Origianlly Published in the Atlantic Quarterly.
Used with permission.
The most northerly continuously occupied site in Canada is the Canadian Forces Station Alert located in the north-east corner of Ellesmere Island at 82.5° N, 62.3° W. In the early days, communications with the south were provided by LF (low frequency) and HF (high frequency) radio systems. The maximum data rates were very limited, and ionospheric conditions often severely limited HF communications.
Due to its far northern location, Alert cannot communicate directly with any geostationary satellite, even one directly south. Fortunately, one of the Canadian High Arctic weather stations is located at Eureka (80.0° N, 85.8° W) on the west coast of Ellesmere Island. This location can 'see' a limited portion of the geostationary satellite arc. The elevation angle to a satellite located due south is approximately 1.3°, and is less to any geostationary satellite either east or west of Eureka. The satellite terminal must be located so that the path to the satellite is not blocked by obstacles, such as buildings or hills.
Even for clear paths, satellite communications at low elevation angles suffer from low-angle fading. Early work at the Communications Research Centre (CRC) showed that the frequency of occurrence and severity both increase with decreasing elevation angle. Measurements by CRC at Resolute (74.7° N, 95.0° W) showed that, although low angle fading occurs less often in the arctic than in the south, it can be as severe. In 1974, as part of the military program at CRC, a temporary terminal was located on one of the hills near Black Top Ridge east of the Eureka weather station. A link was established with CRC at Ottawa. The received signal strength was monitored at both the temporary northern terminal and at CRC for about three weeks. Subsequent data analyses showed that satellite communications with Eureka were completely feasible and that low-angle fading could be severe, at least during the sunny season three-week period of this experiment.
As part of other operations, one of the Telesat satellites was moved somewhat east. At its new location, satellite communications could be established to locations along the eastern half of the Eureka airstrip. Telesat submitted an unsolicited proposal to install two Anikom terminals along the eastern half of the Eureka airstrip and to operate these links for at least one year. The signal strengths at each terminal were recorded and analysed by CRC. This experiment in 1976 provided the first information on the seasonal dependence of low angle fading in the High Arctic and also showed unequivocally that low-angle fading could be well correlated even with a horizontal path separation of 500 m. The experimental results, in conjunction with some data from the radiosondes released twice daily from the weather station, also confirmed certain theoretical conjectures of the cause of low-angle fading.
From these theoretical ideas, it could be shown that the paths to the satellite needed to be vertically spaced to ensure that the low-angle fading at the two earth terminals would be uncorrelated. Subsequently, CRC established two experimental battery-powered terminals at Eureka, one at Skull Point and the other at Upper Paradise, which showed conclusively that the effects of low-angle fading could be almost completely overcome, and that very reliable satellite communications with Eureka could be achieved.
During this time, the problems associated with establishing a link between Eureka and Alert were also being investigated. Although long hops are unusual in the south, there should be no problem with them provided there is adequate clearance. Fortunately, Ellesmere Island has many high peaks that can be used as repeater locations for a link between Alert and Eureka. Alert is located near sea level and is surrounded by high terrain except toward the Arctic Ocean to the north and east. A range of mountains rising to about 1250 m lies about 40 km directly to the west of Alert.
A repeater located on one of these peaks (Grant) forms the first hop from Alert to Eureka. Since Grant is so high above the surrounding terrain, there is good path clearance to a second repeater site 121 km distant. An area of uplifted rock just east of the Eureka weather station and known as Black Top Ridge rises to about 900 m. A repeater located on the southern slope of this ridge links with the satellite terminal at Skull Point only 18 km distant. In the other direction, the Black Top repeater has a clear view to a site about 113 km distant on the north side of Greely Fiord. The path between this location at about 1050 m above the fiord and the repeater at Black Top has excellent clearance.
Therefore, if it could be demonstrated that there are no problems with radio propagation on long links in the High Arctic, it should be possible to link Alert and Eureka with five or six repeaters. The propagation characteristics were investigated by an experimental measurement program conducted by CRC. To measure the propagation characteristics on as long links as possible, this experiment used only five repeaters between Alert and Eureka. Early results showed that there are no unusual propagation characteristics associated with the long links. Shortly thereafter, the High Arctic Data Communications System (HADCS) project was approved. The experimental link was converted to provide six digital voice links and several signalling links between Alert and Eureka. A private automatic branch exchange (PABX) was installed at Eureka and integrated with the existing PABX at Alert. To users in Alert and Eureka, calls to the other location appeared to be internal calls. On an temporary basis, the Eureka PABX was linked by a short experimental link to Skull Point so that not only Eureka but also Alert could access the south by dialing '9' for an outside line. This experimental link proved to be useful during the installation of the original HADCS system.
Based on information available before the HADCS project was approved, it was assumed that the repeater sites would experience low temperatures, possibly below -50° C, and high winds. It was known that the sun would be continuously above the horizon from about the third week in April to about the end of August (the sunny season). The sun would not rise from about mid-October until about the end of February (the sunless season). It was also clear that all repeater locations would be accessible only by helicopter, and therefore accessible only during the sunny season. As a consequence, all equipment would need to be highly reliable and capable of operating under cold and windy conditions.
In preparation for designing the link between Alert and Eureka, the Defence Research Establishment Ottawa had investigated various power sources that could provide suitable power for the radio repeaters. The sources considered included thermal electric generation with various fuels, nuclear sources using a radioactive isotope, such as cobalt-60, and various primary and secondary battery chemistries. Several sources were found to be unsuitable for a variety of reasons, including poor low temperature performance, safety issues, and expense.
For several reasons, he most suitable source was also the most basic. These cells, type AD608, were originally manufactured by Cipel, which was subsequently acquired by SAFT. Although generally considered to be a primary cell, this source is actually a half fuel cell in which one fuel, zinc, is provided within the cell and the other fuel, oxygen, diffuses into the cell through a very porous central carbon rod. The current is limited by the oxygen diffusion rate which, at low temperatures, limits the current to about 200 mA. Since the end-of-life voltage is about 0.9 V, a battery requires 14 cells in series. Since the repeater draws about 2.0 A, ten battery boxes in parallel are required to supply the current. Since each cell is rated at about 2000 Ah, the capacity of ten batteries in parallel is about 20,000 Ah. The charge required by the radio for one year is about 17,500 Ah. Therefore, the AD608 cell provides a very comfortable 'fit' with the radio power requirements and site access restricted to once each year.
As expected, there are some serious operational disadvantages associated with these cells. Each cell must be sealed from the atmosphere until it is placed in service. To prepare a battery box for use, the old cells are removed and replaced with new cells. The cells are wired in series. The seal over the porous carbon rod is removed, as well as two plugs in the cell lid. Approximately 4.5 l of water is added to each cell. This dissolves the potassium hydroxide cake within the cell and the battery is now energized. Since it contains aqueous potassium hydroxide, the battery box must be slung by helicopter to the site. Of course, the batteries that are removed from each site also contain aqueous potassium hydroxide and must be slung from the site to facilities at either Alert or Eureka.
The disposition of spent cells is nontrivial. The highly-caustic electrolyte can be emptied from each cell and neutralized by the addition of a mild acid. Since batteries must be replaced according to the calendar, there will be some zinc remaining in each cell. Some of the early cells included mercury with the zinc to reduce the self-discharge rate. These cells present particular disposal problems.
To provide continuous power to the radio, the batteries are arranged in two banks, A and B, which are placed on either side of the platform at each site. One bank is removed and replaced each year and the other bank placed in service. This ensures that there is always a fresh battery bank at each site. It is clear from this description that the battery re-supply operation is costly and time consuming. The sites are accessible and helicopters available for only a few weeks each year. During this period, old batteries must be removed from each site. This often involves removing snow that has packed around the battery boxes during the winter. New batteries must be assembled and energized, flown to each site, placed in position, and connected. Since the battery boxes must be outside, any cable damage must also be repaired. Various aircraft, ranging from Chinook heavy lift helicopters to civilian Bell 205, 212, and 214 helicopters, and the military Griffon (412) helicopters, have all been used at various times.
Since it is 500 km from Alert to Eureka, a fuel cache must be established and maintained at a suitable midpoint, such as Tanquary Fiord. The helicopters must position themselves in the Arctic or be carried north and returned south by C130 Hercules aircraft. All in all, the battery re-supply operation requires considerable time, personnel and aircraft resources. It has been estimated that the annual re-supply operation costs exceed several million dollars, and this estimate does not include some costs that are 'buried' in other activities. The cost of the replacement cells pales in comparison. Since the self-discharge rate of the AD608 cells is low, it becomes attractive to consider using solar power during the sunny season. In principle, this eliminates the battery re-supply operation every other year.
Diversitel designed, built, and installed a power supply system at each repeater site to supply power to the radio whenever solar power is available and the lead-acid battery has been discharged by less than 15%. At this discharge level, the lead-acid battery electrolyte will not freeze even if the temperature drops to -50°. The system remembers the enabled primary battery bank so that, when solar power is no longer available in the fall, the previously-enabled primary battery bank is reconnected to the repeater radio. During the sunless season, the enabled primary bank battery may become exhausted and its voltage drop below a switching threshold. The other primary battery bank is enabled and supplies power to the repeater radio. Thus, the power system uses solar power whenever it is available, and extracts as much energy from each primary battery as practicable.
While eliminating the battery re-supply operation during alternate years is attractive, it would be highly desirable to eliminate it altogether. In 1991, while installing the semi-annual system described above, Diversitel conceived a system that could provide solar power to the radio during both the sunny and the sunless seasons. The idea is based on the fact that the latent heat of fusion of fresh water is large and that lead-acid batteries operate quite satisfactorily at 0°. Water is enclosed in containers that are designed so that they do not burst when the water expands on freezing. An insulated enclosure houses the lead-acid batteries and the water. During the sunless season, heat flows from the liquid water through the enclosure walls to the environment and the water freezes. If the enclosure is sufficiently insulated, and there is sufficient water, some liquid water will remain when the sun returns in March. The battery is recharged and any excess energy is used to melt the ice. Before the end of the sunny season, the water is all liquid, the battery is fully recharged, and the system is ready to enter the sunless season.
Calculations showed that it should be possible to build an insulated enclosure to house sufficient water and lead-acid batteries that power could be supplied continuously to a load. Diversitel designed and built an experimental system, and installed the system at CFS Alert in August 1992. After one year's operation, the data showed that the concept was completely feasible.
In subsequent years, a program to remote much of the operations at CFS Alert was undertaken. This included replacing the radio and the power systems at the repeater locations. A Request for Proposals was issued in October 2001 for a new power system that could supply up to 2.0 A continuously to the new radio. Among others, Diversitel responded to this request and was awarded a contract to design, build, and install a new power system at the six repeater sites.
In the Diversitel system, lead-acid batteries are contained within a highly-insulated enclosure. The batteries are charged during the sunny season by current from eight 120 W photo-voltaic panels arranged in an octagon. For most of the sunny season, the sun is above the horizon throughout the entire 24-hour day and all panels contribute current to the battery and load. Since the foreground is covered with snow until mid-June, there is appreciable energy backscattered to the panels that are not directly illuminated by the sun. Since there is no advantage in tilting the panels, they are mounted vertically. In comparison with the conventional arrangement in which all panels face south, the octagonal arrangement as seen from above substantially reduces the wind resistance and the peak photo-voltaic current, with almost no reduction in the solar energy received by the panels each day.
At each site, the enclosure rests on a steel frame that is supported by four insulated pads. At three sites, these pads are supported by well-drained rock. At the other three sites, the pads are supported by soil that can thaw each year. The internal pad design minimizes heat conduction into the ground while the white fibreglass-polyester coating reflects solar energy. The pressure on the ground is about 15 kPa.
The battery charging and temperature are controlled by a highly-reliable dual microcontroller. Excess photo-voltaic energy is dissipated by selectable shunting resistors. The current from each photo-voltaic panel and each individual battery current are measured. The dual-controller also calculates the battery current and state of charge. All data, including load current, load voltage, ambient temperature, etc., are returned on request to the repeater radio telemetry system for display by the Network Monitoring System.
During the sunless season, power is continuously supplied from the lead-acid battery to the radio. The batteries are normally discharged by less than 50% of their nominal capacity by the time the sun returns in March. During most of the sunless season, the ambient temperature is lower than the battery temperature and heat energy flows from the batteries to the environment through the highly-insulated enclosure walls. Since the thermal conduction is quite low, and the battery thermal mass large, the battery temperature falls only slowly throughout the sunless season. With the sun's return in March, the batteries are recharged. With time, the ambient temperature rises above the battery temperature, and heat energy flows from the environment into the battery through the highly-insulated enclosure walls.
With the Diversitel power supply system, nothing other than solar energy is consumed and nothing is emitted. There are no moving parts that might fail. The lifetime of the system is expected to exceed 20 years. Throughout this time, no supplies are required, there are no waste products, and the battery re-supply operation is completely eliminated. Relative to the cost of the previous battery re-supply operation, the payback period for the Diversitel power supply system is less than one year.
Credits and References:
1) Dr. John Strickland, Diversitel Communications <jstrickland(at)diversitel.ca>