triangular positioning hardware - help please

Hi, Does anybody have an idea of available hardware for sending some kind of beams and hardware available for measuring the strength of those beams.

I would prefer to set up 3 beamers in a triangle with 50m (150 feet) between each, and then be able to read the position near this triangle.

The robot should move around in a square surrounding the triangle. The square will be 50m x 50m (150feet x 150 feet).

The robot should be able to navigate around with a precision of 10cm (4 inch).

The robot should be be controlled by a Arduino Duemilanove.

Any ideas of suitable hardware for this solution?

Thanks in advance.

Regards, Lars