locating via GSM, revisited

Andy Green andy at openmoko.com
Tue Apr 22 12:52:33 CEST 2008

Hash: SHA1

Somebody in the thread at some point said:

| Highly problematic it seems to me to do this forced cell reselect
thing to get
| TA for other than the current (most nearby) cell, the function named "BTS
| test" on Nokia monitor [Display 17]. Probably that's about as illegal as
| using police frequencies to mess around with, or creating your own
| TV-station. In the end they might accuse you for tearing down the
whole GSM
| network by fraudulent or abusive usage.

Well the impact of it is interesting if you don't have distance
information from all but one transponder... you end up with a highly
variable shape that is the intersection of all the circles (because you
don't know the radius from the centre of each you should use), with one
master annulus cutting through it from the station you are connected to
and have a solid distance estimate for.

Put together the result is an arc of shorter or longer length describing
your location... if the arch length is small because the other stations'
intersection is small, it's as good as a point location.  If there's
only one other base station around anyway, the arc may be 2/3 of the
circle and if your station is the only one, it is the whole annulus.
But normally your location would appear as a little rainbow segment of
one length or another.

- -Andy
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