"Steve Firth" wrote in message
.. .
Wayne.B wrote:

On Sat, 17 Jul 2010 11:22:42 +0100, Boo
wrote:

I understood that "selective availability mk2" (or whatever it's known
as) is
the capability to selectively deny access to GPS signals over limited
geographical areas, maybe this has some bearing ?

It would be limited mostly to "line of sight" distances, and Cuba is
not known for being a technical power house with unlimited financial
resources.

You and the previous poster are giving the appearance of not knowing
what selective availability (SA) is. SA is a feature of GPS that permits
the civilian positioning feature of GPS to be subject to a psuedo random
position error. This error affects all GPS units within a particular
area and is only "line of sight" in that it affects all units within
line of sight of particular satellites, so the area can be huge or
indeed global as it was until 2000 when the feature was turned off. Your
comment makes it appear that you are confused between jamming and SA.

At present GPS is denied in particular areas using other means which are
less susceptible to correction. SA could be defeated by the use of dGPS
and the US have stated it will never be turned back on again.

You seem to have been attempting to claim that GPS cannot be jammed or
denied, in this you are wrong. The methods used are not however public
knowledge although from time to time NOTAMS and similar publications
will indicate in which areas GPS is being denied for trials.


Exactly right Steve,
Washington DC and Fort Hood in upstate NY are just
two places I have personal knowlege of where GPS is
munged for security purposes.
Regards,
JR