SAGE - Semi Automatic Ground Environment - Part 1/2

*** THIS FILM WAS MADE MY THE US GOVERMENT AND IS PUBLIC DOMAIN ***

SAGE, the Semi Automatic Ground Environment, was an automated control system used by NORAD for collecting, tracking and intercepting enemy bomber aircraft from the late 1950s into the 1980s. In later versions, the system could automatically direct aircraft to an interception by sending commands directly to the aircraft’s autopilot.

By the time it was fully operational the Soviet bomber threat had been replaced by the Soviet missile threat, for which SAGE was entirely inadequate. Nevertheless, SAGE was tremendously important; it led to huge advances in online systems and interactive computing, real-time computing, and data communications using modems. It is generally considered to be one of the most advanced and successful large computer systems ever developed.

IBM’s role in SAGE (the design and manufacture of the AN/FSQ-7 computer, a vacuum tube computer with ferrite core memory based on the never-built Whirlwind II) was an important factor leading to IBM’s domination of the computer industry.

Background

Prior to the introduction of SAGE, the task of intercepting bombers was becoming increasingly difficult. This was the latest shift in a balance of power that had been see-sawing since the 1930s.

During the leadup to World War II it was widely believed that the bomber was essentially immune, at least in any practical sense. As speeds approached 200 mph the time between seeing the bomber and it reaching its targets was growing so short that there was no time for interceptor aircraft to climb to altitude. Once the bombs were released the multi-engine bombers often had a performance advantage over the fighters, allowing them to escape with relative ease. The only apparent solution to this problem would be to keep fighters in the air on-station at all times, a practical impossibility due to the short flight times of contemporary fighters. Thousands of fighters would be needed to keep enough of them in the air at any one time to defend against a raid of perhaps a hundred bombers. Most believed “the bomber will always get through”.

The introduction of radar seriously upset this equation. Radar gave just enough warning time for fighters to “scramble” and be at the bomber’s altitude by the time they arrived. In modern terms radar is a “force multiplier”, allowing a small number of fighters to handle the task that would otherwise require many more aircraft. Speeds of the aircraft of the era were such that the rest of the task of intercepting the bombers could be carried out by hand. The RAF, for instance, used a large map with markers representing various radar contacts, with controllers relaying positions and directions to the aircraft by radio.

In the post-war era, the speed of the new jet-powered aircraft increased by a factor of two to three, similarly decreasing the available effective response time. In a general sense this should not have cause a problem; although the bombers were approaching much faster and gave less warning time, the fighters intercepting them were also much faster and could climb to altitude in minutes. But it was all of the other tasks that caused the problem. This included collecting information about the targets from the radar sites, figuring out where they were going (developing a track), and then guiding the fighters to intercept them. A study in the 1950s by the RCAF concluded that it would take on the order of one minute per interception. With flight times on the order of an hour by several hundred aircraft, some were bound to escape interception due to operator overload. The balance shifted toward the attackers again. With nuclear bombs onboard, this was unacceptable.

The problem became even more acute if the bombers attacked at low level. Radar is line-of-sight, so by approaching close to the ground they would remain hidden behind the curvature of the Earth until approaching to within a few tens of miles. With a jet bomber this meant the defenders had only a few minutes to react, far too little time to launch an interceptor, let alone guide it to an intercept.