Mid-Canada Line

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A rough map of the three warning lines

The Mid-Canada Line, also known as the McGill Fence, was a line of radar stations across the "middle" of Canada to provide early warning of a Soviet bomber attack on North America. It was built to supplement the less-advanced Pinetree Line, which was located further south. The majority of Mid-Canada Line stations were used only briefly from the late 1950s to the mid-1960s, as the attack threat changed from bombers to ICBMs; the early warning role passed almost entirely to the more capable DEW Line further north.

Construction of the Pinetree Line had only just started when air planners started to have concerns about its capabilities and siting. By the time it detected a potential attack by jet-powered aircraft, there would be little time to do anything before the attack reached Canadian or northern U.S. cities. Additionally the Pinetree systems used "old" pulse-based radars that were fairly easy to jam and were unable to detect targets close to the ground due to scattering. Although expensive in terms of fuel use, it would be possible for Soviet bombers to evade detection by flying lower. They would eventually be seen as they approached the stations, but possibly so late that there would be no possibility of intercepting them. This made the Pinetree Line's usefulness somewhat suspect even before it had become operational.

In 1951 Professor G. A. Woonton of McGill University had demonstrated a Doppler-based system to U.S. officials that avoided both of these problems. The system used two antennas, a broadcaster and receiver, separated by about 60 km. Any aircraft flying between the antennas would be detected when it "broke" the beam, although its exact location would not be known. He proposed a system in which two overlapping lines of stations would be set up, forming a continuous "fence". This not only covered the "dead points" directly over the antennas, but also allowed for location within 30km depending on which pair of beams were broken – a distance well within the detection ranges of interceptor aircraft radar.

The nature of Doppler radar offered considerably better detection capabilities as well. In the Pinetree and other pulsed radars the antenna spent most of its time turned off, listening for the echos of the signal it sent out. The Doppler system proposed by Woonton separated the sending and receiving antennas, allowing it to broadcast continuously. Its "continuous wave" nature meant that more radio energy fell on the target in a unit of time, meaning that more energy was returned to the detector. Thus for any given detection range requirement, the Doppler radar can use considerably smaller broadcasters, dramatically reducing cost.

More importantly, the Doppler system examined the frequency shift of the signal for detection, as opposed to simply looking for any return at all. This allowed the broadcasting antennas to be aimed with the some of the signal actually hitting the ground, in which case the reflected return signal would not have any Doppler shift (the ground typically isn't moving) and could be filtered out. Any aircraft in its line of sight from the ground up to 65,000ft could be detected, as long as it was flying fast enough to shift the signal out of the filter's band. This was easy to arrange.

In 1952 several test systems were set up by the Canadian Defence Research Board (DRB), and proved the concept worked in theory.

In February 1953 the Canada-U.S. 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 United States to prepare independent briefs on the subject. By July 1953, RCAF Air Defence Command had completed its brief, followed shortly thereafter by its USAF counterpart. Both reports suggested building a Doppler fence further north, along the 55th parallel, roughly at the entrance of James Bay into Hudson Bay.

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 minimum operational requirements. By the end of November 1953, the Mid-Canada Line had been approved in principle. Unlike the jointly-operated Pinetree line and future DEW line, the Mid-Canada line would be funded and operated entirely by the RCAF. The DRB estimated that the system would cost about $69,700,000, while an independent RCAF report placed it at $85,000,000.

In December an effort started to try to understand what sort of problems would be encountered during construction. Several "trains" consisting of tractor-pulled sleighs set out cross-country. One, manned by the RCAF, set out eastward from Fort Nelson, BC in order to link up with a second moving west from Flin Flon, Manitoba, while a third crewed by the Army left Lake Nipigon near Thunder Bay, Ontario for Lansdown House about 200km further north. The missions proved that it was possible to build the new line, but only during the winter when the muskeg was frozen solid. These missions also inspired the U.S. Army to invest in purpose-built overland trains which they experimented with in the 1960s but never put into production.

While that was taking place, efforts were underway to start primary siting studies. It quickly became clear that the areas in question, at least in eastern Canada, were so remote that there was no really accurate topographical information. A huge effort to map the area in a 15-mile wide strip across the entire country was started by Transport Command almost immediately, and ended by the spring of 1954. With this information in hand a construction division was set up, the Systems Engineering Group (SEG), in February 1954, tasked with producing a final report on the system to be submitted on June 1.

Experiments continued on the radar systems and considerably longer range test radar sites were set up, one near Ottawa and another near Clinton,(in southwestern Ontario). The results from these tests were not completed until 1954, but showed that they could reliably detect aircraft at 90 km instead of 60 km, meaning the fence could consist of fewer stations. Nevertheless the price rose, now estimated at about $120,000,000. Although their final report was not yet ready, the SEG put in an interim report in June and it was approved by cabinet by the end of the month.

In their report they outlined the system that would be built almost exactly. It called for eight major Sector Control Centres, numbered from 200 to 900, each of which control up to thirty unmanned radar sites for a total of 90 radar stations. Each of the radar stations consisted of a single tall mast with a number small dishes in fixed positions on top (typically four, two pointed in either direction), with power and electronics located in a building below the mast.

The sector control centres were linked using an advanced microwave communications system developed in part by CADRE, which scattered off the troposphere for long-distance communications. The southernmost site along the eastern portion of the line, at Cape Henrietta Maria on Hudson Bay, was used as the main communications point, and three additional repeater stations transferred data from the line southward to air defense headquarters in North Bay, Ontario. The easternmost station at Hopedale, Labrador was co-located with an existing Pinetree Line station in order to save construction costs.

All aircraft transiting the line would have to file a flight plan through the Mid Identification Zone, or MIDIZ, centred on the fence. The plans also called for the construction of several airbases known as Line Clearance Aerodromes just to the north of the line, where interceptor aircraft could operate in times of heightened alert.

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. Detailed site selection started in 1955, with a major surveying effort running across Canada at the 55th parallel. 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.

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. Operations were shortly integrated into the newly-formed NORAD. Even the SEG's revised estimates turned out to be too low, and the fence's final cost is estimated at $224,566,830.

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 targets detected by the MCL, 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 Canada and the US. However as the Soviet Union moved their offensive capability 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.The operations site located at Cranberry Portage, Manitoba, for example, has been converted into a high school and residence since active operations at the site closed in the mid-1960's.

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