Date: Sat, 6 May 2000 12:40:42 -0600 (MDT) From: Nate Williams <nate@yogotech.com> 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 ) Message-ID: <200005061840.MAA18274@nomad.yogotech.com> In-Reply-To: <200005061847.LAA07298@mass.cdrom.com> References: <200005061607.KAA17627@nomad.yogotech.com> <200005061847.LAA07298@mass.cdrom.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). 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 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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