We thought about infrared, an emetter as a beacon that we couldnt see but the robot could. We wanted to use multiple IR receiver around the robot to understand from which way the signal came from. We used the receiver not to decode a message in IR as we can often see like with a TV and a remote, but to analyse the intensity of the light we received. with some tests this way worked but not as well as we wanted, it couldnt detect far enough or couldnt say if the signal came from one way or an other.