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I have a rough list of topics for future articles, a scratchpad of two-word ideas that I sometimes struggle to interpret. Some items have been on that list for years now. Sometimes, ideas languish because I'm not really interested in them enough to devote the time. Others have the opposite problem: chapters of communications history with which I'm so fascinated that I can't decide where to start and end. They seem almost too big to take on. One of these stories starts in another vast frontier: northeastern Canada.
It was a time, rather unlike our own, of relative unity between Canada and the United States. Both countries had spent the later part of World War II planning around the possibility of an Axis attack on North America, and a ragtag set of radar stations had been built to detect inbound bombers. The US had built a series of stations along the border, and the Canadians had built a few north of Ontario and Quebec to extend coverage north of those population centers. Then the war ended and, as with so many WWII projects, construction stopped. Just a few years later, the USSR demonstrated a nuclear weapon and the Cold War was on. As with so many WWII projects, freshly anxious planners declared the post-war over and blew the dust off of North American air defense plans. In 1950, US and Canadian defense leaders developed a new plan to consolidate and improve the scattershot radar early warning plan.
This agreement would become the Pinetree Line, the first of three trans-Canadian radar fences jointly constructed and operated by the two nations. For the duration of the Cold War, and even to the present day, these radar installations formed the backbone of North American early warning and the locus of extensive military cooperation. The joint defense agreement between the US and Canada, solidified by the Manhattan Project's dependence on Canadian nuclear industry, grew into the 1968 establishment of the North American Air Defense Command (NORAD) as a binational joint military organization.
This joint effort had to rise to many challenges. Radar had earned its place as a revolutionary military technology during the Second World War, but despite the many radar systems that had been fielded, engineer's theoretical understanding of radar and RF propagation were pretty weak. I have written here before about over-the-horizon radar, the pursuit of which significantly improved our scientific understanding of radio propagation in the atmosphere... often by experiment, rather than model. A similar progression in RF physics would also benefit radar early warning in another way: communications.
One of the bigger problems with the Pinetree Line plan was the remote location of the stations. You might find that surprising; the later Mid-Canada and DEW lines were much further north and more remote. The Pinetree Line already involved stations in the far reaches of the maritime provinces, though, and to provide suitable warning to Quebec and the Great Lakes region stations were built well north of the population centers. Construction and operations would rely on aviation, but an important part of an early warning system is the ability to deliver the warning. Besides, ground-controlled interception had become the main doctrine in air defense, and it required not just an alert but real-time updates from radar stations for the most effective response. Each site on the Pinetree Line would require a reliable real-time communications capability, and as the sites were built in the 1950s, some were a very long distance from telephone lines.
Canada had only gained a transcontinental telephone line in 1932, seventeen years behind the United States (which by then had three different transcontinental routes and a fourth in progress), a delay owing mostly to the formidable obstacle of the Canadian Rockies. The leaders in Canadian long-distance communications were Bell Canada and the two railways (Canadian Pacific and Canadian National), and in many cases contracts had been let to these companies to extend telephone service to radar stations. The service was very expensive, though, and the construction of telephone cables in the maritimes was effectively ruled out due to the huge distances involved and uncertainty around the technical feasibility of underwater cables to Newfoundland due to the difficult conditions and extreme tides in the Gulf of St. Lawrence.
The RCAF had faced a similar problem when constructing its piecemeal radar stations in Ontario and Quebec in the 1940s, and had addressed them by applying the nascent technology of point-to-point microwave relays. This system, called ADCOM, was built and owned by RCAF to stretch 1,400 miles between a series of radar stations and other military installations. It worked, but the construction project had run far over budget (and major upgrades performed soon after blew the budget even further), and the Canadian telecom industry had vocally opposed it on the principle that purpose-built military communications systems took government investment away from public telephone infrastructure that could also serve non-military needs.
These pros and cons of ADCOM must have weighed on Pinetree Line planners when they chose to build a system directly based on ADCOM, but to contract its construction and operation to Bell Canada [1]. This was, it turned out, the sort of compromise that made no one happy: the Canadian military's communications research establishment was reluctant to cede its technology to Bell Canada, while Bell Canada objected to deploying the military's system rather than one of the commercial technologies then in use across the Bell System.
The distinct lack of enthusiasm on the part of both parties involved was a bad omen for the future of this Pinetree Line communications system, but as it would happen, the whole plan was overcome by events. One of the great struggles of large communications projects in that era, and even today, is the rapid rate of technological progress. One of ADCOM's faults was that the immense progress Bell Labs and Western Electric made in microwave equipment during the late '40s meant that it was obsolete as soon as it went into service. This mistake would not be repeated, as ADCOM's maritimes successor was obsoleted before it even broke ground. A promising new radio technology offered a much lower cost solution to these long, remote spans.
At the onset of the Second World War, the accepted theory of radio propagation held that HF signals could pass the horizon via ground wave propagation, curving to follow the surface of the Earth, while VHF and UHF signals could not. This meant that the higher-frequency bands, where wideband signals were feasible, were limited to line-of-sight or at least near-line-of-sight links... not more than 50 miles with ideal terrain, often less. We can forgive the misconception, because this still holds true today, as a rule of thumb. The catch is in the exceptions, the nuances, that during the war were already becoming a headache to RF engineers.
First, military radar operators observed mysterious contacts well beyond the theoretical line-of-sight range of their VHF radar sets. These might have been dismissed as faults in the equipment (or the operator), but reports stacked up as more long-range radar systems were fielded. After the war, relaxed restrictions and a booming economy allowed radio to proliferate. UHF television stations, separated by hundreds of miles, unexpectedly interfered with each other. AT&T, well into deployment of a transcontinental microwave network, had to adjust its frequency planning after it was found that microwave stations sometimes received interfering signals from other stations in the chain... stations well over the horizon.
This was the accidental discovery of tropospheric scattering.
The Earth's atmosphere is divided into five layers. We live in the troposhere, the lowest and thinnest of the layers, above which lies the stratosphere. Roughly speaking, the difference between these layers is that the troposphere becomes colder with height (due to increasing distance from the warm surface), while the stratosphere becomes warmer with height (due to decreasing shielding from the sun) [2]. In between is a local minimum of temperature, called the tropopause.
The density gradients around the tropopause create a mirror effect, like the reflections you see when looking at an air-water boundary. The extensive turbulence and, well, weather present in the troposhere also refract signals on their way up and down, making the true course of radio signals reflecting off of the tropopause difficult to predict or analyze. Because of this turbulence, the effect has come to be known as scattering: radio signals sent upwards, towards the troposphere, will be scattered back downwards across a wide area. This effect is noticeable only at high frequencies, so it remained unknown until the widespread use of UHF and microwave, and was still only partially understood in the early 1950s.
The locii of radar technology at the time were Bell Laboratories and the MIT Lincoln Laboratory, and they both studied this effect for possible applications. Presaging one of the repeated problems of early warning radar systems, by the time Pinetree Line construction began in 1951 the Lincoln Laboratory was already writing proposals for systems that would obsolete it. In fact, construction would begin on both of the Pinetree Line's northern replacements before the Pinetree Line itself was completed. Between rapid technological development and military planners in a sort of panic mode, the early 1950s were a very chaotic time. Underscoring the ever-changing nature of early warning was the timeline of Pinetree Line communications: as the Pinetree Line microwave network was in planning, the Lincoln Laboratory was experimenting with troposcatter communications. By the time the first stations in Newfoundland completed construction, Bell Laboratories had developed an experimental troposcatter communications system.
This new means of long-range communications would not be ready in time for the first Pinetree Line stations, so parts of the original ADCOM-based microwave network would have to be built. Still, troposcatter promised to complete the rest of the network at significantly reduced cost. The US Air Force, wary of ADCOM's high costs and more detached from Canadian military politics, aggressively lobbied for the adoption of troposcatter communications for the longest and most challenging Pinetree Line links.
Bell Laboratories, long a close collaborator with the Air Force, was well aware of troposcatter's potential for early warning radar. Bell Canada and Bell Laboratories agreed to evaluate the system under field conditions, and in 1952 experimental sites were installed in Newfoundland. These tests found reliable performance over 150 miles, far longer than achievable by microwave and---rather conveniently---about the distance between Pinetree Line radar stations. These results suggested that the Pinetree Line could go without an expensive communications network in the traditional sense, instead using troposcatter to link the radar stations directly to each other.
Consider a comparison laid out by the Air Force: one of the most complex communications requirements for the Pinetree Line was a string of stations running not east-west like the "main" line, but north-south from St. John's, Newfoundland to Frobisher Bay, Nunavut. These stations were critical for detection of Soviet bombers approaching over the pole from the northeast, otherwise a difficult gap in radar coverage until the introduction of radar sites in Greenland. But the stations covered a span of over 1,000 miles, most of it in formidably rugged and remote arctic coastal terrain. The proposed microwave system would require 50 relay stations, almost all of which would be completely new construction. Each relay's construction would have to be preceded by the construction of a harbor or airfield for access, and then establishment of a power plant, to say nothing of the ongoing logistics of transporting fuel and personnel for maintenance. The proposed troposcatter system, on the other hand, required only ten relays. All ten would be colocated with radar stations, and could share infrastructure and logistical considerations.
Despite the clear advantages of troposcatter and its selection by the USAF, the Canadian establishment remained skeptical. One cannot entirely blame them, considering that troposcatter communications had only just been demonstrated in the last year. Still, the USAF was footing most of the bill for the overall system (and paying entirely for the communications aspect, depending on how you break down the accounting) and had considerable sway. In 1954, well into construction of the radar stations (several had already been commissioned), the Bell Canada contract for communications was amended to add troposcatter relay in addition to the original microwave scheme. Despite the weaselly contracting, the writing was on the wall and progress on microwave relay stations almost stopped. By the latter part of 1954, the microwave network was abandoned entirely. Bell Canada moved at incredible speed to complete the world's first troposcatter long-distance route, code named Pole Vault.
One of the major downsides of troposcatter communications is its inefficiency. Only a very small portion of the RF energy reaching the tropopause is reflected, and of that, only a small portion is reflected in the right direction. Path loss from transmitter to receiver for long links is over -200 dB, compared to say -130 dB for a microwave link. That difference looks smaller than it is; dB is a logarithmic comparison and the decrease from -130 dB to -200 dB is a factor of ten million.
The solution is to go big. Pole Vault's antennas were manufactured as a rush order by D. S. Kennedy Co. of Massachusetts. 36 were required, generally four per site for transmit and receive in each direction. Each antenna was a 60' aluminum parabolic dish held up on edge by truss legs. Because of the extreme weather at the coastal radar sites, the antennas were specified to operate in a 120 knot wind---or a 100 knot wind with an inch of ice buildup. These were operating requirements, so the antenna had not only to survive these winds, but to keep flexing and movements small enough to not adversely impact performance. The design of the antennas was not trivial; even after analysis by both Kennedy Co. and Bell Canada, after installation some of the rear struts supporting the antennas buckled. All high-wind locations received redesigned struts.
To drive the antennas, Radio Engineering Laboratories of Long Island furnished radio sets with 10 kW of transmit power. Both D. S. Kennedy and Radio Engineering Laboratories were established companies, especially for military systems, but were still small compared to Bell System juggernauts like Western Electric and Northern Electric. They had built the equipment for the experimental sites, though, and the timeline for construction of Pole Vault was so short that planners did not feel there was time to contract larger manufacturers. This turn of events made Kennedy Co. and REL the leading experts in troposcatter equipment, which became their key business in the following decade.
The target of the contract, signed in January of 1954, was to have Pole Vault operational by the end of that same year. Winter conditions, and indeed spring and fall conditions, are not conducive to construction on the arctic coast. All of the equipment for Pole Vault had to be manufactured in the first half of the year, and as weather improved and ice cleared in the mid-summer, everything was shipped north and installation work began. Both militaries had turned down involvement in the complex and time-consuming logistics of the project, so Bell Canada chartered ships and aircraft and managed an incredibly complex schedule. To deliver equipment to sites as early as possible, icebreaker CCGS D'Iberville was chartered. C-119 and DC3 aircraft served alongside numerous small boats and airplanes.
All told, it took about seven months to manufacture and deliver equipment to the Pole Vault sites, and six months to complete construction. Construction workers, representing four or five different contractors at each site and reaching about 120 workers to a site during peak activity, had to live in construction camps that could still be located miles from the station. Grounded ships, fires, frostbite, and of course poor morale lead to complications and delays. At one site, Saglek, project engineers recorded a full 24-hour day with winds continuously above 75 miles per hour, and then weeks later, a gust of 135 mph was observed. Repairs had to be made to the antennas and buildings before they were even completed.
In a remarkable feat of engineering and construction, the Pole Vault system was completed and commissioned more or less on schedule: amended into the contract in January of 1954, commissioning tests of the six initial stations were successfully completed February of 1955. Four additional stations were built to complete the chain, and Pole Vault was declared fully operational December of 1956 at a cost of $24.6 million (about $290 million today).
Pole Vault operated at various frequencies between 650 and 800 MHz, the wide range allowing for minimal frequency reuse---interference was fairly severe, since each station's signal scattered and could be received by stations further down the line in ideal (or as the case may be, less than ideal) conditions. Frequency division multiplexing equipment, produced by Northern Electric (Nortel) based on microwave carrier systems, offered up to 36 analog voice circuits. The carrier systems were modular, and some links initially supported only 12 circuits, while later operational requirements lead to an upgrade to 70 circuits.
Over the following decades, the North Atlantic remained a critical challenge for North American air defense. It also became the primary communications barrier between the US and Canada and European NATO allies. Because Pole Vault provided connections across such a difficult span, several later military communications systems relied on Pole Vault as a backhaul connection.
An inventory of the Saglek site, typical of the system, gives an idea of the scope of each of the nine primary stations. This is taken from "Special Contract," a history by former Bell Canada engineer A. G. Lester:
(1) Four parabolic antennas, 60 feet in diameter, each mounted on seven mass concrete footings. (2) An equipment building 62 by 32 feet to house electronic equipment, plus a small (10 by 10 feet) diversity building. (3) A diesel building 54 by 36 feet, to house three 125 KVA (kilovolt amperes) diesel driven generators. (4) Two 2500 gallon fuel storage tanks. (5) Raceways to carry waveguide and cables. (6) Enclosed corridors interconnecting buildings, total length in this case 520 feet.
Since the Pole Vault stations were colocated with radar facilities, barracks and other support facilities for the crews were already provided for. Of course, you can imagine that the overall construction effort at each site was much larger, including the radar systems as well as cantonment for personnel.
Pole Vault would become a key communications system in the maritime provinces, remaining in service until 1975. Its reliable performance in such a challenging environment was a powerful proof of concept for troposcatter, a communications technique first imagined only a handful of years earlier. Even as Pole Vault reached its full operating capability in late 1956, other troposcatter systems were under construction. Much the same, and not unrelated, other radar early warning systems were under construction as well.
The Pinetree Line, for all of its historical interest and its many firsts, ended as a footnote in the history of North American air defense. More sophisticated radar fences were already under design by the time Pinetree Line construction started, leaving some Pinetree stations to operate for just four years. It is a testament to Pole Vault that it outlived much of the radar system it was designed to support, becoming an integral part of not one, or even two, but at least three later radar early warning programs. Moreover, Pole Vault became a template for troposcatter systems elsewhere in Canada, in Europe, and in the United States. But we'll have to talk about those later.
[1] Alexander Graham Bell was Scottish-Canadian-American, and lived for some time in rural Ontario and later Montreal. As a result, Bell Canada is barely younger than its counterpart in the United States and the early history of the two is more one of parallel development than the establishment of a foreign subsidiary. Bell's personal habit of traveling back and forth between Montreal and Boston makes the early interplay of the two companies a bit confusing. In 1955, the TAT-1 telephone cable would conquer the Atlantic ocean to link the US to Scotland via Canada, incidentally making a charming gesture to Bell's personal journey.
[2] If you have studied weather a bit, you might recognize these as positive and negative lapse rates. The positive lapse rate in the troposphere is a major driver in the various phenomenon we call "weather," and the tropopause forms a natural boundary that keeps most weather within the troposphere. Commercial airliners fly in the lower part of the stratosphere, putting them above most (but not all) weather.
