GPS Finally Gets Its Fix
by Elaine Dickinson
Millions of navigators worldwide who use the U.S.-built Global Positioning System (GPS) satellite signals to fix their positions at sea, in the air or on land just got handed a big bonus from Uncle Sam. On May 1, President Clinton ordered the U.S. military to immediately stop “scrambling” the civilian GPS signal.
Since midnight Greenwich Mean Time on that date, every boater with a GPS receiver is getting position accuracy that is 10 times better than it’s been since the early 1980s, all without having to touch a button. Without the deliberate degradation, called “Selective Availability” (SA), the civilian GPS signal went from accuracy of within 300 feet to about 30 feet. In a general sense, it’s the difference between finding your location on an entire football field to finding it on the deck of a 30-boot boat.
The administration also pledged to keep the signal free for all users for the foreseeable future.
“For more than 15 frustrating years, BoatU.S. and many others have asked the military, the Department of Transportation, and the White House to put an end to Selective Availability,” said BoatU.S. Chairman Robert Schwartz. “It was extremely difficult to explain to boaters why a multi-billion-dollar system our government built was being deliberately altered, especially in the 1990s when GPS grew to have so many different consumer uses.”
As civilian uses for GPS exploded in the 1990s and they grew into an $8 billion-a-year industry, many asked why SA remained. The answer from the military, which built and ran the 24 satellites, never varied: “national security.” The Pentagon, which still reserves another Precise Positioning signal just for military use, argued that the civilian signal had to be degraded to deny its use to enemies of the U.S.
That reasoning carried far less water in 1996 when the U.S. Coast Guard launched its own coastal network of corrected GPS signals, called Differential GPS, or DGPS. By buying an additional receiver, anyone could pick up a radio frequency with a GPS position corrected to within several meters of accuracy. This signal covers U.S. coastlines, Great Lakes and major rivers, up to a range of about 125 miles offshore. For consumers, the high cost of a DGPS receiver was prohibitive. Originally they were in the $500 price range but have since come down to about $300.
But also in 1996, after years of pressure, the Clinton Administration, in an effort led by Vice President Al Gore, announced that SA would be done away with as soon as the Pentagon could overcome its security concerns.
The policy on SA was to be reviewed every year, beginning in 2000, and it was a surprise to everyone that Clinton acted to ban SA the very first year of review. In a briefing at the White House, Arthur Money, Assistant Secretary of Defense, said the U.S. now has the ability to deny the GPS signal to specific regions of the world. According to the White House announcement, all agencies from the National Security Council to the CIA were in agreement with the military that SA should end.
So where does that leave boaters who have Differential GPS receivers or were thinking about getting one? Differential GPS, according to at least three companies making DGPS receivers, will continue to to correct errors in the regular GPS signal caused by ionospheric distortions in the earth’s atmosphere, bad data transmissions, or the satellite’s orbit. The corrected DGPS signal should be accurate from three to 15 feet, about twice as good as regular GPS.
This greater pinpoint accuracy is especially important for sportfishermen in returning to an exact fishing location or to divers in locating an underwater wreck or other dive site. DGPS was primarily built for commercial ships in U.S. harbors where being just a few feet out of a channel can spell big trouble.
“DGPS will still provide a higher level of accuracy, even with SA turned off,” said Bob Buzzell, Senior Vice President of the BoatU.S. Product Division. “Those boaters who previously had the need or desire for greater accuracy provided by Differential GPS will still want the heightened accuracy of DGPS continues to provide.”
The Coast Guard has said that Differential GPS service will continue and they have no plans to shut it down.
From Boat/U.S. magazine, July 2000.