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COMMENT: Prof. Per Enge, Aeronautics/Astronautics (415) 723-2853

Precise regional aircraft navigation by satellite demonstrated

STANFORD -- Tests conducted by Stanford researchers indicate that the type of satellite navigation the Federal Aviation Administration (FAA) plans to adopt by the year 2001 can accurately track an aircraft's position as it travels over long distances.

Using an experimental version of the system, called Wide Area Differential GPS, the Stanford scientists successfully have tracked with a 3-meter accuracy a small plane traveling around central California. Per Enge, professor of aeronautics and astronautics, reported their results on Dec. 11 at the annual meeting of the American Geophysical Union in San Francisco.

The acronym GPS stands for Global Positioning System, a satellite-based navigation system created by the U.S. Department of Defense that is finding widespread civilian use. The GPS system consists of 24 satellites that beam special radio signals that allow people with special receivers to determine their position in three dimensions.

Standard GPS, which can be picked up by hand-held receivers costing less than $300, can pinpoint a position to within 100 meters. While useful for many purposes, this accuracy is inadequate for aircraft navigation. So the FAA plans to establish an additional system consisting of a number of ground stations and geosynchronous satellites that will broadcast a correctional signal that substantially improves the accuracy of the standard GPS signal.

The Stanford researchers have put together an experimental version of the system using ground stations in San Diego and Arcata, Calif., and Elko, Nev. Information from these stations is sent to the Stanford campus, where it is processed. Correctional information is then broadcast to the aircraft by UHF radio.

"The big problem with using GPS signals over large geographic areas is atmospheric distortion," Enge said. This comes from two sources: variations in the lower atmosphere, or troposphere, and in the ionosphere. Tropospheric effects appear to be difficult to predict using information from a widely distributed ground system but they also appear to be quite small. Ionospheric effects, by contrast, appear to be very large, up to 50 meters, but predictable using the ground system, he said.

So the researchers are using the information from the ground stations to create a computer model of the ionosphere that predicts its effect on the radio signal from the GPS satellites at any given time. This information is broadcast to a computer on the aircraft that uses it to correct the GPS signals.

Thus far, the researchers tests in the San Francisco Bay Area indicate that their modeling efforts appear to be working quite well, Enge reported. But not as much research has been done on these effects as he would like. One reason he is presenting this research at the AGU meeting is to get criticisms and suggestions for improving their methodology from members of the atmospheric science community.

In addition to using Wide Area Differential GPS for transcontinental navigation, the FAA also would like to use it to replace the current Category I Instrument Landing System (ILS) that is installed in more than 800 airports around the country. Category I ILS is a complex system of radio signals and beacons that guide pilots in for landings when they can see the runway from an altitude of 200 feet or greater.

Annual repair and maintenance of these systems cost the federal government about $100 million per year. Most of the equipment is old and will need replacing by the end of the decade, Enge said.

"But we have a heavy burden if we are going to replace ILS. There is no record of anyone ever being killed using the system. So it has a safety record of 100 percent, which is very difficult to match," he said.



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