In the middle 1950s, the United States and Canada became increasingly concerned about transpolar attack from the Soviet Union by bombers, and later by ballistic missiles. After an MIT study demonstrated the vulnerability of the two countries from such an attack, an agreement was reached to build a massive defense system, part of which would become the Distant Early Warning Line, or DEW Line. Construction of the DEW Line was authorized in 1954, and the actual construction was accomplished from 1955 through 1957. The DEW Line consisted of a chain of radar installations across the far arctic north of Canada, stretching 3000 miles from Alaska to Baffin Island. During construction of the line, and even after it was completed, there were always gaps in the coverage. Those gaps were covered in part by airborne assets: specifically, Lockheed WV-2 Warning Star aircraft of the US Navy, and similar EC-121s of the USAF.
Late in 1954, the Canadian government under Prime Minister Louis St. Laurent ordered the RCAF to provide additional assets to cover gaps in the DEW Line. To augment and complement the US aircraft, the RCAF needed an aircraft that was not limited to the use of established bases with well-groomed concrete runways, but rather, one that could be based on a temporary basis near the various DEW Line facilities to plug gaps caused by outages or periodic maintenance. The aircraft chosen was the rugged and reliable Bristol Freighter that was already in RCAF service in Europe. Significant modification for the role was necessary, however.
First, the Freighter had to be fitted with appropriate radar gear. This involved the fitment of AN/APS-20 search radar and AN/APS-45 height finding radar. The antenna for the AN/APS-20 could not be fitted under the fuselage as it had been for the Lockheed aircraft, but instead was installed as a replacement for the Freighter’s clamshell nose doors, changing the shape and size of the Freighter’s nose (and making it even more bulbous). The AN/APS-45 antenna was installed in a dorsal hump, as in the Lockheeds, although smaller in size (with somewhat diminished capability).
The cargo loading door of the Freighter was replaced by a smaller personnel door, and the interior was fitted with operator stations, removing the fuselage windows.
In order to increase the loiter time for the new variant, fuel load was increased by the addition of a fuselage fuel tank, and additional tanks in the outer wings (not present in any other Freighter variant). Because of the additional weight and drag of these additions, takeoff performance of the aircraft had to be augmented by the addition of a pair of underwing boost engines, in the form of two General Electric J-34 turbojets which were already being considered for the RCAF's Lockheed Neptune fleet. This in turn caused the need for even more fuel, which was met by the addition of two underwing external fuel tanks, dedicated to carry JP-4 for the turbojets.
Because the new aircraft was to operate from austere strips in the arctic, it would be equipped with skis on its fixed landing gear.
The aircraft entered service in September, 1957, and, since it was no longer a “Freighter” was given the new service name of Bristol Watchman. Seven Freighters were converted to Watchmen at the Bristol facilities in Winnipeg, and were homebased at RCAF Station Frobisher Bay on Baffin Island, though they spent most of their lives spread out along the DEW Line where they gave yeoman service for about 5 years.
