From owner-freebsd-hackers  Sat May  6 11:40:52 2000
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Date: Sat, 6 May 2000 12:40:42 -0600 (MDT)
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From: Nate Williams <nate@yogotech.com>
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To: Mike Smith <msmith@FreeBSD.ORG>
Cc: nate@yogotech.com (Nate Williams), hackers@FreeBSD.ORG
Subject: Re: [OT] Finding people with GSM phones (was Re: GPS heads up )
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> > Plus, they can get a fix on the phone in 300ms (good to about 25m),
> > which is far faster than a GPS unit can do it.  Basically, the phone is
> > 'locked on' as soon as you turn it on and it finds a cell tower.  And,
> > apparently they've figured out a way to get a coarse fix on it even
> > where there is only one tower, although when I pressured him, he just
> > smiled and claimed it was a trade secret.
> > 
> > Or so I've been told, but I trust the source since he's one of the
> > smartest guys I ever met. :)
> 
> GSM (which is what all of these systems are based on) depends heavily on
> knowing the flight time from the phone to the cell hardware (and back), in
> order for TDMA to work correctly.  25m is special for a reason I don't 
> recall (possibly flight time for one clock, or something similar).
> 
> Triangulation is typically trivial with only two towers (your phone will 
> generally log into at least the strongest three or four cells) because 
> the towers use directional antennae, so the tower knows where the antenna 
> you're on is pointing and you can eliminate the shadow position (most of 
> the time).

Right, with triangulation it's trivial.

> With one tower, you're down to describing an arc along which 
> the phone is probably located; still pretty good when it comes to finding 
> someone.

He seemed to imply that they could get it within 25m, even with one
phone.  Like I said, I don't understand how, but I didn't question his
ability.  Plus, he knows alot more about the stuff than I do.


Nate


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