Comprehensive Display System

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The Comprehensive Display System (CDS) was a Cold War command, control, and coordination system of the British Royal Navy that worked with the detection/search Type 984 radar.[1] The CDS allowed operators to assign objects on the radar display different IDs, and used simple electronics to predict their future locations. The CDS made it easy for operators to vector friendly fighters onto intercept courses with unknown targets. CDS was very influential in military circles, and led to computerized versions in for form of DATAR, Naval Tactical Data System, and SAGE.

The trackball (known as ball tracker at its time) was invented by Ralph Benjamin as part of his work for the CDS in 1946.[2][3] The prototype, named roller ball, was patented in 1947,[3] but kept as a secret inside the military.[2][3] It laid the foundation to input devices such as the computer mouse.


During early testing of the Chain Home radar system, a bomber was able to make a simulated attack on its target before the radar operators were able to successfully vector the defending fighters to intercept it. Studying the situation, Robert Watson-Watt identified the problem as being one of communications; the radar operators and members of the Observer Corps overloaded the pilots with information, sometimes contradictory. Henry Tizard, with Patrick Blackett and Hugh Dowding immediately set to work on this problem, designing a 'command and control air defence reporting system' with several layers of reporting that were eventually sent to a single large room for mapping. Observers watching the maps would then tell the fighter groups what to do via direct communications. Operators at the group headquarters would use this information to re-create smaller versions of the map, and could then relay information to the pilots by reading their own copies.

This Dowding system worked well in the era when bombers flew in large formations designed to maximize their defensive firepower. This caused the raids be limited to perhaps a few dozen plots at the most, limiting the amount of information that needed to be relayed up and down the chain of command. If those same bombers had attacked in smaller groups they would have overwhelmed the system, which is precisely what the RAF did to the German Kammhuber Line after 1942, flying all of their bombers individually over a single radar control area, a tactic known as a bomber stream.

The system also relied on the relatively low performance of the bombers compared to the fighters. This allowed the operators to guide the fighters into the general area of the bombers and then allow them to hunt visually. Even if they ended up behind the bombers, the fighters extra speed, roughly double that of the bomber's cruise, allowed them to easily chase down the formations. In the case of higher speed bombers, like the Dornier Do 17 or the Focke-Wulf FW 190 used in the "tip-n-run" raids of 1942/43, the fighters needed to be vectored to locations far in front of the attackers, "leading" them. Trained observers could do this by noting the location of travel and projecting that forward in time, but only at the cost of having to receive several radar reports over time to generate a "track".


Much of the work of developing a track and plotting an intercept is purely mechanical, and can be easily sped up by automation of various steps of the process. For instance, a track can be calculated by comparing the X and Y locations of subsequent reports and dividing by the time between them. The result is a rate of change in X and Y, which can also be easily presented as a vector indicating course and speed. With this vector, operators can easily predict future locations and estimate intercept courses "by eye". The Comprehensive Display System was an electromechanical solution to this problem.


  1. ^ Henson, Jason W. "Type 984 3D radar". Harpoon Head Quarters. Retrieved 2013-06-26. 
  2. ^ a b Copping, Jasper (2013-07-11). "Briton: 'I invented the computer mouse 20 years before the Americans'". The Telegraph. Retrieved 2013-07-18. 
  3. ^ a b c Hill, Peter C. J. (2005-09-16). "RALPH BENJAMIN: An Interview Conducted by Peter C. J. Hill" (Interview). Interview #465. IEEE History Center, The Institute of Electrical and Electronics Engineers, Inc. Retrieved 2013-07-18.