go. There's a lot more ahead. And whatever trends you see in the commercial area you can be sure are being led by the military.''

Source: www.query.nytimes.com/gst/fullpage.html

Science >> Science Articles