On Fri, 20 May 2005 17:03:53 +0100, John Rumm
wrote:


The "tracking" of a standard GSM mobile can only be done by
triangulation based on signal levels received at the cell base stations.

No, it is done by the timing component of the GSM signal. A single
base station can provide a fairly accurate range fix and a rough
azimuth. Two base stations provide an unambiguous fix of variable
accuracy and three provide the "cocked hat" beloved of real
navigators.

In a densely populated area (i.e. lots of base stations) that can be
accurate to a few hundred meters under best conditions.

Under best conditions it can be accurate to about 10m.

There is a new breed of phones that does include a GPS receiver.

Not in Europe.

Its main selling point is the ability to provide very local information
via SMS.

It has no selling point - it is a requirement of the American FCC.
Whether they ever appear elsewhere remains to be seen.

I believe that if the phone is off (as opposed to standby) then
these are defeated also.

They are.

--
Peter Parry.
http://www.wpp.ltd.uk/