SA Set to Zero
GPS Selective Availability Turned Off on May 1, 2000
In plain English, we are unscrambling the GPS signal. It's rare that
someone can press a button and make something you own instantly more
valuable, but that's exactly what's going to happen today. All the people
who bought a GPS receiver for a boat or a car, or their riding lawn mower
or whatever, to use in business and in recreation, are going to find that
they're suddenly 10 times more accurate as of midnight tonight. -
Neal Lane, Director of the Office of Science and Technology.
If you take a look at your handheld or automobile Global
Positioning System (GPS) unit today, you'll notice that it's much, much
more accurate now than it was on May 1. The reason? U.S. President Bill
Clinton ordered Selective Availability (SA) turned off at midnight May 1
(Coordinated Universal Time). Now, civilian GPS users around the world will
no longer experience the up to 100 meter (approximate 300 feet) random
errors that SA added to keep GPS a more powerful tool for the military.
Today, GPS units are accurate to within 20 meters (approximately 60 feet);
although in good conditions, units should display an error of less than 10
With SA activated, you really only know if you are on the field or
in the stands at that football stadium; with SA switched off, you know
which yard marker you are standing on.
In 1998, President Clinton directed that SA should be turned off between
2000 and 2006. Fortunately, it happened early in that range of years. The
U.S. military was able to quickly develop and test their ability to
selectively block accurate GPS transmissions in areas of conflict or where
U.S. security was at risk. When the U.S. Air Force Space Command turned off
SA last night, GPS became incredibly accurate for the entire planet.
GPS operates through the use of 24 satellites, paid for by the U.S.
government but free for the world to use, that are orbiting the earth. The
satellites broadcast extremely accurate time signals (accurate to within 40
billionths of a second) using their onboard atomic clocks. GPS units on the
earth triangulate the time signals from the satellites to provide location,
velocity, and elevation of the units themselves. When Selective Availability
was on, GPS units received a scrambled signal from the satellites, which
hindered private and commercial use of GPS.
The current worldwide GPS industry is estimated to be approximately U.S.
$8 billion and there are about four million GPS users worldwide. Now,
experts expect that the demand and use of GPS will skyrocket, leading to $16
billion industry within three years. Use of GPS in a variety of areas has
automatically been vastly improved. For example, automobile GPS units and
mapping software under SA would often place the car one to two blocks from
its actual location; today, GPS can tell which side of the freeway a car is
GPS is actually now more accurate than the accuracy standard for United
States Geological Survey topographic
maps so outdoor enthusiasts should truly appreciate the new accuracy of
their GPS units. Soon, the U.S. Federal Communications Commission will require
location determination technology in cellular phones for use in
emergencies as part of their enhanced 911 service. With a much more accurate
GPS system, GPS might be the technology of choice and emergency responders
will be able to respond more quickly and accurately to these signals from
Future plans for improving the accuracy of GPS include the launching of
eighteen additional satellites that are awaiting launch or are currently in
production. Additionally, two new signals will be broadcast from the
satellites by 2005 to help bypass any distortion from the ionosphere.
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