Date: Sun, 07 May 2000 18:24:04 -0600 From: Wes Peters <wes@softweyr.com> To: Olaf Hoyer <ohoyer@fbwi.fh-wilhelmshaven.de> Cc: Nate Williams <nate@yogotech.com>, freebsd-hackers@FreeBSD.ORG Subject: Re: [OT] Finding people with GSM phones (was Re: GPS heads up ) Message-ID: <39160924.D00CAF40@softweyr.com> References: <200005061847.LAA07298@mass.cdrom.com> <200005061607.KAA17627@nomad.yogotech.com> <200005061847.LAA07298@mass.cdrom.com> <4.1.20000506204714.00cd5290@mail.rz.fh-wilhelmshaven.de>
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Olaf Hoyer wrote:
>
> At 12:40 06.05.00 -0600, you wrote:
> >> > 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.
> >
> Hi!
>
> Well, I've heard reports that they are working on precision calculations
> where you are...
>
> Those numbers only work (at least this is my latest understanding) if you
> are actually doing some calls, so that all towers (in the GSM-900 net, IIRC
> the phone locks to three towers, one for primary data transfer, the other
> two for backup or movement issues to hand over) have some active connection.
> If the phone is only turned on, it sends some data to the tower, so that
> you know that in the area this tower overlooks, (360 degree) it is somewhere.
>
> There were some famous cases where some criminals were located by tracking
> down their cell phone. The police needed some decision from court to do
> that, but after that, it was a short way to go. The GSM nets have some of
> this ability built in, to track phones. The operators only don't want the
> "normal" citizen or user to know about that.
All this discussion of the wonders of GSM is wonderful, but doesn't apply
to the USA where this mandate is happening. This was the stated reason
for Rockwell getting into the GPS chipset market, because the volume is
going through the roof by late 2002. (TDMA and CDMA cover much more of
the USA than GSM, and will probably continue to lead coverage for quite
some time.)
--
"Where am I, and what am I doing in this handbasket?"
Wes Peters Softweyr LLC
wes@softweyr.com http://softweyr.com/
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