Re: GPS

> 1) Has not GPS (civilian side) been given up to
> some sort of open system? (If so, wouldn't
> futzing up parts of it be problematic?)

In the last year of Bill Clinton's administration, he finally authorised
the
de-activation of the "dither factor" - random mathematical errors that
were
purposefully generated to throw a margin of error into the GPS network -
military GPS units with the correct on board tables could filter these
out,
precision units (Airlines mainly) had different tables to give a greater
accuracy and standard civilian units had no correction - all this did
was
make sure that a non-US friendly user couldn't get firing solutions
based on
GPS, nut could still get general use.

> 2) Can differential GPS (multi-receiver) defeat or
> significantly attenuate the futzing up signal?
> (That is, can't differential GPS be used in such a
> way as to amerliorate the effects of signal
> uncertainty introduction)? If so, the only
> receivers that will be seriously futzed by the
> fuzz that can be inserted (fuzz substituting for
> any real technical understanding....) would be
> the non-differential single receiver GPSes,
> correct? (And yes, these are the more common
> variety IIRC)

Differential refers to both multi-reciever (rare outside university
work)
and units that refer back to surface based beacons for higher accuracy -
they have a limited range and are still in use in some areas - coverage
was
mainly restricted to areas where coverage was vital (airports) or no
other
nav data was available (remote straits and passages). This system gave a
finer solution to both precision units and civilian units but no
significant
improvement to military units. Additionally the system is affected
heavily
by weather and the recievers' surrounding terrain - particularly
canyons.

> 3. If the US (or anyone) were to fuzz up a
> signal coming from someplace and this caused
> some manner of disaster (air crash, ship to run
> aground and sink, etc), wouldn't that be a very
> bad thing? (Yes, I know secondary navigation
> methods are meant to verify positions, but
> people ARE becoming GPS dependent).  I
> realize when weighed against a large scale
> offensive, these risks are minimal, however I can
> see someone attempting some form of
> litigation... (Land of the Free, Home of the
> Lawyer)

The impact of such a move would theoretically mean quite a lot of
property
damage, lots of lives lost and many, many law suits - you are very
correct
in the view that we are becoming GPS dependant - I sell and maintain GPS
units in Australia and even our taxis are co-ordinated by GPS.

Brendan Pratt