array9

But a combination of bluetooth, wireless and sound should work fine

1.  discover nearby phones via normal phone 3g etc signals, to switch on Bluetooth

2. Start Bluetooth contact

3. If apps compatible, prepare information for exchange

4. Sync very accurate timers on both machines

5. Make one device a sender and the other the receiver. Agree on a time for transmission

6. Sender sends an coded high frequency chirp, and the receiver measures the time of receipt

7. The time of transmission can then be converted into a distance via speed of sound, and exchanged with the sender.  

8. This can be repeated at agreed preset times, until out of range.

9. When all contacts are out of range, switch off bluetooth

Notes.

You only need to establish the difference between clocks, rather than actually resetting them

Random agreed times for chirps, will mean you can handle a number of  phone contacts simultaneously. And/or the chirps can be at different frequencies.

You probably could do this while having a telephone conversation.

Chirps can have information in it, for verification.

The audio method gives potentially a definitive distance.  This could be used to calibrate a power measurement of the Bluetooth.

The various timing constants, such as the time to perform the audio analysis will be phone dependent, but can be dynamically measured by the phone itself on start up.

Health issues about how loud the chirp can be, would have to be checked.

Might not work in a noisy environment potentially

Fun

Martin