From: Bill Stewart <stewarts@ix.netcom.com>
Date: Sun, 29 Sep 1996 13:59:26 +0800
To: dlv@bwalk.dm.com (Dr.Dimitri Vulis KOTM)
Subject: Re: GPS
Message-ID: <199609290407.VAA20249@dfw-ix3.ix.netcom.com>
MIME-Version: 1.0
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>2. The DoD is doing a study right now on how to make GPS useless to the
>enemy at wartime.
>I think figuring out a way to turn off A/S and getting accurate GPS on
>commercial equipment at all times would make a nifty Cypherpunks project
>- if it really involves breaking some encryption.

Essentially the A/S works by having the publicly-readable signal
contain fuzz fuzz in the low-order bits and the encrypted signal contain 
the real stuff; if they've done decent encryption, you won't crack it.

However, the fuzz _is_ consistent - if you've got two nearby points,
and you really know where one of them is, you can correct for it,
and get better accuracy even than the full GPS with A/S turned off.
The approach the Feds are taking to prevent competition from real
differential GPS is to field a differential GPS system of their own,
located at/near airports (who are the real people who want D-GPS,
so they can do things like better instrumented or automated landings.)
This way, nobody's got much financial incentive to deploy D-GPS
correction transmitters of their own, and manufacturers have an incentive
to deploy equipment tuned to the FAA's correction transmitters,
so they can still turn them off if they want to.  

Except for takeoff/landing, airplanes don't much need differential GPS;
you shouldn't be flying within a hundred meters of other planes anyway,
and if you're doing cropdusting or barnstorming you'd better be able
to see what you're doing or have good radar anyway - most topographic
maps don't have tall trees marked on them.

#			Thanks;  Bill
# Bill Stewart, +1-415-442-2215 stewarts@ix.netcom.com
# <A HREF="http://idiom.com/~wcs"> 	
# You can get PGP software outside the US at ftp.ox.ac.uk/pub/crypto