locating via GSM, revisited

Harald Welte laforge at openmoko.org
Tue Apr 22 10:36:16 CEST 2008

On Sun, Apr 20, 2008 at 08:43:25PM +0800, joerg at openmoko.org wrote:
> Am So  20. April 2008 schrieb andrzej zaborowski:
> > On 20/04/2008, joerg at openmoko.org <joerg at openmoko.org> wrote:
> > >  There is not so wellknown further more detailed information you may get 
> from
> > >  your GSM-modem (MobileStation, MS), which consists of:
> > >  a) The (usually) 6 next nearby BTS (to be correct: next best RF-signal 
> BTS),
> > >  b) The distance to your active BTS, in increments of 550m (Timing 
> Advance, TA)
> > >  This additional info may be used to dramatically improve the precision of
> > >  GSM-based location data.
> > 
> > Wow, didn't know that.  I think the timing advance must be a
> > understood as the virtual distance the signal has travel, i.e. signal
> > strength and not the distance we're interested in.  Or might a GSM
> > modem have a way to know it's physical distance from the BTS? (Maybe
> > the BTS can know, comparing the Signal-quality the phone sees and the
> > quality of the signal *from* the phone to BTS, or even talking to
> > other BTSes)
> TA is generated (decided upon) by the BTS. 

Well, I might be nit-picking, but it is actually decided upon by the
BSC, based on readings it receives from the BTS.  But the BTS just
forwards the TA that the BSC has decided upon.

Doesn't matter for the rest of the discussion.

And yes, you can get neighboring cell information from the Neo, I have
done this myself in the past.

- Harald Welte <laforge at openmoko.org>          	        http://openmoko.org/
Software for the world's first truly open Free Software mobile phone

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