Science / Medicine : Satellites as Sextants : A new $8-billion orbiting navigational system will provide military and mariners alike with accurate positioning all over the globe
There was a quarter moon that night, but a thick fog blotted it from view.
At 5:30 a.m. a computer science technician named Valerie Wong stood in a parking lot in Monrovia and pointed a gray battery-powered plastic box at the heavens and homed in on some military satellites.
She was trying to find out, literally, where she stood.
The screen flashed 34 degrees, 8.38 minutes north; 117 degrees, 59.77 minutes west. “This is going to be accurate within 30 meters,” Wong said.
Punching some buttons on a computerized receiver to find your position doesn’t quite have the romance of Capt. James Cook squinting through a sextant in the 18th Century to guide his ship to the Pacific, or of Charles Lindbergh nosing the Spirit of St. Louis down to within 10 feet of the Atlantic so he could look at the waves to gauge the wind and help keep him on course to Paris.
But the Air Force’s new collection of navigation satellites, called Global Positioning System, will help people find their way around the globe with more precision than ever before. The $8-billion project will provide the military with navigation sitings accurate to about 50 feet anywhere in the world, day or night, in the worst of weather.
This degree of accuracy is available only at the high-level military frequency used by GPS, which eventually will require encryption devices to decode. Another channel, with an accuracy of roughly 300 feet, is planned for broadcast in the open for anyone--foreign, domestic, military or civilian--to use.
Surveyors, oil firms, aerospace companies and the military have tested and generally liked GPS, and in the next few years GPS receivers will be installed in planes, boats and even cars.
At the moment only eight test GPS satellites are in orbit, so there are gaps in coverage around the globe, including certain areas of the South Pacific and some of the polar regions.
At this time of year the satellite coverage is available in Los Angeles about five hours a day, which is why Wong was out so early. She works for Magellan Systems, a small company that plans to begin selling in March $3,000 portable 2-pound GPS receiver that, on the power of six penlight batteries, will help sailors find their way around the world. Until the Air Force starts beaming out two signals, civilian users will have access to the military-quality signal.
The Air Force took the first step to boost GPS to full-time coverage last week with the launch of the first of the next batch of satellites from Cape Canaveral. Several satellites will be fired up each year until the full constellation of 21 to 24 satellites is completed in 1992, which in six orbital planes will blanket the Earth from about 11,000 miles up.
Granted, there are plenty of other navigation tools, including various modern radio beacons. But these have their weaknesses.
“People have found a way to cope with other navigation systems,” said Thomas Stansell, director of advanced programs for Magnavox, which, along with Rockwell International and several other companies, makes GPS receivers for the military. “What they are going to find out about GPS is it does it better than anything else they’ve ever seen.”
The Soviets seem to agree. They are launching their own copycat design called Global Navigation Satellite System, a 24-satellite system for their military, with plans to offer Glonass for civilian use worldwide as well.
Two researchers from the University of Leeds in England have been tracking Glonass and their early reviews aren’t glowing. Depending on who is counting, the Russians have launched from 27 to 39 Glonass satellites, yet only a handful are working properly.
The Air Force’s GPS satellites work by a combination of atomic clocks and radio beacons, but the basic navigation principle is as old as the sextant. It uses time and several points of reference in the sky to plot a fixed position.
Each of the satellites has four atomic clocks that are accurate to within 50-billionths of a second.
A GPS receiver must locate signals from four satellites to calculate its three dimensional position: longitude, latitude and altitude. Because each satellite constantly emits a signal that identifies its position as well as a time check, a GPS receiver can measure how long it took to receive a signal and, by multiplying it by the speed of light with the help of some computer-aided trigonometry, can continuously come up with a precise fix.
The first GPS test satellites were launched in 1978, and the American military is sold on them.
“Soon as you let the troops get their hands on it, they come back with nothing but praise for the system. The equipment is its own best advertisement,” said Air Force Col. Marty T. Runkle, GPS program director.
The military has 23,000 GPS receivers on order. Some infantry soldiers haul around GPS receivers that are a compact 10 by 12 by 4 inches, and the Army wants half of its tanks equipped with GPS receivers.
Other sets will go into fighter planes (the most expensive receivers are for this purpose and cost $45,000), ships, submarines and missiles. Magnavox’s GPS sets, Stansell said, have been used “on every mine-sweeping task force in the Persian Gulf.”
GPS is expected to be particularly helpful to pilots because several established navigation systems have troubling imperfections.
The Federal Aviation Administration operates 970 high-frequency transmitting stations that provide navigational fixes accurate to within 1 nautical mile. The trouble is that the signals travel line of sight and, because of the curvature of the Earth, a pilot must switch from one station to another every 100 miles or so. Even worse, the signals are blocked by mountains and are worthless in rugged areas.
Another common land-based radio signal is Loran, which helps ships and planes find their way within 75 miles of the American and Canadian coastline. But bad weather, notably thunderstorms, scrambles the Loran signal.
On long flights over water, commercial and military jets rely on inertial navigation, which is essentially a combination of a fancy gyroscope that creates a reference point by spinning in the direction of the North Pole and a sophisticated speedometer that charts the distance traveled.
The gyroscope’s bearings create friction that causes it to drift slightly, however, so the longer a flight, the greater the error. Pilots expect an inertial system to be wrong by 1 nautical mile per hour of flight.
It’s also essential that the pilot punch in his correct position before taking off; it he makes a mistake, the inertial system keeps compounding the blunder, which may explain why in 1983 a Korean Air Line jet was 300 miles off course over Soviet airspace when Russian fighters shot it down.
Boeing navigation engineer John Neilsen expects GPS to be commonplace in airliners within 10 years, and he favors GPS as a tool for civilian planes.
The FAA is excited about GPS for both safety and cost reasons. Most airline pilots want to fly with the jet stream to arrive faster and save fuel, but on flights above 29,000 feet, planes must be separated laterally by at least 2,000 feet, so some must fly a less direct route.
“All aircraft want to fly in the same place, and you can’t do that now,” said Clyde Miller, manager of the FAA’s advanced concepts division.
With GPS’s accuracy, he said, more planes “lanes” could be squeezed together, as extra, but narrower, lanes might be added on a freeway. More efficient flying on the busy North Atlantic route, Miller said, could save $450 million a year in fuel.
For all the dazzle of GPS, there remain some bugs to work out. Private industry hates the idea of the military cutting back the civilian accuracy to 100 yards. Why a separate, less-accurate signal for civilians? “You wouldn’t want to provide the enemy the same advantage you are getting yourself,” Runkle said.
The civilian aviation community is also leery about depending solely on GPS because it fears that in a crisis, the military might cut off the civilian signal. Another weakness in both GPS and Glonass is that if a satellite is faulty and sends out bad signals, there is no immediate warning to civilians. The FAA is pressing for a 10-second GPS warning system, and it may set up ground stations to monitor the satellites for faulty signals.
Those problems aside, Chrysler thinks there is a dollar to be made on GPS and has hooked up a GPS test system in a mini-van that plots positions on a television screen with street maps shown in quarter square mile detail. Robert Jensen, production engineering manager for Chrysler’s Acustar division, says that if GPS is on schedule, Chrysler can sell a GPS navigation system as an option its 1993 luxury cars for about $1,500.
As prices drop, Jensen says, GPS could become standard equipment in rental cars. “If you do a lot of traveling and arrive in a city for the first time and don’t know where to begin, GPS is ideal,” he said. “It’s like having somebody next to you who knows where to go.”
