From: pcw@access.digex.net (Peter Wayner)
Date: Thu, 11 Apr 1996 04:44:51 +0800
To: cypherpunks@toad.com
Subject: Re: No matter where you go, there they are.
Message-ID: <v02140b09ad916e6d8a17@[199.125.128.5]>
MIME-Version: 1.0
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Hmm. Here's an interesting question.  Let's say that there are 3
satellites in view broadcasting signals f1(t), f2(t) and f3(t).
The way the system triangulates is to compute the distance from
a location to a satellite by timing the arrival of a signal. So
if signal f2(t) arrives at t+3 milliseconds, then the receiver
is 3 lightmilliseconds away from satellite 2.

For sake of simplicity, let the coordinates be expressed in
distance from the satellites. (2,3,1) would mean a distance of
2, 3 and 1 light milliseconds from satellites 1,2 and 3
respectively.

Okay, so why can't I just tape the signals I get from each of
the three satellites. Let these be T1(t), T2(t) and T3(t).
Assume we can easily synchronize them so that T1(t-o1)=f1(t).
That is, we figure out our coordinates (o1,o2,o3), and subtract
the offset from each tape.

Then if we want to pretend to be at coordinate (a1,a2,a3), we
simply say that we just received values T1(t-o1+a1),
T2(t-o2+a2), T3(t-o3+a3).

Or course, I could be completely missing some neat feature of
DGPS. I really don't know the details of how it works and this
could be completely wrong. Any thoughts?

-Peter