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A rough map of the three warning lines

The Pinetree Line was a series of radar stations located across southern Canada at about the 50th parallel, along with a number of other stations located on the Atlantic and Pacific coasts. Run by NORAD (after its creation), over half were manned by United States Air Force personnel with the balance operated by the Royal Canadian Air Force. The line was the first coordinated system for early detection of a Soviet bomber attack on North America, but the early 1950s radar technology quickly became outdated and the line was in full operation only for a short time.

Plans for what would become the Pinetree Line were underway as early as 1946 within the Permanent Joint Board on Defence (PJBD), a Canadian-U.S. organization. However, the costs of running such a system in the post-war era was too high, and instead Canada concentrated on the areas around Ontario and Quebec, while the United States set up stations in the Midwest and along the eastern seaboard.

With the successful test of an atomic bomb in the USSR, plans changed considerably. In 1949 Congress agreed to a $161 million construction program in co-operation with the RCAF, for a continuous line of stations across southern Canada. The USAF's Continental Air Command and the RCAF met in October 1950 to start planning, and in January 1951 the PJBD presented Recommendation 51/1 for the Extension of the Continental Radar Defence System. The USAF later requested an additional set of six (potentially) mobile stations to provide low-level coverage. It was later learned that the original radar systems performed better than expected, and a number of the mobile sites were never deployed.

The system was eventually deployed as a series of 33 main stations and 6 smaller "gap fillers". The majority of these ran in a line at about the 53rd parallel in the west (to offer coverage of major Canadian cities) and about the 50th parallel in the east. A second line ran up the eastern seaboard from the southern tip of Nova Scotia to the southern tip of Baffin Island. Of these, 22 of the main stations and all of the gap fillers were paid for by the USAF, leaving 11 to the RCAF. However 16 of the main stations were manned by RCAF personnel. On January 1, 1955, the system was officially handed over to RCAF command, and over time an additional 10 stations were added.

The Pinetree Line had several technical problems that limited its usefulness almost immediately. For one, the system used a "classic" pulsed mode radar, which made it unable to detect targets close to the ground due to "clutter". Another was that its location near population centres meant it offered only a "last minute" warning, and as the USSR moved to jet-powered bombers the warning time was falling. Studies were already underway in 1951 to build a much smaller series of much more capable Doppler radar stations somewhat further north, which would develop into the Mid-Canada Line, and just over a year after MCL, a more advanced system in the extreme north was built which would be known as the DEW Line.

The Pinetree stations were kept operational during this period, and most underwent modifications as a part of the deployment of SAGE. SAGE dramatically reduced the workload at the stations, cutting staff requirements by well over half. By the later 1950s some were being mothballed as newer systems came on line to the north. Nevertheless many of the Pinetree stations were kept operational into the 1980s, particularly on the Atlantic and Pacific coasts.

• List of Royal Canadian Air Force stations

• The Pinetree Line