Synchronizing Clocks on multiple phones

Hi All,

I'm new to this forum. I'm working on an application which would require accurate synchronization of clocks (within a few milliseconds) of multiple phones (up to a few hundred) which may be separated by a few hundred yards. It's not necessarily the system clock that needs to be synchronized but each phone will have a counter (timer) running and these need to be synchronized. Any thoughts about how to approach this? One thought would be to use an NTP server. I'm thinking that a synchronization signal via bluetooth would be out of the question due to the distance.

Thanks in advance for your input!

Johnny