Meteor Burst Communications
The Janet System
Code-named Janet, after the Roman god of the doorway, Janus,
who looked both ways at once.
From the beginning, it was clear that if meteor trails were to be used efficiently for communication, it would be necessary to operate in bursts, taking advantage of the relatively large signals when they were present and shutting down the system when no suitable trail existed.
The burst type of operation required that some means be devised to provide for detection of meteor trails and for the subsequent selection of those trails which were suitable for the transmission of information. In order to accomplish the detection and selection functions simultaneously, it was visualized that the system would incorporate both a transmitter and a receiver at each station, with the transmitters radiating carrier continuously. The reception of radio energy from the remote transmitter would indicate the presence of a suitable trail and information would then be transmitted at high speed to the remote station during the lifetime of the trail. The actual control of the system would be performed by "gating" units which would permit the transmission of information only when the received signal level was greater than some predetermined value. With the addition of the necessary information storage facilities at each end, this mode of operation would provide, inherently, a two-way communication system.
When a meteoric particle enters the region of the ionosphere in the height range of 80-120 kilometres, it becomes heated as a result of many collisions with air molecules. Atoms evaporate from its surface with thermal velocities and undergo further collisions with the air molecules. Such collisions result in the release of heat to the surroundings and may cause the excitation or ionization of the atom. Excited atoms may emit light of sufficient intensity to make the meteor perceptible to an observer on the ground. The ionization produces a trail of free electrons in the wake of the meteor, and it is this trail which is detected by radio methods. While only two or three meteors are visible in any given hour, hundreds of trails can be detected in the same period, using sensitive radio equipment. The electrons partially scatter any radio waves incident on a trail, and it is this phenomenon which is utilized in the Janet system.
Apparently D.W.R. McKinley and other discussed the idea of using meteor signals for communication purposes as early as 1950. Previously, various groups at the Radio Physics Laboratory had been interested in scattering from ionospheric irregularities but the experimental work had been confined to back-scatter measurements. In 1952, largely because of the stimulus provided by the Cedar Rapids-Sterling experiment of the National Bureau of Standards, the emphasis was shifted to forward-scatter measurements. These measurements led to a detailed investigation of the forward scattering of radio signals from meteor trails and a study of the utility of these signals for communication purposes. By 1954, communication via VHF signals reflected from individual meteor trails had been achieved and development of equipment for this purpose was under way. The initial studies of forward scattering from meteor trails at the RPL leaned heavily upon similar investigations which were already in progress elsewhere.
DRTE's objectives were:
- To assess the utility of meteor signals for communication purposes.
- To demonstrate that meteor signals could be used for carrying coded information.
- To improve the reliability of long distance communication over the HF/VHF frequencies.
Work began at RPL in the Fall of 1952. JANET was headed by Dr. P.A. Forsyth. A number of other individuals were instrumental, including E.L. Vogan, D.R. Hansen, C.O. Hines, L.L. Campbell, D.W.L. Davis, S.J. Gladys, G.R. Lang, L.M. Luke and M.K. Taylor.
In order to establish the feasibility of the Janet system, three phases of experimentation were contemplated. To begin with, crude measurements of strength and duration were made on the signals received from transmitters at distances of 900 and 1200 kilometres. Made in 1952, these measurements indicated that a sufficient number of meteor signals were observed with a transmitter of modest power to support teletype communication at moderate rates. Secondly, in June 1953, modulated signals were transmitted simultaneously in both directions over a path extending from Ottawa to Port Arthur, a distance of 1050 kilometres. The frequencies used were near 50 mc and were separated by about one mc. This occasion marked the first successful automatic operation of a two-way circuit, in which the modulation of the transmissions was initiated by the occurrence of a suitable meteor trail. Finally, a rudimentary teletype system was set up in which the occurrence of a suitable meteor trail caused teletype signals originating at Ottawa to be transmitted at high speed to Halifax and then retransmitted back to Ottawa where they were decoded and printed for comparison with the original. The system operated successfully in March 1954.
The feasibility of this method was established and a contract was placed with Ferranti Electric Limited (Canada) for the development of equipment suitable for use in a communication system of the Janet type.
The development program moved slowly because of the statistical nature of the meteor phenomenon. At each stage, it was necessary to conduct long tests in order to obtain statistically reliable results. More detailed knowledge about meteors was a major requirement for pursuing the project.
A considerable amount of time was spent on improving the design of the system. Probably the most important consideration in the design of a Janet system is that of gating - the operation of turning the system on and off as suitable meteor trails occur. The problem was one of choosing the criteria to be used in determining when transmission was to be started and stopped.
The average character error rate was 1.5 percent. It was noted at the time that most of the errors occurred toward the end of individual transmissions. As a result, refinements to the gate-off procedure meant an error rate of somewhat less than 0.1 percent. A unique feature of the Janet system was related to the inherent directivity of the meteor scattering process. This ensures that the signal reflected from a particular trail may be received within only a limited area on the ground. At distances of hundreds of kilometres, only a negligible fraction of the total transmitted information is available. The Janet system lent limited privacy to a mode of communication not known for this characteristic.
It may seem that the Janet technique was somewhat wasteful of spectrum space because of the relatively high instantaneous signalling rates which are required to achieve modest average information rates. This defect was offset by the fact that Janet systems, using the same frequency assignments, can be located much closer to each other than can other systems operating over comparable distances.
The Janet system had disadvantages in its complexity of storage and signalling equipment, as well as discontinuous communication with its delays and limited range (2000 kilometres). These were offset by the fact that modest information capacity is obtainable with modest transmitter power and simple antenna systems. If the Janet system had gained wide acceptance, it could have nearly doubled the amount of frequency spectrum space available for long distance communication. However, there were also technical problems that would have made that difficult to achieve.
Much of the initial enthusiasm over Janet was due to its novelty. While the feasibility of reliable communication by this method were demonstrated, it did not compete well with more established techniques. Over the years, there have been resurgences of interest in the Janet system. The Janet system was designed to fill the requirement in Canada for a relatively low power, reliable, point-to-point communication system for use over ranges of 500 to 1500 kilometres.
This project was reported in local newspapers, once it was taken off the secret list of Defence Research.
Forsyth, P.A., Vogan, E.L., Hansen, D.R., and Hines, C.O. "A Meteor-Burst Communication System." Proceedings of the Institute of Radio Engineers. Vol 45, No 12, December 1957.
Campbell, L.L., and Hines, C.O. "Bandwidth Considerations in a Janet System." Proceedings of the Institute of Radio Engineers. Vol 45, No 12, December 1957.
Campbell, L.L. "Storage Capacity in Burst-Type Communication Systems." Proceedings of the Institute of Radio Engineers. Vol 45, No 12, December 1957.
Davis, G.W.L., Gladys, S.J., Lang, G.R., Luke, L.M., and Taylor, M.K. "The Canadian Janet System." Proceedings of the Institute of Radio Engineers. Vol 45, No 12, December 1957.
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Last updated on August 29, 1997 by Cynthia Boyko
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