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SECTION 6
THE SPACE SEGMENT (TRANSMITTER)-SURFACE SEGMENT (RECEIVER)
P OWE R BU DGET FOR AN H F DB S-A SY STE M- SE RV l C E
In an earlier paper, the author outlined an approach to the design of a
DBS-A common user system that could provide the world with reliable,
high-quality audio broadcasting in the HF portion of the electromagnetic
spectrum, specifically in the 26 MHz bandit Because the ionosphere
supports onward radiowave propagation in this high frequency portion of the
HF region for only a relatively small fraction of the time, this band is
only marginally useful for long-distance, surface-based shortwave audio
broadcasting.2 For this very reason, however, this band could be used
effectively by space-based transmitters because essentially all of the time
signals transmitted downward along line-of-sight propagation paths toward
receivers on the Earth's surface would not experience important ionospheric
influences throughout large subsatellite surface areas.
The first time that the possibility of using this portion of the
radiowave spectrum in such a fashion was discussed professionally in the
public literature was in a paper by two British Broadcasting Corporation
(BBC) engineers in 1978.28 U.S. professionals at the National
Telecommunications and Information Administration (NTIA) recently have
explored the influence of the ionosphere on such a use in great detail.29
In the author's earlier report,26 the basic circuit estimations made
by Phillips and Knight28 were used as the basis for a more detailed
consideration of this possibility in overall system design terms.
In one circuit example, Phillips and Knight28 assumed a 26-MHz,
double-sideband AM transmitter to be located in geostationary orbit that
would broadcast downward to state-of-the-art spacewave receivers having a
26. "Modernizing And Expanding International High-Frequency Broadcasting,
T. F. Rogers (unpublished) January 1982. See also: "The Use of
Satellites in Modernizing and Expanding International HF Broadcasting
T. F. Rogers, Report of the International Broadcasting Convention,
Brighton, U.K., September 16-25, 1982; pages 155-157.
27. The 1979 World Administrative Radio Conference reduced the 26 MHz
region spectrum for broadcasting from 500 to 430 KHz.
28. "Use of the 26-MHz band for satellite broadcasting," G. J. Phillips and
P. Knight, E.B.U. Review-Technical Part, August, 1978; pages 173-178.
.
29. "Study of Factors Affecting an HF/VHF Direct Broadcasting Satellite
Serviced C. Rush, J. Aarons' F. Stewart, J. Klobuchar, P. Doherty, M.
PoKempner, and R. Reasoner, N.T.~.A. Report 84-158, September, 1984.
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simple half-wave dipole wire antenna with 5 db gain, or for easily
transported or even mobile purposes a 5 db gain ferrite rod antenna
incorporating improved components then becoming available. A post-detection
audio bandwidth of 5 kHz also was assumed. Phillips and Knight then
estimated the size and character of the in-orbit transmitter required to
provide reliable, high-quality signal reception over a surface area that
would escape almost all ionospheric influences except for the Faraday
rotation effect.
The author assumed that each regional satellite transmitter would
incorporate a large, multisegmented30 or multifeed antenna with an
approximate i,OOQ-foot overall diameter3] that, driven by an array of
individual subtransmitters, could generate several Earth-directed
spot-coverage beams. The selection, frequency, and power output of each
subtransmitter would be controlled by the space segment's surface feeder
station. Each surface footprint would have an area of approximately 2
mi ~ ~ i on square mi ~ es.
In that paper the author concluded that with such a geostationary
transmitter and adequate statistical system allowance for natural and
commercial-industrial electrical noise and ionospheric attenuation (a system
allowance that, in total, is ll db greater than that used by Phillips and
Knight 28 in their single circuit power budget calculation), a 40-db,
30. See "On the Feasibility of Direct Emergency Spot Broadcast from
Satellite to Ordinary Ground Receivers," R. M. Lerner, M.~.T. Lincoln
Laboratory Technical Report 546, 29 December, 1980; see, especially,
pages l3-] 5.
While the physical size of such an antenna would be large, its
electrical size, i.e., the ratio of its area to the radio wavelength
used, and therefore its inherent directivity and gain, would be about
the same as that of the UHF antenna employed on the U.S.-NASA ATS-6 R&D
cornrnunications satellite that was placed in orbit over a decade ago.
Space antenna technology has advanced to the stage where an aerospace
firm is now working under U.S. government contract on an antenna
reflector which, when unfurled in space, would be "nearly twice the
size of a football field" (i.e., almost 20 percent of the area
suggested here). Because of its accuracy and stability of
construction, it could be satisfactorily employed at wavelengths much
shorter than the near 40 feet of 26 MHz radiation. The Soviet Union
reports that it is working on the construction of a parabolic reflector
for use in space that has a diameter of 300 to 350 meters, i.e., at
least the size suggested here. And the possibility of employing very
long end-fire arrays made up of tether elements should not be
overlooked. A fundamental justification advanced by NASA for a
civilian Space Station is that it, including its crew of technicians,
could be used to assemble and test large structures such as this in
ow-Ear~th-orbit.
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post-detection S/N (corresponding to a C/N = 38 db in a pre-detection
bandwidth of 70 kHz) could be expected in 99.9 percent of the surface areas
covered, 99.9 percent of the time, with a radiated RF continuous wave power
of 1,300 watts.
The report by Rush, et. al., 29 (see its Section 3) suggests that the
influence of the ionosphere on the ability of 26-MHz signals to penetrate to
the Earth's surface, especially in the most northern latitudes and/or during
intervals of maximum sun spot activity, may be more important than Phillips
and Knight and this author realized. This might raise a question concerning
the ability of any 26-MHz space-based system to provide an overall 99.9
percent service reliability. As in the 2.5 GHz conceptual system-service
studied here earlier, however, by excluding areas north of the Arctic and
south of the Antarctic circles from service considerations, excluding even a
few additional regions at latitudes closer to the Equator where the
population density is as small as in the Arctic and Antarctic regions, and
insuring that geostationary space segments are optimumly located in orbit to
favor coverage of the more northern regions. But, because of the
ionosphere's influence on surface coverage, the use of more space segments
in an HE DBS-A system-service would be required than in a UHF DBS-A
system-service to ensure that the reliability of service EQUAL be excellent
at all times for the vast bulk of the world's population. In the limit,
if it were judged necessary to provide a higher reliability of service to
some northern regions not easily possible with the use of geostationary
satellites, one or more additional space segments, placed into appropriate
"Molniya"-like orbits could be employed. Of course, almost any desired
reliability of service for almost any surface location could be obtained in
this fashion at increased system cost.
The report by Rush, et. al.,29 (see its Section 7) emphasizes that the
ionosphere would cause short-term fading of 26-MHz signals received after
they had traversed it. In some surface regions (the more intense fading
affecting heavily populated regions would be confined to some 10° north and
south of the geomagnetic equator) and at certain times, particularly during
the local late evening hours at times of high sun spot activity. This
fading would be more important than was judged to be the case by Phillips
and Knight28 and the author.26 But the conclusion reached by Rush, et.
al.,29 (see their page 123) that "It is not reasonable to try to overcome
the nighttime (1900-2400 furs. local standard time) levels of scintillation
activity...with more power" is too pessimistic.
The author's 26-MHz DBS-A conceptual system design characteristics26
incorporate power budget margins (see its Appendix No. 3) that provide
received signal strengths lB db above rural quiet, 11 db above rural, and 4
db above residential area electrical noise levels external to the receiver.
32. More detailed northern region statistical coverage calculations are
required than necessary for the purposes of this paper.
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Except for some high-noise metropolitan areas (i.e., Bogota, Colombia;
Santiago, Chile, Buenos Aires, Argentina; Rio de Janeiro-Sao Paulo, Brazil;
and Calcutta, India) nearly all of the inhabited surface areas that are
likely to experience intense scintillation fading could thus accommodate it
by using various portions of this 4 to lS db range of system power margin.
An additional margin of 9 db to accommodate the peak hours of
commercial-industrial activity, and thus the electrical noise that it
creates, is also contained in the power budget estimate~26 and, since the
peak noise level that occurs late in the business day is over, or nearly
over, by the time any intense scintillation fading is expected to occur,
this margin also could be drawn upon.
Finally, the system's total power could be "taxed," dynamically, to
provide power to the beam covering the region affected during such intense
fading. Recall that a S/N of 45 db would be provided as a Standard Service
in all of the system's radiated beams for most of the time (see Section 3~.
Were an HF DBS-A system to have, for example, six beams that could be used
simultaneously, then a short-term power "tax" of ~ db could be levied upon
each of five beams, increasing the power of the one beam serving the
affected region by 5 db. All regions not affected by the scintillation
fading would then have a S/N of 44 db, rather than 45 db, i.e.' a nominal
decrease during the relatively rare interval of short-tern scintillation
fading in the one region. A short-term power tax that reduced the S/N in
regions not affected by fading to 43 db would provide lO db to combat the
scintillation fading; etc. (If FM were employed in the DBS-A system, then
expectation of this kind of fading would strongly suggest that the surface
receivers be provided with a means of "FM threshold extension.")
Thus because of the large power margins that would be built into an HE
DBS-A system-service, and the way it could be designed to operate
dynamically (it would be able to respond quickly to reports of such fading
from a surface monitoring network), a great and effective power reservoir
could be available to accommodate scintillation fading. More comprehensive
statistical analyses are indicated--analyses that would inquire carefully
into exactly how often 26-MHz scintillation fading could be expected to
occur, where, when, and with what hour-to-hour and short-term intensity.
These statistics would have to be related with analogous statistics
concerning commercial-industrial and naturally occurring electrical
noise.33 But the situation need not be as pessimistic in overall service
reliability and quality as the Rush, et. al., report29 suggests, and
perhaps no further system power margin need be required.
33. It may be that the data now availabJe--as in the building structure,
foliage, and rough terrain attenuation circumstances of comparable
interest at UHF--are not sufficient to allow temporal and spatial
engineering studies to be made satisfactorily on a worldwide,
decades-long basis. If that data are needed, they would have to be
gathered .
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Finally, for any listener who found the received signals unsatisfactory
because of severe scintillation fading, even after the system's entire
power margin had been drawn upon, there would still be the option of
employing increased receiver antenna gain. A fixed, ha~f-wave folded dipole
employed in a five-eJement Yagi (or helix) array, and having outside area
dimensions of about 20 feet x 40 feet, would provide 5 to ~ db more gain
than the assumed ha~f-wave (wire) antenna.
Although a large allowance must be made at HF for the influence of
external electrical noise and ionospheric influences, no allowance is needed
for building structure or foliage attenuation. HF reception in
circumstances of difficult terrain would also generally be superior to that
at UHF because the diffraction field propagation loss rate is much less.
And although the assumed gains of both the space transmitter and surface
receiver antennas are much lower at HF than at UHF, so is the free-space
path loss. There need be essentially no concern for the influence of
receiver antenna directivity. Moreover, the DC-to-RF efficiency of HF
transmitters is higher today. Unlike FM, provision for peak modulation
power must be made for AM. To compare the performance of an HF DBS-A
system-service with that of the UHF system-service outlined in Sections 2
and 3, adjustments must be made to the power budget analysis made in
Section 3.
Three HF service cases are considered:
l. A Baseline Service that would continue to use the world's present
26-MHz receivers designed to receive double-sideband AM signals.
2. A Standard Service that would require either the use of a much
higher power in the space segments than for the Basic Service, or use of FM
by the 26-MHz band system. In the Batter circumstance new receivers would
be required. In each case, delivery of a 45 db SIN in a 5 kHz
post-detection band would be expected.
3. A Superior Service that assumes the 26-M1z band would be used for
wide-band FM, and 50 db SIN in 15 kHz, post-detection, would be delivered by
· ~
US1 ng new rece1 vers.
The author's earlier study included a power budget estimate 26 that
concluded that a space segment power of 1,300 continuous wave RF watts would
be required. General adjustments to that estimate are as follows:
1. An ordinary system design and operating margin to accommodate less
than optimum overall systems performance: + 5 db.
2. For the conversion of DC to RF and for other space segment power
needs: + 3 db.
3. Although it is not clear what influence the use of the
compandor-expandor technique would have on the allowance for accommodating
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to AM modulating peaks, the peak-to-average power ratio adjustment to be
made, probably conservatively, is: + 6 db.
4. Use of the compandor-expandor technique: - 16 db.
5. Allowance for multi-channel use statistics: - 5 db.
Total:
- 7 db.
Therefore, the basic, adjusted, power budget now suggests that the
initial space segment power estimate is 1,300/5 = about 260 DC
watts/channel/beam.
Baseline Service
As did Phillips and Knight,28 it is assumed here that either a
horizontal dipole receiving antenna is used or that more efficient
components are used in a newly purchased, low-cost, ferrite rod antenna.
Thus the approximate DC power required in a space segment in order to
provide one reliable 5 kHz channel of double sideband AM service to
receivers in a surface area of 2 million square miles at various levels of
received S/N is as follows:
Received S/N Space DC Power
45 db
40 db
35 db
30 db
25 db
820 watts
260 watts
80 watts
26 watts
8 watts
If one channel were provided to a maximum of six such surface areas
simultaneously--areas extending over approximately 1,500 miles in latitude
in the subsatellite area, and approximately 9,000 miles in longitude for
one, 1-~/2 hour, interval of three such intervals in the morning and in the
evening each day (six in-orbit space segments are assumed to be required at
26-MHz re 4 at 2.5 GHz to eliminate any important ionospheric influence on
surface coverage), the total space segment DC power required would be:
Received S/N Space DC Power
-
45 db
40 db
35 db
30 db
25 db
- 50 -
5,000 watts
1,600 watts
500 watts
160 watts
50 watts
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Thus if, for comparative purposes, the DC power available in the space
segment were taken as the same 5,000 watts necessary to provide the quality
and reliability of service desired at 2.5 GHz, then only one channel could
be provided at a Baseline (and Standard) Service quality, i.e., 45 db S/N in
5 kHz, if this channel were delivered throughout the entire surface area
served by one space segment. The maximum number of channels that could be
provided at various lower S/N levels in covering this total area would be:
Received S/N Number of Channels
45 db
40 db
35 db
30 db
25 db
3
JO
30
100
As observed in the 2.5 GHz case, it is not possible to judge what
maximum number of channels would be expected to serve what maximum number of
surface areas simultaneously. It is also more difficult to judge the
acceptance of a lower than Standard Service S/N level by an HE AM listening
audience. A 25 db S/N might seem unacceptably low to a person who,
accustomed to listening to radio only under circumstances where high-quality
signals are broadcast over local over-the-air AM and FM stations, would be
faced with deciding to purchase a new receiver to listen to programs
broadcast from space. But for a shortwave broadcasting listening audience
able to receive a steady, fixed-frequency, undistorted signal--one not
marred by interference--a 25 db S/N might be quite acceptable. (~t should
be noted that HE shortwave broadcasters are now seriously discussing the
acceptability of signal-to-inter~ference ratios of only 27 db or even less.)
If the latter is the case, then lOO channels, i.e., a number of channels
commensurate with those that could be delivered by a 2.5 GHz system-service,
albeit with Standard Service quality in the latter case, could be delivered
at 26-MHz. The capacity of a 26-MHz AM system-service that would otherwise
utilize today's receivers would be much less than that of a 2.5 GHz
system-service unless the power of its space segment were increased to well
above 5,000 DC watts and/or unless the comparative capacity required of a
2.5 GHz system-service were scaled down considerably.
Of course, because of the sharply lower number of beams at 26-MHz, the
use of AM, and no possibility of using polarization discrimination because
of the Faraday effect, particular care would be required to guard against
channel-to-channel interference when using a large number of channels.
Standard Service
AM If a Standard Service were provided with a 26-MHz AM signal, much
more power would have to be provided in the space segment. Because one
Standard Service channel serving all areas of a given region simultaneously
would require 5,000 DC watts, lOO channels or more could require hundreds of
thousands of DC watts, depending upon the maximum simultaneous number of
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channels required to serve the maximum number of any region's 2
million-square-mile footprint.
Until recently, many might have viewed this amount of space power to be
unreasonable. But it is important to recall that Skylab's solar
cell-battery space power supply provided over 12 kw a dozen years ago and
NASA has just tested a modern array on the Shuttle that is designed to
produce 12 kw. The civilian Space Station also expects to be operating with
lOO kw within a decade. A U.S. multi-Federal department program is
exploring space nuclear power supply options ranging from 50 kw to a
megawatt or more, the SD] program is reportedly actively considering the
generation of lO megawatts of electromagnetic radiation and its transmission
across thousands of miles in space, and NASA demonstrated many years ago
that 50 kw average could be transmitted through the atmosphere with high
reliability and efficiency using a collimated microwave beam--thus
emphasizing that large amounts of electricity could be obtained at, or
shipped to, geostationary orbit from the surface. And NASA, with the
Department of Energy, is readying a program to conduct an early
experiment-demonstration that could see an unattended, Jong-flight,
perpetual aerostat platform (HAPP) and its payload stationed at 70,000 feet,
powered from the ground via a beam of microwave electricity. So, there is
no doubt that hundreds of kilowatts, or more, could be employed by DBS-A
space segments a decade from now if the involved organizations considered
such activity important. The question is not one of fundamental
technological feasibility, but one of the relative newness of the concept of
using such high in-space power in the satellite communications field and,
perhaps, its high initial development cost.
It should be noted that the use of extremely high effective radiated
powers in space to provide a large capacity, high-quality DBS-A service does
not raise the same hazard of unwelcome surface flux densities at HE that it
might at UHF, there is no expressed concern in international agreements
about space-generated high surface flux densities in the 26 MHz band.
FM As in the 2.5 GHz conceptual system design outline, 60 kHz of RF
bandwidth could be used per channel to allow FM to produce a SIN of 45 db.
With a deviation ratio of 5X, a C/N of 23 db would be required (i.e., 45 db
(S/N) - 10 logy (3) (M)2 _ 3 db (pre-emphasis) = 45 db - 19 db - 3 db =
23 db), since the 16 db of effective S/N improvement provided by
compandor-expandor use is already allowed for here in the basic adjusted
power budget estimate. The Baseline Service 26-MHz space segment DC power
estimate of 260 watts/channel/unit surface area assumes a SIN = 38 db
delivered in a lO kHz RF bandwidth. This FM case requires a C/N of 23 db in
60 kHz and therefore 260 watts would be too high by 38 db - ~ db - 23 db = 7
db. Also, the 6 db allowance made earlier for peak AM power is not needed.
Thus employment of FM rather than AM, 60 kHz of spectrum/channel rather than
lO kHz, and new receivers would allow the delivery of a highly reliable
channel for 260/20 = 13 watts of space segment DC power/channel/per beam at
a Standard Service quality.
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Given a maximum of six beams to be served simultaneously, the maximum
number of simultaneous channels that could be provided with the 5,000 DC
watts required for the UHF DBS-A system outlined earlier would be
(5,000~/~6~13) = about 60. In the basic AM case, however, only one channel
could be provided at Standard Service quality. Even so, this would be a
lesser Standard Service system capacity, by a factor of about three, than
would be available in a 2.5 GHz service in which the system's space segment
power and RF bandwidth were the same. In order to provide the same capacity
arrived at in studying the 2.5-GHz conceptual system-service, lO to 15 kw of
space segment DC power would be required. Given the great difference in the
surface footprint area, however, 200X greater at HE than at UHF (2
mi]lion/10,000), the minimum number of receivers served per channel would be
much greater at [`F.
While the surface area-to-area self-interference situation would be
sharply improved with the use of FM, it might still be difficult to
accommodate 100 channels in the 430 kHz now used for shortwave broadcasting
in the 26-MHz band. This would be the case unless a considerable re-use of
frequencies in each region is found to be feasible even with the
simultaneous use of only six beams and, more importantly, unless much or all
of the other 2.7 MHz used for such broadcasting in the other, lower
frequency, HE bands, also were allocated to a 26-MHz DBS-A system-service.
If l/2 of the 2.7 MHz were to be so used, for instance, and a frequency
re-use factor of 3X were achievable, then about 75 Standard Service channels
could be accommodated/space segment; if all were so used, 140 channels could
be accommodated. If not, and if a large number of channels were nonetheless
required, then a smaller FM deviation ratio would have to be used and the
lower resulting S/N compensated for with even more space segment DC power.
Superior Service
A Superior Service, one providing 50 db S/N in 15 kHz in each reliable
channel would require an additional space segment power of 10 db. Unlike
the 2.5 GHz situation, however, the employment of greater receiver antenna
gain at 26-MHz generally does not appear to be a practical course because
the radio wavelength is nearly 40 feet. If the RF bandwidth were not
increased, one channel could be provided to one beam for (13~10) = 130
watts of space segment DC power, and to all six beams for 800 DC watts. Of
course, this would leave less capacity for the provision of many Standard
Service channels; lO Standard Service channels/beam would have to be
sacrificed for each Superior Service channel/beam. If the bandwidth were
increased and a larger deviation ratio employed, the DC power and,
consequently, the number of Standard Service channels sacrificed would be
reduced. The prospects of such a move are not now particularly
encouraging. Considering the space power and 26-MHz spectrum required, it
might weld be that a Superior Service could only be delivered economically
at times outside the prime dinner and breakfast Standard Service delivery
intervals.
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Representative terms from entire chapter:
standard service
