THE IMPLICATIONS OF SATELLITE TECHNOLOGY FOR TELEVISION BROADCASTING IN CANADA
B.C. Blevis and M.L. Card
Department of Communications
This paper was presented at the AIAA 4th Communications Satellite Systems Conference, Washington, D.C./April 24-26, 1972.
Paper No. 72-553
This paper discusses, from a technological point of view, the particular advantages of using satellites to provide television broadcasting service to the remote areas of Canada. It addresses the specific problems of extending broadcasting cover
age by terrestrial means that arise because of the unique Canadian demography.
The paper also considers problems of more general concern, including the impact of the frequency allocations made by the W.A.R.C. on the future development of operational satellite broadcasting systems for individual and community reception. The possible use of multi-purpose satellites to provide additional services of a complementary nature is discussed briefly.
To explore the use of satellites for the extension of broadcasting and other services to remote regions, Canada has signed an agreement with N.A.S.A. to carry out a joint project to launch a Communications Technology Satellite. This satellite incorporates an SHF transponder with a high power TWT which will make available an early capability for television broadcasting to community receivers for experimental purposes.
The major federal legislation governing broadcasting in Canada today is the 1968 Broadcasting Act which defines broadcasting as "any radio-communication in which the transmissions are intended for direct reception by the public".
The 1968 Act goes on to state that: "Broadcasting activities in Canada make use of radio frequencies that are public property and Such undertakings constitute a single system (called) the Canadian Broadcasting System. (This) system should be effectively owned and controlled by Canadians so as to safeguard, enrich and strengthen the cultural, political, social and economic fabric of Canada. All Canadians are entitled to a broadcasting service in English and French (which) should contribute to the development of Canadian unity and provide for a continuing expression of Canadian identity. Facilities should be provided ..... for educational broadcasting".
In its continuing efforts to achieve these broad and challenging objectives, the Canadian television broadcasting service, which began in 1952, includes (as of January 1971) 350 stations comprising originating stations, repeaters, low power relay transmitters, and frontier coverage television stations, operated by the Canadian Broadcasting Corporation (CBC), which broadcast a four-hour video-taped program package in remote communities. At least 90% of the Canadian population have access, in one way or another, to television broadcasting service, although there are serious deficiencies in this coverage.
In addition, in January 1971, over 300 CATV systems were operating in Canada. Several of these were much larger than any in the U.S.A. The overall market penetration of cable systems is considerably greater in Canada.
However, as shown in Fig. 1, the area coverage of Canadian territory by television broadcasting is far from complete.
The Canadian system is well developed in the south, where most Canadians and their major cities are located, but service to remote areas, particularly the territories, and the northern parts of the provinces, suffers from deficiencies such as sparse geographical coverage and lack of local and live programs. Thus over 99% of the population of Ontario have access to television in some form. In Newfoundland about 85% have television service, and in the Northwest Territories only about 40% have coverage.
In the speech which opened the current session of the Canadian Parliament, on February 17, 1972, it was noted that some one million Canadians in about 260 communities are now without service in their own language. The Canadian government has therefore authorized the CBC to extend coverage to areas not yet served.
When Telesat Canada commences operations early next year with the first Canadian domestic satellite, called ANIK (an Eskimo word for brother), service will be improved immediately through the system's ability to provide live programs to certain remote communities where ground terminals are being installed. Fig. 2 shows the Telesat system for television as now planned. The system includes 25 remote TV installations which will be capable of receiving programs and broadcasting them locally. Programs will be distributed initially from the 3 up-link stations to be located near Vancouver, Toronto and Montreal (1).
ANIK operates in the 4 and 6 GHz bands allocated to the Fixed-Satellite Service, and therefore is subject to the sharing constraints, and power flux density limits imposed by International agreement at these frequencies. ANIK will perform the television network distribution role for which it was designed, but there are economic limits on the ultimate coverage because of the cost of additional terrestrial stations.
The Yukon and the N.W.T. together cover some 1.5 million square miles, but their population is only about 50,000. Many of the 260 communities mentioned previously cannot, over the next few years, be economically reached either by terrestrial microwave or by satellite distribution systems. Therefore, even after maximum extension of the conventional terrestrial broadcasting networks has been achieved using the facilities provided by ANIK at 4 and 6 GHz, there is still a problem of providing service effectively to the remainder.
In the few years since satellite systems operating at 4 and 6 GHz were conceived, technology has advanced significantly. This was recognized at World Administrative Radio Conference for Space Telecommunications (WARC-ST) held in Geneva last summer, when new frequency bands were allocated for satellite broadcasting. This new technology will not simply add to TV distribution capacity, but will enable hundreds of remote communities to obtain access to television programming, through small, relatively inexpensive receiving installations. It will also permit the input of local program material to satisfy the desire for regional programming and social communications.
The number of television channels will be determined by the needs for time zone or regional coverage, as well as the requirements for a number of separate and simultaneous programs for entertainment, educational, public informational and specialized (narrowcasting) purposes. Multiple audio channels to accommodate different languages are a possibility in an operational system. Certainly the programming must take into consideration the social and cultural needs of the various peoples to whom the service is directed and permit, to the greatest extent possible, the exchange of programs among groups with common interests.
II. Extension of Coverage
It is the ability to offer essentially uniform quality to an unlimited number of viewers anywhere within the large area enclosed by the coverage patterns of the spacecraft antennas that makes satellites particularly attractive in the Canadian environment as just described.
Unfortunately satellite coverage patterns do not conveniently follow jurisdictional boundaries nor time zones. However, it is interesting to observe the extent to which shaping of "footprints" can be obtained by proper selection of satellite locations in geostationary orbit and boresight position. Fig. 3 shows possible coverage of Canada by the use of five antenna beams, each having a circular beamwidth of 2 degrees after allowance for uncertainties in satellite position and antenna pointing. Such coverage also minimizes, to some extent, the spillover of energy into adjacent areas
Location of a satellite at 115° west longitude is attractive in terms of the form of the coverage contours in Eastern Canada and the fact that satellite eclipse will occur after local midnight for a large part of the country. In a sophisticated operational system, in which more than a single satellite would be used to provide the required redundancy at minimum cost, the system could be configured so that, in the primary mode, eclipse problems would be fflitigated in all areas.
In spite of the Canadian requirement for northern coverage, there appears to be no overwhelming need for satellites in other than geostationary orbit. It would be possible to improve northern coverage by use of a system comprising a number of satellites in highly elliptical 12 hour orbits such as in the Russian Orbita system. The use of inclined elliptical, but synchronous, orbits has also been proposed as a means of improving utilization for domestic satellites (see, for example, (2)). However, the necessity for tracking by the receiving terminals discourages their use for television broadcasting to small low-cost terminals.
A geostationary satellite whose sub-satellite point is on the same longitude as that of the receiving terminal will appear at an elevation angle of 5° for a terminal location at 76° latitude. The horizon is at 81.3° latitude, or more if the effects of atmospheric refraction are taken into consideration. There are only 6 communities in Canada at latitudes greater than 76°N; these have a total population of about 150.
III. Frequency Allocations
Until July of last year, there had been considerable speculation concerning the outcome of the World Administrative Radio Conference on Space Telecommunications, which dealt with frequency allocations to the Broadcasting-Satellite Service. Much of the literature had been concerned with the choice of optimum frequencies in respect to the relative costs of systems, state-of-the-art, feasibility of sharing, bandwidth requirements for various types of service, etc.
The Conference is now history; the results are embodied in the Final Acts^. The reasons for the decisions, not always obvious even at the time, will soon be forgotten, and speculation has now given way to the more important tasks of interpreting the regulations and designing compatible systems to operate within the constraints which were agreed.
First of all, a definition of the Broadcasting-Satellite Service was adopted, namely, "a radio communication service in which signals transmitted or retransmitted by space stations are intended for direct reception by the general public". Direct reception includes both individual and community reception which are separately defined. Therefore, while excluding the network distribution of programs by satellite, which falls within the Fixed-Satellite Service, the Broadcasting-Satellite Service appears to encompass reception by individual receivers augmented to varying degrees, community receivers for local distribution and even more elaborate community antenna and redistribution systems.
Two frequency bands totalling 690 MHz [in Region 2 which comprises North and South America) were allocated on a shared primary basis to the Broadcasting-Satellite Service: these extend from 2500 to 2690 MHz and from 11.7 to 12.2 GHz as shown in Table I which summarizes the relevant allocations made by the W.A.R.C. The lower frequency band is restricted to use for community reception from domestic and regional systems, and the spacecraft e.i.r.p. is constrained by the power flux density limitations indicated, except by agreement amongst Administrations affected. There are no similar constraints on the use of the 12 GHz band, other than that for domestic systems, although Res. No. Spa 2-2 C3) directs the setting up of World or Regional Administrative Conferences to develop agreements and associated plans governing the establishment and operation of stations in the Broadcasting-Satellite Service.
Two additional bands at 42 and 85 GHz were allocated exclusively to the Broadcasting-Satellite Service. While these comprise a total of 4 GHz, they lie at frequencies which are not likely to be of immediate interest in the development of systems.
One further allocation was made which gives "footnote status" to the use of the band from 620 to 790 MHz for FM television broadcasting subject to agreement among the Administrations concerned and affected. Power flux density limitations apply generally, except by agreement, and will require the use of energy dispersal techniques. However, the difficulties of sharing between terrestrial and space services will seriously inhibit any early development of the service in this band in Region 2. At the very least, such sharing will require a high degree of cooperation between the separate services, and will create regions near terrestrial transmitters where neither terrestrial nor satellite transmissions could be received without interference^).
An accommodation is possible in the 2.5 GHz band, which is restricted in any case to community reception, since current usage of the band is largely by terrestrial ITV systems. Regions of mutual interference will be greatly reduced and use may be more easily coordinated.
Even in the 12 GHz band, it will be necessary to agree, at least regionally, on the allotment of the band or portions thereof, on the practical aspects of sharing between services, orbit utilization, and on RF channel bandwidths and other system characteristics so that orderly development may proceed.
Frequencies were not specifically allocated to up-links for broadcasting satellites, which are to make use of bands already allocated to up-links within the Fixed-Satellite Service.
IV. Technological Considerations
In considering the design of systems to operate at the various frequencies allocated to the Broadcasting-Satellite Service, it is necessary to take into account the effects of propagation and environmental noise.
At 800 MHz, ionospheric fading and, particularly in urban noisy locations, man-made (indigenous) noise are importantC5). The effects of ionospheric absorption and of cosmic noise background radiation can be neglected in practical systems.
At 12 GHz, the effects of attenuation and noise due to precipitation become significant. In temperate climates, they are not likely to be especially restrictive for a required reliability, considering propagation effects alone, of 99.9%. Specified reliabilities of 99.99% would, however, require large system margins to take into account the effects of precipitation. The importance of tropospheric scintillation increases rapidly with decreasing elevation angle and will be of greater importance than precipitation in the design of systems primarily intended for the extension of northern coverage.
Propagation effects are minimal at frequencies near 2.5 GHz.
Fig. 4, derived in part from Ref. (6), illustrates the variation with frequency of the system margin required to achieve a reliability, as determined by propagation effects, of 99.9% for various angles of elevation corresponding to the given latitudes at the longitude of the sub-satellite point. The curves include the effects of absorption by atmospheric gases and of ionospheric and tropospheric fading, and are based on the extrapolation of measurements taken at discrete frequencies at Resolute Bay, Churchill, Ottawa and Boston. A linear dependence of tropospheric fading on frequency has been assumed but this has not been verified experimentally. The curves do not take into account the attenuation due to rain, or the effects of ionospheric absorption, although these will not substantially change the form of the curves
The figure clearly indicates a minimum in the required system margin for frequencies in the range from one to several gigahertz, and demonstrates the difficulty of obtaining satisfactory coverage at elevation angles of 5° or less near the limits of satellite visibility.
Little is known about the joint probability of occurrence of rainfall attenuation and tropospheric fading. Where individual probabilities are small, however, the loss in signal corresponding to a given percentage of the time should be given, to a first approximantion, by the greater of the two values.
Fortunately, heavy rainfall does not normally occur at northerly locations in Canada, where a geostationary satellite presents a low angle of elevation. At Resolute Bay, for example, the total annual rainfall is about 2^ inches.
The use of the bands at 42 and 85 GHz will be more seriously affected by the attenuation due to precipitation and by low-angle fading. Also, although these allocations fall within so-called atmospheric windows, absorption by atmospheric gases becomes significant especially in the higher frequency band.
A number of papers have been published which deal in detail with the design of ground receivers and the various options which are available (see, for example. References (7) and (8)). While receiver design is not the subject of this paper, some comments on receiving antennas are in order - since the antenna is a critical element which affects the future development of satellite broadcasting systems.
At UHF frequencies and for antenna gains up to about 20 dB, a variety of antennas is available. These include the Yagi antenna, corner reflectors, broadside arrays and helical antennas. Helical antennas have an advantage over the more common Yagi array in terms of the much greater bandwidths which can be provided, an important consideration in the reception of multiple channel FM television. Circular polarization is considered to be a requirement for receiving antennas at UHF frequencies because of the variation of the plane of polarization of the received signal due to Faraday rotation in the ionosphere.
At higher frequencies and for higher gains, the conventional parabolic reflector antenna appears to be the most appropriate. To a first approximation (i.e., neglecting the variation with frequency of propagation and noise effects), the effective apertures of ground terminal antennas required for a given quality of reception are independent of frequency. However, the directivity increases proportionally to the frequency as does the required accuracy of the reflecting surface. Thus as the diameter of the antenna or the frequency increases, the requirements become more stringent on the stability and accuracy of the antenna mount and on the tracking and pointing sub-systems. This is particularly important if the satellite with which the antenna operates varies position or if the antenna is used to receive signals from more than one satellite.
Antenna diameters of from 2 to 4 feet, accompanied by satellite e.i.r.p. of 70 to 80 dBw per channel, are considered reasonable for individual reception. (A 4 ft. antenna has a half-power beamwidth of 1.5° at 12 GHz.) For community viewing, antenna diameters of 4 to 6 feet, with satellite e.i.r.p. per channel of 55 to 65 dBw, may be appropriate. In the case of community reception for subsequent redistribution, antenna diameters of from 8 to 12 feet, corresponding to a satellite e.i.r.p. of 40 to 50 dBw may be acceptable. It should be noted, however, that the smaller antenna diameters, accompanied by higher satellite e.i.r.p. may be desirable at 12 GHz because of the stringent requirements on mechanical stability and antenna pointing accuracy necessitated by the narrow beamwidths.
As in the case of the ground segment, no attempt is made here to examine in detail the current state-of-the-art in spacecraft technology. Only those general aspects affecting the overall concepts of satellite broadcasting systems are discussed.
Assuming that the technical problems can be solved with an appropriate effort in research and development, the costs of systems of comparable performance are expected to be appreciably less at 2.5 GHz than at either 800 MHz or 12 GHz. For a given quality of service, space segment costs decrease rapidly with increasing diameter, or effective aperture, of ground receiving antennas.
Te provide service to individual receivers having very small antennas and moderate noise temperatures, even at low picture quality, systems at 12 GHz will entail much higher space segment costs than those of systems for community reception. However, considering only the recurring costs of the space segment (i.e., excluding research and development), satellites providing a number of channels to several small regions should not be much more expensive than spacecraft supplying fewer channels to larger coverage areas. This assumes that the problems of antenna design and mounting and accurate spacecraft orientation can be overcome. It occurs because the r.f. power required per channel can be traded against the satellite antenna gain, while maintaining constant e.i.r.p.
V. Experimental Systems
Several experiments are already under way to examine the use of the frequency bands recently allocated to the Broadcasting-Satellite Service. Table II summarizes the characteristics of various U.S. and Canadian experimental systems which will investigate the use of satellites for broadcasting as defined by the W.A.R.C.
At 800 MHz, there are two experiments which will be carried on the NASA ATS-F satellite. One of these is the Instructional Television Experiment being undertaken jointly between NASA and the India Department of Atomic Energy; the second, similar in design if not in intent, is the NASA experiment to demonstrate Television Relay using Small Terminals (TRUST).
The ATS-F will also carry an Educational Television Experiment operating in the 2.5 GHz band. This is a cooperative venture of NASA, the Corporation of Public Broadcasting and the Department of Health, Education and Welfare.
At 12 GHz, a television broadcasting experiment will be the principal experiment to be performed using the Communications Technology Satellite (CTS) which is a joint project of NASA and the Canadian Department of Communications. The satellite incorporates a high-power TWT, capable of giving 200 watts RF output, and two 2.5 degree gimballed antennas. A complete description of the spacecraft is given elsewhereC^). in addition to television broadcast to 8 ft. diameter ground terminals, a number of related experiments are proposed. These are listed in Table III. One of these involves the transmission of live television from a 10 ft. transportable terminal in a remote area via the satellite for reception by the main 30 ft. control terminal at Ottawa, at a quality sufficient for use by the networks. Another is two-way voice communications with small terminals (approximately 4 ft. diameter). This facility may be used in conjunction with the television broadcasting capability to provide an order wire or to carry out experiments in information retrieval and interactive educational television.
Multiple purpose satellites are of interest for service to remote areas since many of the comments already made in respect to satellite broadcasting in Canada apply equally well to other service requirements. For example, it may be possible to combine the use of instructional television broadcasting in the band from 2535 to 2655 MHz with two-way voice communications with and between small terminals in remote areas, the latter operating in the neighbouring bands at 2500 to 2535 MHz for space-earth and 2655 to 2690 MHz for earth-space.
It is interesting to note the similarities that exist among the various television broadcast experiments listed in Table II. The satellite e.i.r.p.'s are all within a few dB of one another. Antenna diameters of small ground terminals for community use are of the order of 10 ft., the increasing gain with frequency compensating for the increasing free space loss (but not any additional propagation losses). System noise temperatures are of the order of 1000°K and the proposed quality of service on-axis approaches the equivalent of TASO Grade 1.
Some of the proposals to the FCC for U.S. domestic systems include provision for services which fall within the definition of satellite broadcasting.
- The 2.5 GHz band is particularly attractive from a technological point of view for the development of future systems to provide satellite broadcasting to small low-cost terminals. However, the amount of spectrum space available is limited, and sharing with terrestrial services imposes a number of constraints including the use of the band only for community reception. Nevertheless, the possible use of the band to provide educational or public informational broadcasting in conjunction with two-way voice communications to remote areas requires serious consideration. Satellite broadcasting to community receivers with facilities for local distribution or rebroadcast will enable a balance to be effected among local, regional and national programming.
The 12 GHz band, while allocated to a number of services on a shared basis, has not yet been extensively developed, and no constraints have yet been imposed on its use by the Broadcasting-Satellite Service. Thus, it is the only band, other than those at 42 and 85 GHz, for which the Radio Regulations permit the development of systems primarily for individual reception. Individual reception is a vague term, in spite of attempts by the ITuC^) and others at clarification, and does permit some degree of augmentation of home receivers.
On the other hand, while the trend will be towards higher power satellites to reduce the cost of ground terminals, there does not, in general, appear to the authors at this time to be any compelling reasons, at least in Canada, for the development of television systems primarily intended for individual reception. The introduction of high-power satellites will, of course, permit trade-offs between quality of service and cost, and enable anyone willing to make a modest investment to receive television signals directly from a satellite.
Except for the exclusive bands at even higher frequencies, the exploitation of which is expected to be even farther off, the allocation at 12 GHz is the only one with sufficient spectrum space to accommodate in a single band all of the. likely social, cultural and commercial requirements for television broadcasting from satellites within any Region. The use of narrow beam antennas at this frequency will permit reasonable control of coverage areas, and facilitates sharing between services and systems.
It seems likely that there will be an increasing tendency toward the design and use of multiple-purpose satellites to provide complementary or supplementary services within the same coverage areas. This could permit advantage to be taken of common ground terminal and local distribution facilities, a reduction in space segment costs, and an increase in the social benefits of new services to remote areas (for example, by the use of two-way voice communications to provide a capability for interactive or information retrieval television).
The probable requirement for a number of television channels and coverage areas will encourage the use of multiple channel satellites with individual power amplifiers for each channel.
In the context of the Canadian environment, a satellite broadcasting system could provide:
- extension of existing real time service to all regions where such service is not alreadyavailable
- choice of alternative service, for communities now served dinadequately; including availableility of programming in a second language
- expansion of educational and/or instructional television with provision for information retrieval and interctive channels
- increased xsope of program material through the use of transportable uplinks in remote areas.
(1) Communications capability of the Canadian Domestic Satellite System, J. Almond and R.M. Lester, International Conference on Communications, Montreal, June 1971.
(2) Orbit allocations of domestic communications satellites, M. Shini and Y. Kurose, Proc. IEEE, Vol. 48, pp. 165S-56, September 1969.
(3) Final Acts of the World Administrative Radio Conference for Space Telecommunications, International Telecommunications Union, Geneva, 1971.
(4) Frequency sharing between FM and AM-VSB
television transmission systems, E.F. Miller and R.W. Myhre, NASA Technical Memorandum TM X-52755, April 1970.
(5) Radiofrequency noise measurements in urban areas at 480 and 950 MHz, G. Anzic, NASA Technical Memorandum TM X-1972, March 1970.
(6) Low angle tropospheric fading in relation to satellite communications and broadcasting, K.S. McCormick and L.A. Maynard, International Conference on Communications, Montreal, June 1971.
(7) High power spaceborne TV transmitter design trade-offs for the 1970-1985 period, E.T. Lipscomb, Paper No. 70-434, AIAA 3rd Communications Satellite Systems Conference, April 1970.
(8) A television broadcast-satellite system for ETV-ITV, P.A. Bergin, Paper No. 70-452, AIAA 3rd Communications Satellite Systems Conference, April 1970.
(9) A high-power communications technology satellite for the 11-14 GHz band, C. Franklin and E.H. Davidson, AIAA 4th Communications Satellite Systems Conference, April 1972.