There is a book on it[1]. But suffice it to say that the radar is computing an "exit vector" when the target it moving away from center and applying a correction to the pointer. I built a similar system with LEDs when building a tracking system for two moving robots. In my case I used a parabolic dish (a solar cigarette lighter) and a line of LEDs (bar graph display) as detectors rather than lights. Since the target robot and the pursuing robot had their beacon and search dish in the same plane I could reduce the problem to a managable bit of 2D geometry. As the signal went off axis you could turn the robot to re-align by applying the opposite rotation.

[1] "Multiple Target Tracking with Radar Applications" - Sam Blackman (http://books.google.com/books?id=Ag9TAAAAMAAJ&q=tracking+mul...)