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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?

   

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