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Successful test of satellite-based landing system may open new era in aircraft navigation

STANFORD -- Stanford graduate students have successfully tested what may be the most accurate aircraft navigation system ever placed on a commercial jetliner.

The system, which is designed to allow autopilot landings in even the worst weather conditions, employs a military satellite navigation system -- the Global Positioning Satellite System (GPS) -- to track the position and attitude of an airplane with centimeter- level precision.

Other groups have shown that satellite navigation can be used to guide aircraft across long distances and in the vicinity of airports. But the Stanford "integrity beacon" system is the first to demonstrate that a satellite-based system can reliably land an aircraft even in conditions of near-zero visibility.

After exhaustive testing and development in a small Piper Dakota, the system was tested in a United Airlines Boeing 737-300. The system allowed the plane to make 110 successful automatic landings out of 111 attempts. The airliner tests took place Oct. 11- 14 at Crows Landing, a NASA test field in central California.

Officials from the Federal Aviation Administration, which funded development of the system, say that the Stanford success may speed the adoption of GPS satellite navigation as the standard for the next generation of ir traffic control systems, with the potential to make flying safer, to make on-time landings easier and to save billions of dollars in fuel by making aircraft navigation simpler and more direct.

FAA administrator David Hinson called it “a landmark for GPS development . . . it provides both accuracy and integrity.” Hinson personally flew the agency's Beechcraft King Air equipped with the Stanford system as part of earlier tests at the FAA Technical Center in Atlantic City, N.J.

The origin of the system dates back to four years ago, when Clark Cohen started work on the idea that earned him his Ph.D. in aeronautics and astronautics at Stanford. That idea was a small but ingenious improvement to one of the most basic research projects at the university. He invented a way to determine the exact tilt of a satellite within one centimeter. The invention will mean more accurate data-gathering for Gravity Probe B, the NASA-sponsored project that will test Einstein's theory of general relativity sometime late in this decade.

By early this month, Cohen's invention had undergone a near-complete technology transfer - it had been adapted to control a commercial airliner. The plane's flight controls were linked up to a computer programmed by Cohen and graduate students Stu Cobb, Dave Lawrence and Boris Pervan that translated GPS navigation signals into information about the aircraft's position.

With added data from a pair of credit-card-sized ground transmitters invented by the Stanford team, the on-board computer tracked the plane's position, altitude and speed in real time to an accuracy of two centimeters.

The success of the test flights helps prove that commercial airliners soon will be able to safely use GPS satellite navigation, according to Ray Swider, project manager for GPS research at the FAA.

Stanford's contribution to this future navigation system has several unique characteristics:

A matter of attitude

The story began with an attitude problem. Researchers with the Gravity Probe B project needed to know the precise attitude, or tilt, of the satellite that will carry their test of Einstein's theory into space. For his dissertation research, Cohen proposed using the high-flying GPS network of satellites to determine Gravity Probe B's attitude.

The 24 GPS satellites circle the Earth in geosynchronous orbit. Four to six of them are in sight of a person standing virtually anywhere on the surface, and a simple receiver can compare the data from several satellites to fix the receiver's location.

However, the signals available to non-military users from GPS satellites are not precise enough for Gravity Probe B's purposes. Even when refined with differential GPS, normal satellite navigation systems can locate an object's position only within a couple of yards. Cohen took advantage of the radio carrier wave that GPS satellites use to transmit navigation information to refine that location down to a few centimeters.

Cohen's faculty adviser, aeronautics and astronautics Professor Bradford Parkinson, was a leader in development of the GPS satellite network when he was a colonel in the Air Force. Parkinson is now co-director of Stanford's Gravity Probe B project. He also leads a group of 22 graduate students investigating various uses for GPS. Parkinson and Cohen quickly realized the value for aircraft of this low-cost navigation aid. With a grant from the FAA, they organized the “flying quartet” - Cohen, Cobb, Lawrence and Pervan - to develop the Stanford integrity beacon aircraft landing system.

Their initial concept was developed in less than two years and tested on a Piper Dakota owned and piloted by Stanford aeronautics and astronautics Professor J. David Powell. It consisted of a few hundred dollars' worth of parts, a portable computer and thousands of hours of graduate student labor.

Navigating by (satellite) stars

Here's how the system works: Two flat antennas, each the size of a pack of cards, are fitted to the top and belly of the plane. The first receives position data from satellites in the sky; the second picks up signals from two pseudo-satellites, or pseudolites, credit-card-sized transmitters that beam GPS data from the ground. These are placed on the approach to a runway. Their signals reach only about 1000 feet in the air, so they do not interfere with high- flying planes that use satellite navigation (see illustration).

As the plane approaches the runway, it flies through the overlapping “bubbles” of radio signals sent out by the pseudolites. The on-board computer uses those signals to calculate the plane's exact position with centimeter-level accuracy in three dimensions. For this reason, the team calls the pseudolites integrity beacons.

Once locked on to this position, the computer continues to read satellite signals with the same accuracy throughout the plane's descent. These precise data guide the autopilot as it lands the plane.

In addition to Cohen, others in the quartet each contributed to a key aspect of the process. Cobb was in charge of the electronic interface between the computer and the aircraft.

He also met a challenge from the others to fit all the electronics for the integrity beacon pseudolite onto a board the size of a credit card. Pervan wrote the software that converts the pseudolite signals to a precise position reading as the plane flies through a bubble.. Lawrence wrote the computer code that filters the GPS measurements and keeps accurate track of the plane's position in real time as it continues on to its landing.

“The team has spent countless hours in close quarters to ensure that each element of the system meshes seamlessly. We've also had help from many other students in the aero/astro department,” Cohen said.

The 737 test

Other groups have shown that satellite navigation could be practical to guide aircraft across long distances and in the vicinity of airports. With cooperation from United Airlines test director Gerald Aubrey and engineering flight test Captain Bill Loewe, the Stanford technology was tried against the FAA's most difficult criterion: a Category III (autopilot) landing, accurate and reliable enough so the pilot could trust his instruments in almost-zero visibility.

Loewe says he was convinced that the system would work after the first few successful touch and go landings. The other 100 landings were just a matter of folding his arms and watching the plane land.

More development and more tests will be needed to meet FAA requirements for a practical landing system. The test results will be statistically analyzed and estimates of the probability of failure will be refined. For Category III landings, the requirements are very high: The statistical chance of a system error should be no more than once in a billion landings.

This reliability, or integrity, is the reason that Cohen believes his team's super-accurate system may be practical for future use. No pilot needs to know a plane's position within a few centimeters, and not many care about landing on a dime. But Stanford's system sounds the alarm if it detects an error of even a few centimeters: It has built-in integrity.

“Based on our data analysis so far, we're confident that it will be duck soup to surpass the FAA's standard,” Cohen says.

This flight test has political significance. Swider says it is one of several the FAA is sponsoring as it prepares for an international meeting next March of the United Nations-based International Civil Aeronautics Commission. At that meeting, governments will decide which new technology should replace the world's current standard air traffic control system, which is based on World War II vintage methods. U.S. government representatives will propose using GPS, instead of a microwave system that many U.S. airlines oppose as less versatile and more expensive than satellite-based navigation. Skeptics must be convinced that satellite guidance is as safe, or safer, than the alternatives.

Proving that it could be practical is one reason that United and other airlines are supporting tests of GPS navigation, says Aubrey. “GPS will revolutionize avionics,” he predicts, because each plane will have the capability to pinpoint its own position in the air and avoid all others. That may lead to big fuel savings as airliners are permitted to choose more direct air routes instead of following in a few designated air lanes. At busy airports, instrument-assisted GPS landing systems may allow airplanes to follow each other onto the runway more quickly - and get more passengers to the terminal on time.

NASA administrator Daniel S. Goldin took a test flight as a passenger in Stanford's Piper Dakota test plane on Aug. 19. NASA is the chief supporter of the Gravity Probe B project, and Goldin was especially impressed with the four-year turnaround from one problem involving basic research to a practical test of a prototype that solves very different problems.

He told the students, “It's interesting. Columbus came to the new world in search of gold and spices, and went back with corn and potatoes. Here you are, reaching to understand Einstein's general theory of relativity, and you may end up saving the nation billions of dollars in revolutionized air travel.”

Cohen is still amazed by that sequence of events. “So many dimensions of the process are amazing,” he said. “That we're receiving signals from satellites 11,000 miles away, yet we can get a position within centimeters from that information. That as far as we know, this is the most accurate precision landing system yet devised, and the lowest cost system yet devised.

“It also has the potential to become the world's safest landing system. And fortunately, in this case safety can also be very affordable.”



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