From: "Pat Farrell" <pfarrell@netcom.com>
Date: Tue, 28 Dec 93 14:22:42 PST
To: cypherpunks@toad.com
Subject: RE: GPS and security
Message-ID: <62594.pfarrell@netcom.com>
MIME-Version: 1.0
Content-Type: text/plain


This is a bit off list topics. Press D now if you don't like it....

In message Tue, 28 Dec 93 14:01:15 CST,
  m5@vail.tivoli.com (Mike McNally)  writes:
> *	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).

   The GPS system was designed to have two modes, one highly acurate for
military use, and a low accuracy version for commerce. Since initially
no one had receivers, the whole system used military grade for the first
few years.

Before the Gulf war started, the military needed zillions of GPS units, so
they bought commercial marine navigrtion units. After the war, there
was talk of turning on the division, but that was made
impractical with 'differential' calibration.

To get military accuracy, all you have to do is take the commercial GPS to
known places. Such as the surveying marks at the US Naval Acadamy, or
"ground zero" in the Pentagon. Get your "imprecise" reading, calculate the
difference between it at the known value, and use it for subsequent
calculations.

I expect that triangulation with a few known sites is better than a single
one.

Last I hear from the Sailing trade press was that the Gov still talked about
going to the split setup, but there was little belief that they would.

BTW, two satellites is enuff for a general location fix. Three identifies a
place on the globe, four adds altitude (handy for planes, bombs, and
missles)

Pat

Pat Farrell      Grad Student                 pfarrell@netcom.com
Department of Computer Science    George Mason University, Fairfax, VA
Public key availble via finger          #include <standard.disclaimer>