Lawn mower Robot - Position Detection?

I am thinking of launching a project of a DIY lawn mower robot like http://www.youtube.com/watch?v=HgUxh44jGbs

The downside of existing solutions is that it requires you to define/surround the yard with a cable to prevent the mower from escaping the area.

My idea would be having a couple of RF antenna poles and a multi-channel receiver on the mower which reads the phase shift of those antennas for position calculation.

Accuracy would need to be about 2-3 inches in a normal-sized lawn yard.

Is there any existing project that include position detection already (without GPS with its rough resolution)?

Your feedback would be very much appreciated.

Knubbi: My idea would be having a couple of RF antenna poles and a multi-channel receiver on the mower which reads the phase shift of those antennas for position calculation.

Do you have any reason to believe that's feasible? I admit it's possible in theory, but it would be extraordinarily hard to take it from theory to practice. In effect, it'd require re-inventing a lot of what GPS does, and would involve tackling some darned hard problems.

Yep, you're right. That's why I asked whether anybody has invented "this wheel" already.