McGill Fence
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 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. As the MCL was closed down, the early warning role passed almost entirely to the more capable DEW Line further 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 resulting change in signal is very easy to detect using simple electronics, and the transmitter is not pulsed so 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.
Throughout its history, the MCL suffered from a problem that was never solved; because of the way bistatic radar works, any object near the transmitter or receiver makes a large signal, in contrast to conventional monostatic (single site) radars where this effect is limited to the area around the site. In the case of 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.
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 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 and plotting a course between the stations.
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