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GPS: The
federal government has stopped scrambling the
signals, rendering devices using the navigation system
far more precise.
By Kevin Washington
Sun Staff
The federal government gave hikers, boaters, truckers and
other navigators a surprise present last week when it made
their Global Positioning System receivers 10 times more
accurate than they were the week before.
On May 1, the military stopped scrambling the GPS signals
broadcast from a constellation of 27 satellites orbiting the
Earth from a little more than 12,000 miles away. The Defense
Department, which had been adding fuzz to the signals to keep
enemies from using them in operations against U.S. forces, now
says it can rescramble the signals on a regional basis if war
breaks out.
Before the switch, GPS devices, which help sailors find their
locations and ambulance crews locate accident scenes, could
usually pinpoint their location within 325 feet - a variation
larger than a football field. With the scrambling gone,
GPS units will be accurate to between 48 and 60 feet.
People who sell and use GPS devices are ecstatic.
"This was one of those things that was a
no-brainer," says Alain L. Kornhauser, a professor at
Princeton University and CEO of TravRoute, which makes
GPS-based navigation systems used in cars, trucks, boats and
other vehicles. "This is a realization that we've been
making a mistake. ... The Cold War is over. Why not let the
public that has paid for this get the full benefit of
it?"
Neal Lane, President Clinton's science adviser, put a slightly
different spin on the decision: "It's rare that someone
can press a button and make something you own instantly more
valuable." Popular with outdoorsmen and used for
navigation, surveying, construction and tracking of truck
fleets, GPS has been helping people find their way for
more than a decade. Born in 1973 as Navtech GPS, the system
went into operation in the late 1980s once the 24 required
satellites were launched into orbit.
Those satellites, equipped with accurate atomic clocks,
broadcast a series of precisely timed signals. A GPS device
reads the signals from four satellites, then uses mathematical
equations to compute location. The satellites have been
sending signals on two frequencies.
The first frequency, which can be read only by military GPS
receivers, is precise but encrypted. It's used for tracking
missiles, coordinating forces and guiding munitions, GPS
experts say. The second, for civilian use, had been degraded
through a technique called "selective availability"
by the Department of Defense.
That scrambling involved throwing random nanoseconds into the
precise timing of the signals, says JoAnn Cummings,
president of Adventure GPS, an Internet retailer based in
Decatur, Ala. In turn, the location of a building, reef or
tree would appear to move around within a 50- to 100-meter
radius when plotted on a map.
Given the growing use of GPS among civilians - the White House
estimates that about 4 million people globally use the system
- pressure from businesses and private citizens finally
persuaded the government to change the policy. No one expected
the decision last month.
"We knew it was going to end early, but not this
early," said Percy Aquino, who works in sales at Navtech
Seminars and GPS Supply. "A lot of people in the
GPS community have been waiting for this," says Derek
Reiber, associate editor of GPS World magazine.
"Originally, the president signed a directive putting the
time line where they were going to turn off [scrambling] by
2006, and they would start looking at it this year."
The real winners are those using GPS for recreation and
businesses that cater to them with low-cost units.
Manufacturers of cellular telephones, who will be required by
the Federal Communications Commission next ear to make sure
all cell phones are capable of revealing their positions, will
benefit from the increased accuracy as well.
In-vehicle navigation may improve the most. For example, under
the scrambled system, a driver on Guilford Avenue near
Interstate 83 in downtown Baltimore might have appeared in his
navigation system to be on I-83 itself. That's far less likely
to happen now.
Kornhauser says his job just got easier. For years, he has
come up with algorithms - a series of mathematical equations
used to correct for the scrambling in his PC-based Co-Pilot
navigation systems. "As it was, the navigation systems
rarely made a mistake" because of the use of map-matching
algorithms, he says. "It will make fewer mistakes fewer
times because it won't be spoofed here and spoofed
there." Ambulance crews, police and firefighters
who use GPS should be able to shave valuable minutes off their
response times, says Richard Langley, a columnist for GPS
World and professor of geodesy and precision navigation at the
University of New Brunswick. "Sometimes, in the past,
they have gotten a position [through GPS], but been on the
wrong side of the freeway," he says.
Even with the change, the GPS signal broadcast to average
users needs refining before it's good enough for businesses
that demand precision. In fact, the accuracy of GPS
depends upon several factors, including the quality of the
receiver, says Langley. For example, atmospheric interference
can throw off the timing, and dense tropical forests with high
moisture content aren't good for receiving GPS signals at all.
In cities, signals bounce off tall buildings, and the urban
canyon phenomenon - think of the narrow streets and
skyscrapers of Wall Street - can block a GPS receiver's view
of the sky.
For that reason, precise navigators have been using something
called differential GPS for years. DGPS requires an add-on
receiver, available for as little as $300, to take
measurements from so-called "correction" signals -
one of which is broadcast by the U.S. Coast Guard. If a ship
is navigating a busy harbor, the pilot needs DGPS to get
accuracy as close as one to three meters. That way, says Cummings,
"you don't get an Exxon Valdez."
But things will get better for those who buy cheaper units,
too. Langley says the demise of scrambling will give
manufacturers an incentive to make all their GPS receivers
more precise.
Originally published on May 8 2000
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