Space-Based Broadcasting The Future of Worldwide Audio Broadcasting (1985) / Chapter Skim
Currently Skimming:
Section 6: The Space Segment (Transmitter) - Surface Segment (Receiver) Power Budget for an HF DBS-A System-Service
Section 6: The Space Segment (Transmitter) - Surface Segment (Receiver) Power Budget for an HF DBS-A System-Service
Pages 45-53
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 45... ...
-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.
|
From page 46... ...
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)
|
From page 47... ...
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.
|
From page 48... ...
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.
|
From page 49... ...
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.
|
From page 50... ...
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)
|
From page 51... ...
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 - 5l WORKING PAPER
|
From page 52... ...
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.
|
From page 53... ...
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.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.