On Thu, 18 Dec 2003 10:32:11 -0800, pervect wrote:
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.

No, it doesn't, by design. SA only affects the accuracy of the
GPS satellites' coarse positioning signal. Military GPS receivers
can receive additional signals from the satellites that allow
more precise position determination.

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.

The military GPS signal is encrypted. A receiver needs to know
the current encryption key to decrypt the signals and use them
to compute its position, so just duplicating the hardware somehow
won't do you any good.

Encrypting the signal also makes it very difficult for
an enemy to spoof GPS signals -- spoofed signals would have
to be encrypted with the correct key to fool a receiver.

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).

Encryption key, not "auxiliary code".

I don't know any details of how the keys are distributed, but
I suspect the people who designed the current GPS system thought
a lot about the issue and came up with a solution that is
relatively secure and not terribly inconvenient.

[...]

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.

See

http://www.globalsecurity.org/military/library/report/2003/iraq-and-gps_faq.pdf

for a discussion of these issues. According to the author, the
use of selective availability (SA) to prevent opponents from
using civilian GPS receivers to get accurate position fixes is
a thing of the past:

"The technique the U.S. military developed which allowed them to
switch off SA is selective in-theatre jamming of the SPS signal."

Another poster suggested that SA could be turned on and off on
GPS satellites individually as they overfly the area of conflict --
this is rather unlikely. The orbital altitude of the GPS
constellation is approximately 10,000 miles. This means that
at any given time, a single GPS satellite is above the horizon
and visible to GPS receivers over almost half of the earth's
surface. It would be possible to selectively impose SA on one
hemisphere of the earth at a time, but not in an area much
smaller than that.

To return to the original topic of this thread, I think the
Elbonians would be better off spending money on developing
cheap inertial navigation systems for their hypothetical
low-cost cruise missiles (HLCCMs) than going to any effort to
try to outsmart the U.S. Air Force so they can use GPS.

Inertial navigation systems can't be jammed or spoofed, and
are accurate enough to get HLCCMs within hypothetical low-cost
terminal seeker range of their targets.


ljd