Mid-Canada Line

The Mid-Canada Line, also known as the McGill Fence was a line of radar sites across Canada intended to provide early warning of a Soviet bomber attack on North America, supplanting the less advanced Pinetree Line. The line was used only briefly from the late 1950s to the mid-1960s, as the bomber threat moved to ICBMs, the early warning role passed entirely to the much more capable DEW Line much further north.

In February 1953 the Canada/US Military Study Group (MSG) was asked "to study those aspects of the North American Air Defence System in general, and the early warning system in particular, which are of mutual concern to the two countries". The MSG then asked the air defence commanders of Canada and the US to prepare independent briefs on the subject. By July 1953, RCAF Air Defence Command had completed its brief, followed shortly thereafter by its US counterpart. Both suggested the construction of an new early warning line along the 55th parallel.

In October 1953 the MSG recommended to both governments "That there be established at the earliest practicable date, an early warning line located generally along the 55th parallel between Alaska and Newfoundland", and outlined their consideration as to the minimum operational requirements. By the end of November 1953, the Mid-Canada Line had been approved in principle. Unlike the jointly-operated DEW and Pinetree lines, the Mid-Canada line would be funded and operated entirely by the RCAF.

At this point construction of the Pinetree Line was still in its early stages. The radar systems used in the Pinetree systems were "old" pulse-based systems that would be fairly easy to jam, and had no ability to detect targets close to the ground. Professor G. A. Woonton of McGill University designed a Doppler-based system that avoided both of these problems, as well as having considerably longer range due to its "continuous wave" nature -- more radio energy fell on the targets in a unit time, making them easier to detect. The system used two antennas, a broadcaster and receiver, separated by about 50km; any aircraft flying between the antennas would be detected, even very close to the ground.

Another problem with the Pinetree Line is that it was situated too far south. By the time it detected a potential attack by jet-powered aircraft, there would be little time to do anything before the reached Canadian cities. The Mid-Canada Line thus offered an opportunity to push the detection line further north, away from the cities, without upsetting the operation of the Pinetree Line at the same time.

Eight major Sector Control Centers, each numbered from 200 to 900, would each control up to thirty unmanned radar sites for a total of 90 stations. It turned out that the radar systems worked even better than expected, and could be sited up to 90km apart instead of 60, saving a considerably amount of money. The sector control centers were linked using an advanced microwave communications system developed in part by CADRE, which scattered off the troposphere for long-distance communications. All aircraft transiting the line would have to file a flight plan through the Mid Identification Zone, or MIDIZ, and a number of airbases would be constructed on the north of the line to support interceptor aircraft operations for identification.

At about this time another huge civil engineering project was underway in Canada, the construction of a cross-Canada microwave relay telephone system. Since many of the logistics problems were similar, the construction group, led by Bell Canada, was selected as a major contractor for the base construction. Site selection started in 1955, with a major surveying effort running across Canada at the 55th parallel (roughly at the entrance of James Bay into Hudson Bay). The sites were so remote that the RCAF had to form up its first all-helicopter squadron in order to provide flight support for the survey teams. One interesting project involved the use of two huge overland wheeled trains, a US Army experimental logistics vehicle which could haul the equivalent of several semi-trailers across almost any terrain.

Construction started in 1956 and proceeded quickly. By April 1957 the eastern half was operational, and the line was declared fully operational on January 1, 1958.

It was not long before the RCAF started to have reservations about the costs of maintaining the Line. Although technically more capable than Pinetree, by the time interceptor aircraft had reached their targets they were within range of the Pinetree radars anyway. The extra time offered by the MCL was not considered worth the trouble. The USAF disagreed, but the western half of the line was shut down in January 1964, leaving the eastern half to help defend the industrial areas of the US. However as the USSR moved their defences to ICBMs it became clear that both the MCL and Pinetree systems were of limited use, and the entire Mid-Canada line was shut down in April 1965.

Interestingly the MIDIZ remains administratively operational, although transit across it is not enforced.