From: jerry@terminus.dell.com (Jeremy Porter)
Date: Thu, 30 Dec 93 11:38:13 PST
To: m5@vail.tivoli.com
Subject: Re: GPS and security
In-Reply-To: <2fq4rk$t78@uudell.us.dell.com>
Message-ID: <9312301933.AA11484@terminus.us.dell.com>
MIME-Version: 1.0
Content-Type: text/plain


In article <2fq4rk$t78@uudell.us.dell.com> you write:
>From: m5@vail.tivoli.com (Mike McNally)
>Subject: GPS and security
>
>
>CBS's "60 Minutes" had a "Oooh Scarey" segment about the Global
>Positioning System last Sunday.  The scarey part is, according to 60
>Minutes, that any old terrorist government can now put together
>accurate guided missles and wreak death and destruction upon our
>homeland.  The crux of the problem is that the system broadcasts out
>positioning information to one and all indiscriminately, and according
>to the show there's no way we can stop it.
A couple of points here.  Any terrorist group can get pin-point accuracy
with a suitcase bomb.  Someone could have launched an ICBM at the
world trade center, but it was much cheaper to just rent a truck.
Terriorist don't need high-tech.

GPS has two modes, civilian and military modes.  When the GPS is operating
in "secure" mode civilian access gives 10meter accuracy.  The military
codes give accuarcy down to ~1meter.  The GPS broadcasts using a spread
spectrum mode and the encoding is based on the frequency hopping.  

Now if a military unit wanted to blow up your tank, they would need the
1meter accuarcy.

During desert storm, the u.s. military turned off the secure mode,
because all of the equipment they had use the civilian mode.
>Because my mind is regularly exposed to wild conspiracy theories and
...
>they could be told to do several things in case of national emergency:
>
>*	Shut down (probably possible; they may have actually mentioned
>this on the show).  Problem is that lots of friendlies may grow to
>rely on the data for life-critical things, like guiding commercial
>airliners.
All of the systems I know of, including airplanes and ocean going
ships has LORAN C, GPS and inertial nagivation systems.  Currently
there are still holes in GPS coverage, to get a "good" fix, you
need three satilites overhead.   Currently there are still gaps where
only two satilites are available.

>*	Shut down normal transmission and begin strongly encrypted
>transmission.  No mention of this; apparently, the satellites were
>originally designed with some sort of weak system that made the data
>difficult to use for high-accuracy purposes, but that's been defeated
>(by the FAA or someone contracted thereto).

Actually the system was designed to offer the two modes, but advances
in technology have allowed the GPS reciever people to take into account
things like relativity(high speed satilites, freq shifts etc.), using
more than 3 satilites to get a better fix, averaging techniques etc.
With some more complex equipment and a "known" reference you can
use GPS to measure down to centimeter accuarcy.  Not at very high
speeds though...

>I don't know much about how this system works, so I don't know whether
>any of my thoughts are relevant.  It's probably most likely that the
>government indeed blasted these things up into space without
>considering using encryption technology to enhance security.

Actually the government put a lot of thought into the security, but
seeing as the actual data is just timecode pulses, its not real easy
to secure, but with a good encryption and a frequency hopping setup
like GPS, you can do a lot.

I think that there are provisions for disabling the civilian channel
altogether during wartime.  It would be interesting to see what
happens to MCI, Sprint, etc that use GPS to syncronize they're 
high speed syncronis optical networks...  The should work fine for
several hours but after a couple of days, the phase a frequency errors
in the timecode equipment would lead to a high error rate.
MCI and sprint may also use LORAN C for backup, I don't remember.

-- 
| Jeremy Porter  -----------------  Dell Computer Corp. ----
| Systems Engineering --- jerry@terminus.us.dell.com --------
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