Potential help;
Potential help; I have cut / paste this post, as i entered it earlier in response to another members question - i hope it helps.
I have just been working with a friend who has developed a belt worn system, using GPS / mobile phone technology. It can also be used as a keyring type device.
A "field" can be programmed into it, so if the wearer breaks that field - your phone rings to alert you, then sends a grid reference, which in turn will show their exact position on google maps.
it was originally developed for naughty dogs who run off on walks, it pinpoints where they are down to a few yards.
It is rechargeable, and lasts in "sleep" mode for days on end.
hope this helps, if you are interested, let me know and I'll put you in touch.
Happy days.