Editing Semi-Automatic Ground Environment (section)

===Earlier systems===
{{More sources|subsection|date=November 2024}}
Just prior to [[World War II]], [[Royal Air Force]] (RAF) tests with the new [[Chain Home]] (CH) radars had demonstrated that relaying information to the [[fighter aircraft]] directly from the radar sites was not feasible. The radars determined the map coordinates of the enemy, but could generally not see the fighters at the same time. This meant the fighters had to be able to determine where to fly to perform an interception but were often unaware of their own exact location and unable to calculate an interception while also flying their aircraft.

[[File:Seattle-ADS-map.png|thumb|SAGE radar stations were grouped by Air Defense Sectors (Air Divisions after 1966).  The SAGE System networked the radar stations in over 20 of the sectors using AN/FSQ-7 centrals in Direction Center.]]

The solution was to send all of the radar information to a central control station where operators collated the reports into single ''tracks'', and then reported these tracks to the airbases, or ''sectors''. The sectors used additional systems to track their own aircraft, plotting both on a single large map. Operators viewing the map could then see what direction their fighters would have to fly to approach their targets and relay that simply by telling them to fly along a certain heading or ''vector''. This [[Dowding system]] was the first [[ground-controlled interception]] (GCI) system of large scale, covering the entirety of the UK. It proved enormously successful during the [[Battle of Britain]], and is credited as being a key part of the RAF's success.

The system was slow, often providing information that was up to five minutes out of date. Against propeller driven bombers flying at perhaps {{convert|225|mph}} this was not a serious concern, but it was clear the system would be of little use against jet-powered bombers flying at perhaps {{convert|600|mph}}. The system was extremely expensive in manpower terms, requiring hundreds of telephone operators, plotters and trackers in addition to the radar operators. This was a serious drain on manpower, making it difficult to expand the network.

The idea of using a computer to handle the task of taking reports and developing tracks had been explored beginning late in the war. By 1944, [[analog computer]]s had been installed at the CH stations to automatically convert radar readings into map locations, eliminating two people. Meanwhile, the [[Royal Navy]] began experimenting with the [[Comprehensive Display System]] (CDS), another analog computer that took X and Y locations from a map and automatically generated tracks from repeated inputs. Similar systems began development with the [[Royal Canadian Navy]], [[DATAR]], and the [[US Navy]], the [[Naval Tactical Data System]]. A similar system was also specified for the [[MIM-3 Nike Ajax|Nike]] SAM project, specifically referring to a US version of CDS,<ref>{{Cite journal |type=letter |last=Nelson |first=Maj Gen Morris R. |date=June 12, 1950 |title=subj: Employment of an American Version of CDS |location=USAFHRC microfilm }} (cited by Schaffel pdf p. 311)</ref> coordinating the defense over a battle area so that multiple batteries did not fire on a single target. All of these systems were relatively small in geographic scale, generally tracking within a city-sized area.