Editing Semi-Automatic Ground Environment (section)

==Background==

===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The system was slow, often providing information that was up to five minutes out of date.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.

===Valley Committee===
[[File:Museum of Science, Boston, MA - IMG 3168.JPG|thumb|right|Whirlwind computer elements: [[Magnetic-core memory|core memory]] (left) and operator console]]
[[File:Module from a SAGE computer.jpg|right|thumb|Module from a SAGE]]
[[File:RCA 6499 Radechon tube.jpg|right|thumb|The RCA #6499 Radechon tube was used for [[random-access memory]] in the computers.]]

When the [[Soviet Union]] tested [[RDS-1|its first atomic bomb]] in August 1949, the topic of [[air defense]] of the US became important for the first time. A study group, the "Air Defense Systems Engineering Committee", was set up under the direction of Dr. [[George Valley]] to consider the problem and is known to history as the "Valley Committee".<ref name=valley>
{{cite web
 | title= The Valley Committee
 | website= Lincoln Laboratory
 | date= 1995
 | url= https://www.ll.mit.edu/about/History/origins.html
 | url-status= live
 | archive-url= https://web.archive.org/web/20160401230140/http://www.ll.mit.edu/about/History/origins.html
 | archive-date= 2016-04-01
}}
</ref>

Their December report noted a key problem in air defense using ground-based radars. A bomber approaching a radar station would detect the signals from the radar long before the reflection off the bomber was strong enough to be detected by the station. The committee suggested that when this occurred, the bomber would descend to low altitude, thereby greatly limiting the [[radar horizon]], allowing the bomber to fly past the station undetected. Although flying at low altitude greatly increased [[fuel consumption]], the team calculated that the bomber would only need to do this for about 10% of its flight, making the fuel penalty acceptable.<ref name=valley/>

The only solution to this problem was to build a huge number of stations with overlapping coverage. At that point the problem became one of managing the information. Manual plotting was ruled out as too slow, and a computerized solution was the only possibility. To handle this task, the computer would need to be fed information directly, eliminating any manual translation by phone operators, and it would have to be able to analyze that information and automatically develop tracks.<ref name=valley/> A system tasked with defending cities against the predicted future Soviet bomber fleet would have to be dramatically more powerful than the models used in the NTDS or DATAR.<ref name="Quarterly Progress Report">
{{Cite report
 | title= Quarterly Progress Report
 | date= June 1952
 | publisher= Lincoln Laboratories
}}
 (cited by Schaffel p. 197)
</ref><ref name="web.mit.edu">
{{Cite news
 | title= Physicist George Valley Jr. is dead at 86
 | date= October 20, 1999
 | newspaper= [[MIT Tech Talk]]
 | format= MITnews webpage
 | url= http://web.mit.edu/newsoffice/1999/valley-1020.html
 | access-date= 2012-07-15
 | url-status= live
 | archive-url= https://web.archive.org/web/20121018210958/http://web.mit.edu/newsoffice/1999/valley-1020.html
 | archive-date= October 18, 2012
}}
</ref>

The Committee then had to consider whether or not such a computer was possible. The Valley Committee was introduced to [[Jerome Wiesner]], associate director of the Research Laboratory of Electronics at [[MIT]]. Wiesner noted that the [[Servomechanisms Laboratory]] had already begun development of a machine that might be fast enough. This was the [[Whirlwind I]], originally developed for the [[Office of Naval Research]]<ref>{{cite web| url = https://www.computer.org/csdl/proceedings-article/afips/1951/50400070/12OmNBvkdmJ| title = Project Whirlwind is a high-speed computer activity sponsored at the Digital Computer Laboratory, formerly a part of the Servomechanisms Laboratory, of the Massachusetts Institute of Technology (MIT) by the US Office of Naval Research (ONR) and the United States Air Force. IEEE Computer Society}}</ref> as a general purpose [[flight simulator]] that could simulate any current or future aircraft by changing its software.<ref name=valley/>

Wiesner introduced the Valley Committee to Whirlwind's project lead, [[Jay Forrester]], who convinced him that Whirlwind was sufficiently capable. In September 1950, an early microwave [[early-warning radar]] system at [[Hanscom Field]] was connected to Whirlwind using a custom interface developed by Forrester's team. An aircraft was flown past the site, and the system digitized the radar information and successfully sent it to Whirlwind. With this demonstration, the technical concept was proven. Forrester was invited to join the committee.<ref name=valley/>

===Project Charles===
With this successful demonstration, [[Louis Ridenour]], chief scientist of the Air Force, wrote a memo stating "It is now apparent that the experimental work necessary to develop, test, and evaluate the systems proposals made by ADSEC will require a substantial amount of laboratory and field effort."<ref name=valley/> Ridenour approached [[Massachusetts Institute of Technology|MIT]] President [[James Killian]] with the aim of beginning a development lab similar to the war-era [[Radiation Laboratory]] that made enormous progress in radar technology. Killian was initially uninterested, desiring to return the school to its peacetime civilian charter. Ridenour eventually convinced Killian the idea was sound by describing the way the lab would lead to the development of a local electronics industry based on the needs of the lab and the students who would leave the lab to start their own companies. Killian agreed to at least consider the issue, and began Project Charles to consider the size and scope of such a lab.<ref name=charles>{{cite web |url= https://www.ll.mit.edu/about/History/projectcharles.html |title= Project Charles |website= Lincoln Laboratory |date= 1995 |url-status= live |archive-url= https://web.archive.org/web/20160224153432/http://www.ll.mit.edu/about/History/projectcharles.html |archive-date= 2016-02-24 }}</ref>

Project Charles was placed under the direction of [[Francis Wheeler Loomis]] and included 28 scientists, about half of whom were already associated with MIT. Their study ran from February to August 1951, and in their final report they stated that "We endorse the concept of a centralized system as proposed by the Air Defense Systems Engineering Committee, and we agree that the central coordinating apparatus of this system should be a high-speed electronic digital computer."<ref name=charles/> The report went on to describe a new lab that would be used for generic technology development for the Air Force, Army and Navy, and would be known as Project Lincoln.<ref name=charles/>

===Project Lincoln===
Loomis took over direction of Project Lincoln and began planning by following the lead of the earlier RadLab. By September 1951, only months after the Charles report, Project Lincoln had more than 300 employees. By the end of the summer of 1952 this had risen to 1300, and after another year, 1800. The only building suitable for classified work at that point was Building 22, suitable for a few hundred people at most, although some relief was found by moving the non-classified portions of the project, administration and similar, to Building 20. But this was clearly insufficient space. After considering a variety of suitable locations, a site at [[Hanscom Air Force Base|Laurence G. Hanscom Field]] was selected, with the groundbreaking taking place in 1951.<ref name=charles/>

The terms of the [[National Security Act of 1947|National Security Act]] were formulated during 1947, leading to the creation of the US Air Force out of the former [[US Army Air Force]]. During April of the same year,  US Air Force staff were identifying specifically the requirement for the creation of automatic equipment for radar-detection which would relay information to an air defence control system, a system which would function without the inclusion of persons for its operation.<ref name="Kent C. Redmond"/> The December 1949 "Air Defense Systems Engineering Committee" led by Dr. [[George Valley]] had recommended computerized networking<ref name="Quarterly Progress Report"/> for "radar stations guarding the northern air approaches to the United States"<ref name="web.mit.edu"/> (e.g., in Canada). <!--In 1949 the USAF had funded [[Project Charles]] to develop a demonstration system for automating Air Defense{{Citation needed|date=July 2011}}--> After a January 1950 meeting, Valley and [[Jay Forrester]] proposed using the [[Whirlwind I]] (completed 1951) for air defense.<ref>{{cite web | url=https://www.technologyreview.com/2015/06/23/167538/the-many-careers-of-jay-forrester/ | title=The Many Careers of Jay Forrester }}</ref> On August 18, 1950, when the "[[1954 Interceptor]]" requirements were issued, the USAF "noted that manual techniques of aircraft warning and control would impose "intolerable" delays"<ref name=Futrell>{{Cite report |last=Futrell |first=Robert Frank |date=June 1971 |title=Ideas, Concepts, Doctrine: A History of Basic Thinking in the United States Air Force 1907–1964 |volume=1 |publisher=Aerospace Studies Institute, Air University}} (cited by Volume I p. 187)</ref>{{rp|484}} ([[Air Materiel Command]] (AMC) published ''Electronic Air Defense Environment for 1954'' in December <!--1950-->.)<ref name=McRee>{{Cite report |last=McRee |first={{Who|reason=McRee's first name needs specified.|date=April 2013}}|date=15 December 1950 |title=…Electronic Air Defense Environment for 1954 |publisher=Headquarters, Air Materiel Command}}</ref>  During February–August 1951 at the new [[Lincoln Laboratory]], the USAF conducted [[Project Claude]] which concluded an improved air defense system was needed.{{Citation needed|reason=data from Lincoln Lab article|date=July 2011}}
[[File:Texas Tower 4.jpg|thumb|250px|right|To increase warning time, radar systems called [[Texas Towers]] were placed in the Atlantic Ocean using technology similar to Texas-style offshore oil platforms]]
In a test for the US military at [[Bedford, Massachusetts]] on 20 April 1951, data produced by a radar was transmitted through telephone lines to a computer for the first time, showing the detection of a mock enemy aircraft. This first test was directed by [[C. Robert Wieser]].<ref name="Kent C. Redmond">{{cite book |url=https://books.google.com/books?id=dxZVbxcf_IoC |author1=Kent C. Redmond  |author2=Thomas M. Smith |name-list-style=amp |title=From Whirlwind to MITRE: The R&D Story of the SAGE Air Defense Computer|publisher=[[MIT Press]] |year=2000 |isbn=978-0-262-26426-6}}(''20th of April 1951'' - p.1, ''National Security Act 1947'' - p.12, ''April 1947'' - p.13)</ref>

The "Summer Study Group" of scientists in 1952 recommended "computerized air direction centers…to be ready by 1954."<ref>{{Cite news |last1=Lapp |last2=Alsop |date=March 21, 1953 |title=We Can Smash the Red A-Bombers |newspaper=Saturday Evening Post |page=19}} (citation 29 of Volume I, p. 25)</ref>

[[International Business Machines|IBM]]'s "Project High" assisted under their October 1952 Whirlwind subcontract with [[Lincoln Laboratory]],{{r|Pugh}}{{rp|210}} and a 1952 USAF [[Project Lincoln]] "fullscale study" of "a large scale integrated ground control system" resulted in the SAGE approval<!--Schaffel p. 207--> "first on a trial basis in 1953".{{r|VolumeI}}{{rp|128}}  The USAF had decided by April 10, 1953, to cancel the competing [[Wizard Program|ADIS]]<!--Schaffel p. 199--> (based on CDS), and the University of Michigan's [[Aeronautical Research Center]] withdrew in the spring.{{r|RedmondSmith}}{{rp|289}} [[Air Research and Development Command]] (ARDC) planned to "finalize a production contract for the Lincoln Transition System".{{r|Schaffel}}{{rp|201}}  Similarly, the July 22, 1953, report by the [[Harold H. Bull|Bull Committee]] ([[United States National Security Council|NSC]] 159) identified completing the [[Mid-Canada Line]] radars as the top priority and "on a second-priority-basis: the Lincoln automated system"<ref>quote from Schaffel p. 191; Condit p. 259 footnote 1 cites: "''CCS 381 US (5-23-46) sec 37.''"</ref>  (the decision to control Bomarc with the automated system was also in 1953.)<ref>{{Cite report |last=McVeigh |first=D. R. |date=January 1956 |title=The Development of the Bomarc Guided Missile 1950–1953 |publisher=Western Air Development Center}} (cited by Volume I p. 108 footnote 69: "''Before the end of 1953, it was also decided that the Sage system being developed by Lincoln Laboratories would be used to control the Bomarc.<sup>69</sup>''")</ref>

The Priority Permanent System with the initial (priority) radar stations was completed in 1952{{r|Schaffel}}{{rp|223}} as a "manual air defense system"<!-- (Col John Morton)--><ref name=IYD/> (e.g., [[North American Aerospace Defense Command|NORAD]]/[[Air Defense Command|ADC]] used a "[[Plexiglas]] plotting board" at the [[Ent Air Force Base|Ent command center]].)<!--{{r|Schaffel}}{{rp|151}}-->  The [[Permanent System radar stations]] included 3 subsequent phases of deployments and by June 30, 1957, had 119 "Fixed CONUS" radars, 29 "Gap-filler low altitude" radars, and 23 control centers".{{r|Condit}}  At "the end of 1957, ADC operated 182 radar stations [and] [[Manual Air Defense Control Center|17 control centers]] … 32 [stations] had been added during the last half of the year as low-altitude, unmanned gap-filler radars.  The total consisted of 47 gap-filler stations, 75 Permanent System radars, 39 semimobile radars, 19 [[Pinetree Line|Pinetree stations]],…1 [[Lashup Radar Network|Lashup -era radar]] and a single [[Texas Tower]]".{{r|Schaffel}}{{rp|223}}  "On 31 December 1958, USAF ADC had 187 operational land-based radar stations" (74 were "P-sites", 29 "M-sites", 13 "SM-sites", & 68 "[[Zone of Interior|ZI]] Gap Fillers").{{r|NORAD1958B}}