Yeah, I thought about the GPS thing, but that would be too inaccurate. I was thinking about this mainly for a small RC/Autonomous robot I'm designing, but I'm trying to keep it simple and cheap so I'm not going to start trying to design a way to do this. I'll save that for a later project! Thanks for speedy replies. To answer Richard's questions, I guess it'd be around 100 feet max. I'd like pretty good accuracy, it wouldn't have to be exact, but within like a 1-5 foot radius. The limitations, as far as I can think of would be through walls. The unit's would be Arduino boards-Arduino Duemilanove and Arduino Uno. The amount of power available will be a 9V battery attached to each one. And the last two aren't really known. I think I'm not going to add this feature to the preliminary design. If I find a easy or economical solution then I'll add it later. This is my first big Arduino project so I'm trying to keep it simple. Thanks again for the responses.