From: Paul_Koning/US/3Com%3COM@smtp1.isd.3com.com
Date: Sun, 14 Apr 1996 00:33:29 +0800
To: cypherpunks@toad.com
Subject: Re: No matter where you go, there they are.
Message-ID: <9604112350.AA9349@>
MIME-Version: 1.0
Content-Type: text/plain


>From: perry @ piermont.com ("Perry E. Metzger")
>Aren't things even worse? Since the satelite signals are not
>authenticated with anything like public key methods, couldn't I just
>synthesize a signal appropriate to any spot on the planet, knowing the
>positions of the satelites relative to that spot?

In the case of the C/A (civilian) code, absolutely.  In the case of the
P code (military) only if you have the key, but in that case, yes.

As Jim Bell pointed out, there are boxes you can buy for
suitable amounts of money that are "GPS simulators" -- they construct
out of whole cloth the signals you would receive if you were at
location X with satellites {Y1, Y2, ...} overhead.

However, if all you set out to do is fool a location authenticator,
deriving shifted location data from the actual satellites is far
easier and bound to be much cheaper.  There is one limitation
this has that the simulator approach doesn't -- delaying real signals
requires having access to the same satellites (or a large enough
subset, i.e., 3-4 of them) that the checking station has overhead.

 paul