"pervect" wrote in message
...
On Thu, 18 Dec 2003 08:21:03 GMT, "Kevin Brooks"
wrote:


That is a decent description of the selective availability (SA)
function of
GPS. SA renders the average (non-US military) receiver incapable of
determining a precise fix, and you need precision for the kind of
weapons
the poster was postulating. SA was shut down a couple of years back
so that
civil users (i.e., surveyors, commercial aircraft, etc.) could take
advantage of its precision (prior to that occuring surveyors had to
use what
is known as "differential GPS", a more time consuming method of
achieving a
precise location), but according to the official USG website on the
subject
it can be reinstituted over a particular region at will.


Denying the US use of GPS would have a negative impact on US
military
capability, but it would not eliminate it.

Actually, I don't think SA adversely affects US military systems.

Brooks

Processors and computing power are getting cheaper every year - and
there are a lot of US weapons with military GPS around - so it's
conceivable to me that someone could obtain one of these weapons and
reverse-engineer the GPS system on them.

They can, but the signal is encrypted. The military can change the key
at will. In fact, I suspect that the keys are changed at least daily.

Even if you know all about the lock, you won't get a thing if you don't
have the key.



If there is no sort of "auxiliary code input" to the weapon (i.e. some
sort of activation code that has to be input) the reverse engineered
weapons would work just as well as the US weapons, so the US would
have to make the choice of whether it was better for everyone to have
(accurate) GPS or nobody to have GPS.

Without knowing for sure, I would personally expect that current
weapons would have some sort of auxiliary code, and that this code
would have to be entered as part of the target programming process
(which is quite long according to news reports, though it's getting
shorter).

I'm sure it's a private key system. The US issues keys (probably 1k
bits or more) to all units that need them. Contingency keys are also
issued. If someone captures the keys for next month, the contingency
keys are used.



Ray Drouillard