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Assuming the list members are not unhappy about GPS discussions (if so, I can
take this off list), here's my reply:
As far as I know, individual satellites cannot be (easily?) operated in the
suggested way. After all, such operation would have to be reversed by so-caled
authorized (military) users. So, it's S/A, jamming, or nothing.
As far as the foot print of a GPS satellite is concerned, go to
 and select one of the GPS
satellites. Click "no night" and update to get a better view. That will show
you the coverage area of a GPS satellite. Every GPS user in that region will
see this satellite in their sky somewhere above the horizon. The GPS satellite
antennas are designed to cover the whole visisble hemisphere (and a bit
beyond).
-- Richard Langley
   Professor of Geodesy and Precision Navigation

On Mon, 5 Jun 2006, Red wrote:

>Richard-
> "1) S/A cannot be selectively applied in a satellite to affect only a small
>region on the Earth's surface. The foot print of a GPS satellite covers
>virtually the whole hemisphere below the satellite."
>
>I think you misread my earlier comments to produce this. I was not and am not
>referring to SA. What I had been told was that DoD were telling specific
>satellites, at specific times, to deliver specifically incorrect information in
>their data streams. This is not the distortion that is used for SA, but rather,
>they were being fed gross misinformation. Perhaps it is SA--I don't know. But
>the programming that was originally done in/with the system apparently was
>fairly simple compared to what is now being done in real time.
>
>As to a region being small or covering a complete hemisphere...Small is a
>relative thing, but I don't think a single GPS satellite actually covers
>anything near a hemisphere on the ground, does it?  Suppose you told the
>satellites "When you are more than 30 degrees over the horizon for location X,
>change your data stream and lie about where you are, and go off frequency to
>create false doppler results while you are at it."
>
>Wouldn't that have the effect of effectively junking the data--but only doing so
>during the time those satellites were likely to be used in a much smaller area,
>since GPSes routinely try to acquire the birds directly overhead rather than at
>the horizon?
>
>And that would accomplish the goal, which is to allow domestic use of the system
>while simultaneously severely degrading it overseas. If you can keep the system
>"substantially" functional over the domestic US, while degrading it for a 5,000
>mile footprint or even a 10,000 miles footprint someplace else--that's much
>better than degrading it everywhere.
>
>I don't know how big the footprint they target is, the remarks I had seen
>indicated it was something much tighter, i.e. in the thousand-mile range. Can
>that be done by diddling satellites? Perhaps so, if you only diddle the ones
>more than 40? 60? degrees above the local horizon?
>


===============================================================================
 Richard B. Langley                            E-mail: lang@unb.ca
 Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
 Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
 University of New Brunswick                   Fax:      +1 506 453-4943
 Fredericton, N.B., Canada  E3B 5A3
     Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
===============================================================================

   

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