Finding Your Way Home From Anywhere by Satellite

For Bill Harkins, a constellation of 24 satellites orbiting 12,000 miles above Earth has brought renewed independence.

An avid backpacker who frequently took solo outings into the Connecticut hills or New Hampshire's White Mountains, Harkins says he had to sharply curtail his outdoor activities in 1992 when injuries from an auto accident left him with memory and orientation problems. ''I had to cut out solo backpacking completely,'' he says.

But three months ago, he bought a $300 hand-held receiver for signals from the United States Air Force's global positioning system (GPS). This device will give him his precise location via signals from four satellites. He can enter the location of his home or his car and keep track of directions and distances from wherever he is.

Prior to the accident, ''I'd always had a feeling of freedom, of being able to jump into the car and go,'' says the resident of South Meriden, Conn. Now, he adds, that feeling has returned. After parking near a trailhead or on a roadside, ''I enter the car's position and wander to my heart's content. And I've returned to solo camping.''

While not everyone has the same need to know their latitude and longitude as Bill Harkin does, civilian use of GPS technology is exploding, according to industry analysts.

''Two to three years ago, the end product was either military or civilian navigation equipment,'' says Mike Swiek, executive secretary of the US GPS Industry Council, a four-year-old trade association in Washington. ''But GPS is information. Integrating it with other information technologies is expanding the market.''

Other civilian uses

Other widely publicized civilian uses include building GPS technologies into automobiles to give drivers directions, locate stolen cars, or alert emergency services to auto accidents the moment an air bag is deployed. In California, geophysicists are using GPS systems to measure the buildup of stress along faults in the Los Angeles Basin.

Efforts are under way to incorporate GPS systems into other applications, including:

r Aiding the blind: Last December, the European Union voted to spend more than 1 million ($1.6 million) to help a consortium of private companies develop a GPS-based package for the blind. It would combine video, infrared, and voice-synthesis technologies. A fully integrated package is expected to be available beginning in 1998.

r Heavy construction: Researchers at Ohio State University are developing a GPS-based system for surveying and grading construction sites. The ultimate aim: robotic bulldozers that can grade a site to within 1 centimeter of specifications.

r Farming: Using GPS systems, companies are developing ways to deliver highly tailored amounts of seed, fertilizer, pesticides, or water to specific patches of field, which can increase yields and efficiency.

Before long, says Mr. Swiek, the technology will be included in cellular telephones and in laptop computers, helping companies keep track of sales forces and more efficiently use their resources.

As the price of the technology continues to fall and the accuracy increases, civilian GPS navigation systems will reach a broader market.

Late last month at a meeting of the International Civil Aviation Organization in Montreal, President Clinton reiterated the US's commitment to provide international access to GPS for peaceful purposes.

The worldwide market for GPS equipment jumped from roughly $40 million in 1989 to $600 million last year, according to the Industry Council. By the year 2000, the market could reach $8 billion.

The first four GPS satellites were launched in 1978. A total of 40 have been built, with an average design life of 7-1/2 years.

The satellites get information on their positions from ground stations controlling the system. Since the satellites' positions are known and the distances to them determined by highly accurate timing signals, a user's GPS receiver takes data from the four satellites within the receiver's horizon that are farthest apart. The data are then used to triangulate the receiver's position.

For the military, which uses a separate set of signals from civilian users, positions can be determined to less than 25 to 40 meters (75 to 125 feet). Civilian receivers using the basic signals give accuracies no worse than 100 meters (300 feet), although they often do better than that, Swiek says. But by combining the signals from the satellites with one from a ground-based transmitter whose precise position is known, civilian accuracy improves dramatically.

Improving accuracy

SATLOC Inc., a Tempe, Ariz., company, announced yesterday a GPS-based navigation system for crop-dusters. It uses data from GPS satellites along with positioning information from 10 ground stations in North America. The data are relayed to a computer in the airplane. The signals correct inaccuracies in the GPS data, guiding the crop-dusters to within roughly three feet of their destinations, according to the company.

Meanwhile, two teams of companies -- one led by Rockwell International, the other by Martin Marietta and Hughes Space and Communications Company -- are getting ready to bid on the next-generation GPS system, which is expected to cost $6 billion. The Pentagon is expected to award the contract in December.