The University of Zurich just released their monocular pose estimation system as open-source ROS package. Infrared LEDs are used to estimate the relative position of two robots. The system therefore is highly accurate, efficient and robust.

In the video shown below, the youBot is equipped with a camera with an infrared-pass filter, the quadrotor with infrared LEDs. The positions of the LEDs on the robots are known, making it possible to precisely estimate the position and orientation of the robots. 

read more, watch the video and get the code, here