Date: Sat, 06 May 2000 11:47:37 -0700 From: Mike Smith <msmith@freebsd.org> To: nate@yogotech.com (Nate Williams) Cc: hackers@FreeBSD.ORG Subject: [OT] Finding people with GSM phones (was Re: GPS heads up ) Message-ID: <200005061847.LAA07298@mass.cdrom.com> In-Reply-To: Your message of "Sat, 06 May 2000 10:07:30 MDT." <200005061607.KAA17627@nomad.yogotech.com>
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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). 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. -- \\ Give a man a fish, and you feed him for a day. \\ Mike Smith \\ Tell him he should learn how to fish himself, \\ msmith@freebsd.org \\ and he'll hate you for a lifetime. \\ msmith@cdrom.com To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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