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.

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).

Even without auxiliary codes, the US could still activate the system
at random times. Unless the enemy knew when the US was going to
activate the system, the US might be able to have some "windows of
opportunity" to use their GPS weapons. Of course the enemy might be
able to take advantage of these same windows if they could respond
quickly enough and if they could detect the US launches.

Assuming (as I suspect) that "auxiliary code input" to the weapon is
required, things get more complicated. Basically the question is how
long it would take for the enemy to figure out what the auxiliary code
was to activate their weapons.

One extreme scenario to illustrate the concept - the satellites could
send out random hash until, say, 6:03 am when a major US strike was
planned. At this point, the satellites would start transmitting valid
information according to some specific agreed upon code which the
enemy didn't know in advance. When all US weapons reach their target,
the satellites would go back to sending random hash.

The enemy would have to figure out what the code was in a very short
time period, and program and launch their weapons before the code
expired. This would be extremely difficult.

Pessimistically assuming that the current military GPS system does get
compromised, and that the code breaking process could be done in
minutes, the US is of course free to build a better one with more
modern (and longer) codes. Of course, retrofitting existing weapons
to use the new GPS system might be a bit involved. OTOH, it could be
as simple as pulling out a modular "black box", and replacing it with
a new improved one.