Relative Location Sensor?

I was wondering if there was a sensor that exists that allows the sensor to be able to figure out the distance from one unit to the other. Like say one of the sensors was 12 yards from the other one. This could probably be done with wireless communication somehow, but I can't think of anything.

Thanks, Michael

This does come up regularly here and I have yet to see a robust method of doing this.

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.

-Michael

My bad, I meant a 9V battery for the transmitter and a 7.4V RC/Robot battery for the actual robot. Sorry about that.

is this Bot for home use ?
i think you can use a overhead cam to know the distance b/w the two robots and somehow communicate with the bots

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How would that work? Are you talking about IR sensing?