Cape Cod SAGE Prototype

While the Digital Computer Division at Lincoln Laboratory wrestled with Whirlwind, the Aircraft Control and Warning Division concentrated its efforts on verifying the underlying concepts of air defense. A key recommendation in the Project Charles Final Report was that a small air defense system should be constructed and evaluated before work on a more extensive system began. The report proposed that the experimental network be established in eastern Massachusetts, that it include 10 to 15 radars, and that all radars be connected to Whirlwind. This methodology of system prototyping, first established during SAGE, became core to Lincoln Laboratory's approach to the development of new system concepts, and is still very much part of the Laboratory's culture today.

As soon as the air defense program began, Lincoln Laboratory started to set up an experimental system and named it, for its location, the Cape Cod System. It was functionally complete; all air defense functions could be demonstrated, tested, and modified. The Cape Cod System was a model air defense system, scaled down in size but realistically embodying all operational functions. 

Map showing the locations of the Cape Cod system radars.

Cape Cod, which was chosen because of its convenience to the Laboratory, was a good test site. It covered an area large enough for realistic testing of air defense functions. In addition, its location was challenging—hilly and bounded on two sides by the ocean, with highly variable weather and a considerable amount of air traffic.

Every aspect of the Cape Cod System called for innovation. Not only did it require radar netting, but radar data filtering was also needed to remove clutter that was not canceled by the moving-target indicator (MTI). Phone-line noise also had to be held within acceptable limits.

A long-range AN/FPS-3 radar, the workhorse of the operational air defense net, was installed at South Truro, Massachusetts, near the tip of Cape Cod, and equipped with an improved digital radar relay. Less powerful radars, known as gapfillers, were installed to enhance the coverage provided by the long-range system. Because near-total coverage was required, the beams of the radars in the network had to overlap, meaning that the radars could be separated by no more than 25 miles.

Cape Cod radar tower

AN/FPS-3 Cape Cod radar tower in South Truro, Massachusetts. Picture used with the permission of the MITRE Corporation (copyright © The MITRE Corporation. All rights reserved).

gapfiller radar

Cape Cod gapfiller radar.

Initially, two SCR-584 radars that had been developed during World War II by the MIT Radiation Laboratory were installed as gapfillers at Scituate and Rockport, Massachusetts. Early tests of these radars showed much shorter detection ranges than expected, however, and improvements in the components and the test equipment were required to resolve the problems and make the radars operate as needed. As new radars became operational, each included a Mark-X identification-friend-or-foe (IFF) system, and reports were multiplexed with the radar data. Dedicated telephone circuits to the Barta Building in Cambridge were leased and tested.

Buffer storage had to be added to Whirlwind to handle the insertion of data from the asynchronous radar network, and the software had to be expanded considerably. A direction center needed to be designed and constructed to permit Air Force personnel to operate the system: to control the radar data filtering, initiate and monitor tracks, identify aircraft, and assign and monitor interceptors.

Construction of a realistic direction center depended heavily on the development of an interactive display console. Nothing comparable had ever been done before, and the technology was primitive. What was needed was a computer-generated display that would include alphanumeric characters (to act as labels on aircraft) and a separate electronic tote-board status display. Then, the console operator could select display categories of information (for example, hostile aircraft tracks) without being distracted by all of the information available.

SAGE display

SAGE system radar display. Picture used with the permission of the MITRE Corporation (copyright © The MITRE Corporation. All rights reserved).

SAGE operator

SAGE operator uses the light-sensing gun.

The display console was developed around the Stromberg-Carlson Charactron cathode-ray tube (CRT). The tube contained an alphanumeric mask in the path of the electron beam. The beam was deflected to pass through the desired character on the mask, refocused, and then deflected a second time to the desired location on the tube face. This process was electronically complex, but it worked. The console operator had a keyboard for data input and a light-sensing gun, which was used to recognize positional information and enter it into the computer. This novel means of control for high-speed computers was invented at the Laboratory by Robert Everett.

All these complex engineering tasks were carried out in parallel, on schedule, and with little reworking. By September 1953, just two years and five months after the go-ahead, the Cape Cod System was fully operational. The radar network consisted of gapfiller radars, height-finding radars, and long-range radars.

The software program could handle, in abbreviated form, most of the air defense tasks of an operational system. Facilities were in place to initiate and track 48 aircraft, identify and find the height of targets, control 10 simultaneous interceptions from two air bases, and give early warning and transmit data on 12 tracks to an antiaircraft operations center. Now the system had to be tested.

Next, part 2: MIT Lincoln Laboratory tests and expands the Cape Cod System

Adapted from E.C. Freeman, ed., Technology in the National Interest, Lexington, Mass.: MIT Lincoln Laboratory, 1995.

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