Peter Todd wrote:

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> On Thu, Aug 30, 2007 at 01:17:09AM +0100, Howard Winter wrote:
>>>...
>>>Although, if anyone off-hand knows the
>>>frequency of such transmissions for the GSM standard I'd be interested.
>>>Google searches didn't turn that spec up.
>>
>>There are 4, depending on where you are - 850, 900, 1800, 1900 MHz.
> Lol, see this is why google searched didn't turn it up.
> 
> I'm looking for the time between "heartbeat" signals, not the fequency
> of the RF transmissions themselves.
> 
> Maybe period would be a better word.

The "tower polling interval" varies by technology. 5 minutes to 60 minutes if
my memory is correct (read a book about cell tower technologies about a decade ago).
The newer phones poll less often to prolong battery life.
The do wake up many times a second to see if there is a page call for them.

The quick way to find out for yourself is to monitor your phone's battery
current using a scope and small series resistance. You'll see huge spikes for
a poll response, and a regular train for the 'listen to polling channel'.

Your bigger issue with using RF detection will be false triggers by much
stronger, but far away transmissions (police radios (5-15W),
nearby paging transmitters (500W), passing cabs (5W), etc.

Wi-Fi in a park is highly unlikely. That is way beyond the range limit of most cheap
systems. And is very hard to detect because of it's short transmission time (milliseconds)
and low density, not to mention the frequency hopping.

Two way paging? WHY? Then it's a cell phone or blackberry.

I would just go with KISS and use passive IR. Or possibly microwave door openers
to sense motion without being easily recognized. If you look at the beat frequency
by tapping the motion detector internals, you can also get velocity and coming/going.

You can get both sensing methods in some PIR security sensors.
They use PIR to confirm the size of a moving object they detect with
microwave doppler to prevent false alarms. About $40 US. "Pet resistant".

Robert



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