Someone stole my Neo Freerunner... :(
helge.hafting at hist.no
Mon Jan 26 14:19:08 CET 2009
The Digital Pioneer wrote:
> Indeed, GPS fixes are tough to get, but they can be done. Just out of
> curiosity, can the telco really do all that passive triangulation (or
> more importantly, can I) they talk about in the movies? :P
They have to do some of that anyway, just to make mobile roaming work at
all. A phone that is "on" but not in use, still talk to the towers
regularly. This so they know what tower to use, _if_ a call suddenly
comes in. Let the phone rest atop a bad FM radio, and you will hear this
in the form on radio noise now and then. You will also hear noise
immediately before a call or sms comes in.
When the phone is "heard" by one or more towers, the telco knows that it
is in an area near all those towers. The more towers that hear the
phone, the smaller the area because towers are spread and their range
overlap only partially. Some towers are directional. Some towers gets a
better connection to the phone than others - the phone is likely closer
This information alone gives you a good idea of where it is, especially
in cities where towers are packed densely.
I seem to recall that the gsm protocol lets the telco adjust a phone's
signal strength. They generally go as low as possible, so it won't cause
unnecessary interference elsewhere. They can tell the phone to vary the
strength in order to measure from several towers.
If they have sufficiently precise timers, then they can measure the
distance from several towers too. This takes advantage of the fact that
radio waves move at the speed of light, and so they arrive at different
towers at a different time. 3 or more towers can then pinpoint the phone
location with great precision, using exactly the same sort of
calculation as a GPS unit uses when finding its position based on timing
differences between 3 or more satellites.
A third option is highly directional antennas. I don't think telcos
bother with that though. Expensive installations and not needed for
I don't know if they use precision timers, but they can definitely see
how the thief roam around. When he goes home, the police may have an
idea about who has a criminal record in that area.
There is a program around that is supposed to look for a special keyword
in a sms, and send a gps reading back. It didn't work when I tried it,
but this appraoch can be developed into something more robust. Another
other security idea: Send an SMS to put the phone in "stolen" modus. (Do
that quickly, before they change the sim card.)
* Send gps coordinates regularly, by SMS to a configured number.
* Send a copy of every sms sent and received to the same place.
* Send the phone log whenever a call is made
* Send details about any new SIM card inserted.
* If there is enough disk space, record conversations and play them back
to a configured number when the thief isn't using the phone. Could
Also, a stolen phone could wait for a special message. If you give it up
because the telco and police won't bother - have the phone brick
itself by wiping out its flash memory. Or better, change the boot to display
"This phone is stolen from . . ." The thief throws it away - with luck,
someone else finds and returns it.
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