Damon Cassell
Thu Dec 16 13:00:09 EST 2004

Selective accuracy was eliminated in 2000. That has not changed.


On Thu, 16 Dec 2004 11:23:23 -0500, Sid Whitaker <> wrote:
> Shawn is right about the military's use of GPS. My understanding is that
> civilian GPS is bascially the same as military GPS, with something called
> "selective accuracy" (SA) thrown in. In other words, military GPS receivers are
> accurate to within a few feet (or less), while the military adds SA to civilian
> GPS receivers to make them less accurate (say, to within 50 feet or so). SA was
> designed and added specifically to counter enemy use of the GPS system. I
> thought that SA was removed prior to 9/11 as GPS became more widely accepted,
> but perhaps not. Seems to me that instead of shutting down the GPS system, the
> military easily could increase the SA on civilian GPS to a level of inaccuracy
> (e.g., to one mile) that would render the system useless.
> Sid Whitaker
> Quoting Shawn Mamros <>:
> > >Interesting news that says the White House is keeping open the option of
> > >being able to shut down GPS over the U.S. in the event of a major crisis
> > >so as to prevent the GPS to be used against us.  I was unaware that the
> > >GPS system is so neatly linked that it could be brought off-line.  Would
> > >the entire set of birds be shut down?  If so, what else shares those
> > >birds and would be similarly affected, broadcast, financial links,
> > >weather, other critical services?
> >
> > IIRC, GPS was originally developed by and for the U.S. military, and
> > they still use it.  In fact, they have access to a greater degree of
> > pinpoint accuracy than what they allow to be available for civilian
> > use.  So they likely wouldn't shut down GPS altogether, but instead
> > lock out civilian access, while they still would use it themselves.
> >
> > And I'm pretty certain GPS uses its own set of satellites, rather
> > than shared space on civilian communications satellites, for the
> > very same reason.  But even if they didn't, I would think satellites
> > these days would provide control over individual transponders, such
> > that you could turn one or more off while keeping others on.
> >
> > -Shawn Mamros
> > E-mail to: mamros -at- mit dot edu
> >

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