[ToddF]

Extrapolating from Jason's idea of using ambient light, there are also ambient radio waves being transmitted from fixed locations around the field. Though wireless communications are not permitted, there isn't any rule against passively detecting sources of transmitted radio waves. Just like a cell phone can triangulate it's location from cell towers, it might be possible to triangulate robot position and orientation. You could use one of your practice matches to drive around the field and make a calibrated map.

It would be a heck of an engineering challenge, though. The necessary equipment might also put you way over budget.

Todd F.