http://www.nytimes.com/library/tech/00/06/circuits/articles/15next.html
June 15, 2000
WHAT'S NEXT
Pentagon Lets Civilians Use the Best G.P.S. Data
By MATT LAKE
It seems as though almost no car advertisement can be broadcast on television without
some mention of a feature that can pinpoint exactly where you are, even when you are
in the middle of nowhere. And it is getting hard to rent a car in a strange town
without being offered an extra (for around $40 a day) that will give you turn-by-turn
directions to wherever you want to go.
Both types of offers take advantage of a gift from the United States government called
the Global Positioning System, or G.P.S. It uses time signals that are transmitted
from satellites in geosynchronous orbits around Earth and picked up by receivers,
which use that information to calculate those receivers' locations. The math used to
calculate location is complex, based on the time it takes for the signals to travel
through space and the difference in transmission time for signals from as many as four
satellites.
But until May 2, the system was flawed by inaccurate timing signals from the
satellites -- errors that were deliberately introduced. The positioning system was
designed to have two tiers of service: the Precise Positioning Service and the
Standard Positioning Service.
If you were part of the military of the United States or of one of the nation's
allies, or were in an approved civil agency, and if you had big-ticket decoders and
cryptographic passkeys, you could take advantage of the more accurate G.P.S. data from
the Precise Positioning Service. That can pinpoint your latitude and longitude to
within roughly 72 feet (22 meters) -- or less than half that in some cases -- and your
elevation, relative to sea level, to within roughly 90 feet.
But everyone else -- like hikers boaters and people paying Avis for a NeverLost system
in their rental cars -- got much less precise information. The Standard Positioning
Service measurements were accurate to only within about 330 feet (100 meters) for
locations and within about 510 feet for elevation. And what's more to the point, that
inaccuracy in position information was introduced at some cost by an encrypting system
called selective availability.
There were good reasons for introducing selective availability. In the early 1980's
when the Global Positioning System was introduced, its primary function was military,
though there were undoubted benefits to other organizations like commercial airlines
and coast guards.
The typical military approach (which makes sense in wartime, even if it's a cold war)
is to keep anything that may be construed as a threat under tight wraps.
So the government decided that there were good military reasons to limit access to the
most accurate positioning information.
But by 1996, in the post-cold war world, the United States government had decided that
there was less need to spend money on encrypting the precise positioning signal.
So the Clinton administration announced that the government would end the policy of
selective availability to "encourage acceptance and integration of G.P.S. into
peaceful civil, commercial and scientific applications worldwide and to encourage
private sector investment in and use of United States G.P.S. technologies and
services."
The change was planned for 2006, but the timetable was speeded up because the
positioning system has exploded in popularity since the announcement in 1996.
Worldwide, there are now more than four million G.P.S. users, using commercial
receivers that not only calculate latitudes and longitudes but also superimpose those
details onto computerized maps and calculate directions to desired locations. Not only
are these G.P.S. receivers used in car, air and marine transportation, but they are
also becoming critical for mining, oil exploration and agriculture and for emergency
response teams.
And the United States Commerce Department has predicted that worldwide sales of G.P.S.
products and services will reach $8 billion this year, a number that could double by
2003.
With that kind of demand -- coupled with a competing Russian system called Glonass and
talk of another from the European Union -- it made sense to move the deadline forward.
So six years ahead of schedule, the White House ended the policy of selective
availability at midnight on May 1.
The response from G.P.S. receiver manufacturers was quick and positive. The move was
commended on the following day by John Huyett, president and chief executive of the
Magellan Corporation in Santa Clara, Calif., the company that manufactured the first
handheld G.P.S. receiver and also makes Avis's NeverLost system.
"Our products have become more valuable and useful overnight," Mr. Huyett said. "The
same G.P.S. receiver that provided accuracy within 100 meters of a user's location
yesterday is providing position fixes with as good as 10-meter accuracy today."
Dr. Alain L. Kornhauser, founder of TravRoute, which makes navigation systems for
laptops and Pocket PC devices, said: "We should praise the government for doing
something right. Even if the only reason they're doing it is because they did
something wrong before." Dr. Kornhauser is director of the Interdepartmental
Transportation Research Program at Princeton University.
Even though removing the deliberate errors from G.P.S. theoretically makes the system
10 times as accurate, drivers using G.P.S. navigation systems should not expect to see
that much improvement.
Although a receiver is less likely to think that you are on a road parallel to the one
you are actually on -- and give you impossible turning directions -- the G.P.S. is
prone to other errors.
Since G.P.S. receivers need signals from four satellites to calculate position
precisely, accuracy will suffer if signals from one or two satellites are lost.
That can happen to someone driving along tree-lined roads or next to power lines. But
in many cases, the algorithms built into a G.P.S. receiver's navigation system will
cope with short interruptions.
"The software can extrapolate forward," Dr. Kornhauser said. "You'll be in the car for
an hour, pass under trees for three or four seconds, and the software takes that into
account, based on the speed and direction of travel. You don't need that level of
precision."
A greater source of error is atmospheric conditions, which can degrade the microwave
carrier signals that G.P.S. satellites transmit and introduce errors.
Any error is very likely to be compounded, creating larger positioning errors, because
the G.P.S. uses calculations that are based on signals sent from as many as four
satellites. And even small signal errors can result in large positioning errors
because position calculations are based on minute time differences; the problem
becomes worse if more than one satellite's signals contain errors.
To overcome the errors introduced by signal noise and create more consistently
accurate positioning, another enhancement to the G.P.S., called Differential G.P.S.,
is under development and already partly rolled out.
Differential G.P.S. uses stationary ground-based receivers to monitor the signals from
the constellation of G.P.S. satellites.
These stationary receivers do not cover nearly as much of the world as the G.P.S.
satellites, but they are becoming widespread in coastal areas, where they are of most
value to ocean shipping and transportation. The Differential G.P.S. stations are
strategically located so they can spot signal errors generated by G.P.S. satellites
and figure out how to correct them.
To correct the signal error, the station transmits the correction formula, which
specially equipped G.P.S. receivers can receive and use to correct the signal. Using
that trick, the Differential G.P.S. can provide accuracy to within about three feet.
The result of such increased accuracy has freed G.P.S. navigation companies to
concentrate on their customers' needs. Instead of dedicating computing cycles to
removing errors caused by either the government or the ionosphere, G.P.S. navigation
products can concentrate on faster and better directions.
"Now that the computer doesn't have to do all that," Dr. Kornhauser said, "the
processing power can be used to suggest, 'You might be better off trying this -- what
do you think?' The in-vehicle navigation system becomes an advisor, not a commander."
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krys, you live on in our memories
your life's promise merged with our own dreams
but when we heard you died we cried
no one could answer the question, "why?"
-- jeradonah, Oct 30 '99
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