From: "Meindert Sprang"
References: <email@example.com> <firstname.lastname@example.org>
Subject: Re: Extracting time from GSM / mobile phone transmissions?
Date: Mon, 23 Sep 2002 16:14:59 +0200
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"Peter" wrote in message
> Jack Smith wrote:
> >GSM synch signal uses frame numbers, not absolute time.
> >Also, base stations need not be synchronized to each other, although
> >it can be done to reduce handover times. Hence, measuring arrival
> >times from multiple BTSs (assuming you know where the BTSs are
> >located) won't necessarily let you locate yourself.
> >That being said, there are some interesting locating schemes that have
> >been developed for locating the handset, as required by rules in USA
> >for emergency call location finding (enhanced 911). In addition to GPS
> >chip in the receiver, someone (Ericsson?, can't recall now) is
> >producing a system that measures time of arrival of MS uplink at
> >several Base Stations and thereby can locate the MS to within the
> >required distance.
Base stations send a TA (Timing Advance) parameter to the handset, telling
the handset how many bittimes earlier in it's timeslot to transmit to the
basestation. This is done to be able to stuff at much timeslots in a
channel, without being dependent on the varying propagation time from
handset to basestation and vice versa. Since a handset mostly sees more than
one basestation at a time, there are several TA values available at any
time, each telling the distance from the handset to the basestation in 550