For Liars and Loafers, Cellphones Offer an Alibi

[Alan Barrett]

On Sat, 26 Jun 2004, Major Variola (ret) wrote:
> Eventually the cellphones will be able to tell another phone approx
> where they are. [...] The marketing reason would be to help people
> find others geographically.

At least with GSM, the base station always knows the approximate
distance to the phone (this is needed by the GSM protocol, for reasons
related to time slot management in the presence of finite speed of
light, but it might be possible to hack the phone's firmware to fool it,
or to register with fewer base stations than usual).  The GSM network's
database knows the exact locations of all the base stations.  Add a
little software to do triangulation from multiple base stations, and the
GSM network knows the location of the phone, to an accuracy that depends
chiefly on the base station density.  Add a layer of user interface
software, and you're done.  No cooperation from the phone is necessary,
except what the phone would normally do in order to register itself with
base stations so that it can receive calls.  No GPS or other non-GSM
protocols are necessary.

This is already offered as an extra cost service (branded "Look for me")
by Vodacom in South Africa.  It's targeted at parents who want to know
where their children are, and the phrase "with their permission" is
included in current advertising.  As the seeker, you send an SMS (text
message) to a special number to "register" your phone as a user of the
locator service, and to ask for the location of another phone.  The
network sends a message to the target phone, and the user must reply to
give permission to be located.  Then the network sends a text message
to the seeker, telling them the location of the target.  I don't know
whether the target's permission is asked every time, or just once per
seeker; I do know that it's not just once globally.  In any case, the
"permission" is just a flag in a database, and is not really needed by
anybody with back-door access to the GSM provider.

--apb (Alan Barrett)