Awhile back I had an idea for tracking a push type lawn mower.
I'd put IR LED transmitters in the 4 corners of a square lot, each with it's own pulsing IR code. On the top of the mower would be an IR detector that could move 360 degrees. I slowly go 'round until I saw 1 of the IR LEDs, mark that angle, and keep going until I saw at least 2 of the 4 IR transmitters. As long as I had two angles to work with, I could triangulate my position inside the square lot (ignoring the fact that there are really 2 places an object could be in 2D space using that method, one inside the square, one outside). The more the IR LED transmitters, the better the accuracy, and the better the chances I'll see them if the platform was to somehow get off level.