Someone stole my Neo Freerunner... :(

Helge Hafting helge.hafting at
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 
to those.

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 
normal operation.

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.)

Stolen mode:
* 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
   be interesting.

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.

Helge Hafting

More information about the community mailing list