In the environment, I have two robots and a couple of fixed obstacles. In order to detect obstacles I am using ultrasonic sensors. For robots, they need to detect each other and from which side other robot is coming (front, left, right or back), and do this during the motion. For this purpose, I cannot use PIR sensors, because robots are constantly moving. Also, I need to differentiate between moving robots and stationary obstacles, so ultrasonic sensors are also not helpful.
So I came up with idea to somehow mark the robots with some property unique to environment, so when we detect object with that property, we know that it is a robot, and not another obstacle. One of ideas might be to put lasers on one robot, and on other robot put four laser sensors, one for each side, so we precisely can say from which side the other robot came from. Another option might be to use IR transmitters on one robot and four IR receivers on other robot? What do you suggest, is there any other type of sensors that might help?