Subject: GPS SA shutdown

From: Miguel <[email-masked]>
Date: Thu, 04 May 2000 09:15:42 +0200

Hi all,

I've read a few days ago about the US government disabling the
Selective Availability feature on civilian code GPS system (that
provided an artificial error in measured location). Because some routing
algorithms are using GPS information I think this can be good news for
these algorithms.

I'd like to know if this is really true (I was not able to track it
down) and if true, the expected location precision.

Thanks,

--
Miguel Sanchez Lopez                            |       [email-masked]
Universidad Politecnica de Valencia             | voice:+3496 387 9700
Camino de Vera, 14                              | fax:  +3496 387 7579
46071 VALENCIA (Spain)                          |

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From: Ron Orr <[email-masked]>
Date: Thu, 04 May 2000 12:11:16 +0200

Here is the scoop:
Here in Bosnia the accurcy of my Garmin etrex went from 90 meters to
less than 10.
-----------------------------
Clinton Unscrambles GPS Signals

12:50 p.m. May. 1, 2000 PDT

WASHINGTON -- President Clinton on Monday gave the go-ahead for letting
boaters, motorists, and hikers use a satellite-navigation system with
the same pinpoint accuracy as the military has long enjoyed.

Clinton ordered that at 8 p.m., EDT on Monday night, the U.S. military
stop intentionally scrambling the satellite signals used by civilians to
improve the accuracy of Global Position System receivers tenfold.

The decision, based on four years of deliberations, is likely to be a
boon to the GPS industry, which is already expected to double in the
next three years, from $8 billion to more than $16 billion.

The possibilities for increased commercial use are wide: air, road,
rail, and marine navigation, precision agriculture and mining, oil
exploration, telecommunications, electronic data transfer, construction,
recreation, and emergency response.

Consumers who have bought GPS receivers for boats, cars, or recreation
will find that they are 10 times more accurate when the military ceases
disruption of the signal beamed down from orbiting GPS satellites. The
White House said it should generate greater acceptance of the system.

"In plain English, we are unscrambling the GPS signal," said White House
science adviser Neal Lane. "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."

"This is a very significant step forward in furthering the worldwide
utility of GPS for peaceful civil, commercial, and scientific pursuits,"
he said.

Emergency teams responding to a crisis will be able to determine what
side of the highway they must respond to, saving precious minutes.

Currently, a GPS-based car navigation could give the location of the
vehicle to within 100 yards, a problem where multiple highways run in
parallel. The signal change will eliminate these problems and possibly
lead to greater consumer confidence in these devices.

Outdoor enthusiasts already use GPS but the greater accuracy will
provide accuracy of 20 or better, allowing hikers, campers and hunters
to navigate their way through unmarked wilderness terrain.

In fact, the White House said, the accuracy of GPS will exceed the
resolution of U.S. Geological Survey topographical quad maps.

Recreational boaters will enjoy safer, more accurate navigation around
sandbars and rocks. Fishing aficionados will find more quickly a good
fishing hole on lakes and rivers, and commercial lobster boats will find
it easier to recover their traps.

The GPS system is a product of super-reliable atomic clocks that emerged
from Nobel prize-winning physics. It can calculate position, velocity,
and time anywhere on the globe, any time of the day or night, in any
kind of weather.

Artificial errors intentionally have been introduced into the satellite
signals for years for national security reasons, giving the military a
far more accurate system than civilians.

Lane told reporters the military would be able to scramble the signals
on a regional basis should it need to do so for national security
reasons.

Copyright © 1999-2000 Reuters Limited.
--------------------------------------------------------------
May 1, 2000

STATEMENT BY THE PRESIDENT REGARDING THE UNITED STATES? DECISION TO STOP
DEGRADING GLOBAL POSITIONING SYSTEM ACCURACY

Today, I am pleased to announce that the United States will stop the
intentional degradation of the Global Positioning System (GPS) signals
available to the public beginning at midnight tonight.  We call this
degradation feature Selective Availability (SA). This will mean that
civilian users of GPS will be able to pinpoint locations up to ten times
more accurately than they do now.  GPS is a dual-use, satellite-based
system that provides accurate location and timing data to users
worldwide. My March 1996 Presidential Decision Directive included in the
goals for GPS to: ?encourage acceptance and integration of GPS into
peaceful civil, commercial and scientific applications worldwide; and to
encourage private sector investment in and use of U.S. GPS technologies
and services.?  To meet these goals, I committed the U.S. to
discontinuing the use of SA by 2006 with an annual assessment of its
continued use beginning this year.

The decision to discontinue SA is the latest measure in an on-going
effort to make GPS more responsive to civil and commercial users
worldwide.  Last year, Vice President Gore announced our plans to
modernize GPS by adding two new civilian signals to enhance the civil
and commercial service.  This initiative is on-track and the budget
further advances modernization by incorporating some of the new features
on up to 18 additional satellites that are already awaiting launch or
are in production.  We will continue to provide all of these
capabilities to worldwide users free of charge.

My decision to discontinue SA was based upon a recommendation by the
Secretary of Defense in coordination with the Departments of State,
Transportation, Commerce, the Director of Central Intelligence, and
other Executive Branch Departments and Agencies.  They realized that
worldwide transportation safety, scientific, and commercial interests
could best be served by discontinuation of SA.  Along with our
commitment to enhance GPS for peaceful applications, my administration
is committed to preserving fully the military utility of GPS.  The
decision to discontinue SA is coupled with our continuing efforts to
upgrade the military utility of our systems that use GPS, and is
supported by threat assessments which conclude that setting SA to zero
at this time would have minimal impact on national security.
Additionally, we have demonstrated the capability to selectively deny
GPS signals on a regional basis when our national security is
threatened.  This regional approach to denying navigation services is
consistent with the 1996 plan to discontinue the degradation of civil
and commercial GPS service globally through the SA technique.

Originally developed by the Department of Defense as a military system,
GPS has become a global utility.  It benefits users around the world in
many different applications, including air, road, marine, and rail
navigation, telecommunications, emergency response, oil exploration,
mining, and many more.  Civilian users will realize a dramatic
improvement in GPS accuracy with the discontinuation of SA.  For
example, emergency teams responding to a cry for help can now determine
what side of the highway they must respond to, thereby saving precious
minutes.  This increase in accuracy will allow new GPS applications to
emerge and continue to enhance the lives of people around the world.

Miguel wrote:

> provided an artificial error in measured location). Because some
> routing algorithms are using GPS information I think this can be good
> news for these algorithms.
>

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From: Rex Buddenberg <[email-masked]>
Date: Thu, 4 May 2000 07:44:13 -0700

One should note that there are several errors in the GPS error budget;
selective availability is only one of them.

The meta-error is that GPS lacks inherent integrity.  Like other navigation
systems it can provide bad data and advertise it as good.

Within the error budget, we have five causes in addition to selective
availability:
 - ephemeris errors.  The satellites aren't quite where they were predicted
to be.
 - clock drift.  The Cesiums do indeed drift; they are observed by USNO and
they post the data to their web page.
 - ionospheric error.  Distortion of the signal as it transits the ionosphere.
 - tropospheric error.  Ditto for troposphere.
 - receiver noise.  The largest component here is usually multipath which
is always bad in radionav systems.
 - selective availability can be lumped in with the rest of these.

For navigation purposes, there still is compelling logic to differential
GPS -- it fixes all of the above list -- especially including integrity --
except for receiver noise.

Most of the list is not terribly critical for time users -- many average
out over time and you don't need the low altitude satellites for that
purpose (where most of the tropo error lies).  So turning off S/A does
indeed make that nuisance go away.

b