The Mid-Canada Line (MCL), also known as the McGill Fence, was a line of radar stations running east–west across the middle of Canada, used to provide early warning of a Soviet bomber attack on North America. It was built to supplement the Pinetree Line, which was located farther 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. As the MCL was closed down, the early warning role passed almost entirely to the newer and more capable DEW Line farther north.
The MCL was based on the bistatic radar principle, using separated transmitters and receivers. An aircraft flying anywhere between the stations would reflect some of the transmitted signal towards the receiver, where it would mix with the signal travelling directly from the transmitter. The mixing of the two signals produces a pattern that is very easy to detect using simple electronics. As the transmitter is not pulsed, it does not require high voltages and is very simple as well. This leads to a very low-cost system that can cover huge areas, at the cost of providing no information about the precise location of the target, only its presence.
Throughout its history, the MCL suffered from a problem that was never solved; because of the way bistatic radar works, any object relatively close to either station produces a large signal, in contrast to conventional monostatic (single site) radars where this effect is limited to the area immediately around the site. In the case of the MCL, this caused problems when flocks of birds would fly anywhere near either station and swamp the signal of a more distant aircraft. Solving this problem using the Doppler effect was a major design criterion for the AN/FPS-23 "Fluttar" that filled a similar role in the DEW line.
History
Impetus 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 pulsed radars that were fairly easy to jam and were unable to detect targets close to the ground due to "clutter." Although expensive in terms of fuel use, it would be possible for Soviet bombers to evade detection by flying lower and plotting a course between the stations.
Bennett Lewis, head of the AECL Chalk River Laboratories and former Chief Superintendent of the UK Telecommunications Research Establishment (TRE) had proposed to the Defence Research Board (DRB) a system that avoided both of these problems. Known today as a forward scatter bistatic radar, it used two antennas, a transmitter and receiver, separated by some distance. The antennas were positioned and aimed so that the signal from the transmitter filled the space above the line between the two stations. An aircraft flying into this region would reflect some signal back towards the receiver, allowing detection at altitudes as great as 65,000 ft.
A major advantage of the system is that it requires much less power to operate effectively. In a conventional radar, the radio signal has to travel to the target and back again. As each leg of the journey is subject to the inverse square law, the resulting radar equation contains a fourth-power dependence. In contrast, a forward-scatter radar signal always travels about the same total distance, from the transmitter to the receiver, modified only by the altitude of the target. This means it is dependent on the square root of range and not the fourth root, and thus delivers considerably more energy onto the receiver than a conventional radar over the same range. Also, unlike a conventional "monostatic" radar, the transmitter did not have to turn off to allow the receiver to listen for the signal. Since the total amount of energy received at the receiver is a function of both the peak power and the length of the pulse, using a continual signal means the same total energy will be deposited using much lower peak transmitter power. As a result, Lewis' system would require smaller sites and much less power than conventional radars like those of the Pinetree Line.
The major disadvantage of the system is that it did not indicate the aircraft's location within the beam, unlike a pulsed system where pulse timing can be used to determine range. This means the forward-scatter concept is useful for making a "radar fence" or "trip wire" that indicates that something is approaching, but not exactly where it is. To help address locating the target to a degree, the proposal was to build two interlinked fences, so that each pair of stations was perhaps apart, a short enough distance that the radar on an interceptor aircraft would be able to find the target within that area. Using two overlapping sets also allowed one pair to cover the dead zone directly above the towers of the other.
Lewis' initial concept was to place the transmitters and receivers on telephone poles and electric power transmission towers, which provided both a convenient location as well as the small amount of power needed to run the electronics. In the case of the telephone poles, the lines would also be used to send the data back to the tracking stations. This concept generated a considerable amount of interest, although it was abandoned for reasons that are not entirely clear. Willis and Griffiths speculate it might be the need for 1,000 such radars, but it is also likely that the desired to locate the line further north than the heavily settled areas in southern Canada was likely significant as well. In any event, the simplicity of the concept helped bring it to the attention of air planners.
Spider Web
The DRB decided to pursue Lewis’ idea in 1950–51 by directing a research contract to the Eaton Electronics Research Laboratories of McGill University, headed by Professor Garfield Woonton. Lewis suggested to DRB and Woonton that he put the project in the hands of associate professor, J. Rennie Whitehead as project leader, a former colleague of his from the TRE days in the UK who had recently taken a position at the Lab. Some preliminary tests were made in 1952 with breadboard hardware built by a graduate student, Hugh Hamilton, in order to confirm the validity of the idea.
In the meantime, RCA Victor had been brought in by the DRB to design and produce the receivers, transmitters and antennas for tests on a substantial scale. The testing was performed in the summer of 1953, when Whitehead and his team of RCA Victor and RCAF personnel installed and operated a string of seven stations stretching from Ottawa to Mattawa along the Ottawa River valley. Known under the code name of "Spider Web" at the suggestion of Hamilton, the tests were made with aircraft from CFB St. Hubert, near Montreal. All observations were transmitted to and made in the line HQ, which was set up in the equipment hut of one of the seven stations, located in Deep River.
Flight Lieutenant Andrew Matthews of the 104 Communications Flight at RCAF St. Hubert arranged for a series of different aircraft to fly through the network, including an Auster light aircraft, a T-33 Shooting Star, an Avro Lancaster bomber and even a recently acquired de Havilland Comet jet transport. The tests revealed a great deal about the spectral ‘signatures’ of aircraft crossing the line at different points, and demonstrated the capability to detect all sizes of aircraft from 100 ft to over 40,000 ft in altitude. During this time Dr. Ross Warren of RCA Victor and Dr. Whitehead jointly developed the theoretical background for the work in a major report to DRB.
The Spider Web trials were followed in 1954 by intensive tests on a single wide link, built in the Eastern Townships by Bell Canada, who had by this time been given the go-ahead for the implementation of the Mid-Canada Line. When Whitehead inquired why RCA had not been given the contract, a colleague replied "Who do you think runs Canada?" The trials on this prototype link were also conducted by Whitehead and a small team in collaboration with Air Defence Command, St. Hubert, this time on behalf of Bell. The trials involved the flyover of numbers of B-52 bombers by arrangement with Strategic Air Command and a local bombplot unit. They also had full-time use of an Avro Lancaster from CFB Greenwood for the important low-level tests.
Deployment studies
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 Defense Command had completed its brief, followed shortly thereafter by its USAF counterpart. Both reports suggested building a Doppler fence farther 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 Lansdowne House about 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.
Finalizing the plans
As the experiments continued it became clear that by using taller masts, tall, the radar stations could be located further apart, up to . This reduced the number of stations required. 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.