Sid Whitaker
Thu Dec 16 11:23:23 EST 2004

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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