Andy Stamp Department of Communication & Electronic Engineering UoP 2002 1
CHOICE OF OPERATIONAL FREQUENCY
At the risk of generalising, an operational satellite system needs the following: 1. HIGH BANDWIDTH This permits efficient multiplexing of base-band signals (eg voice, audio, video, data). This suggests the use of higher frequency carriers to allow for sufficient bandspread without aliasing. Note also that the ionosphere effectively reflects signals below VHF. Conclusion: Use a frequency higher than HF for penetration of the ionosphere and much higher than that for wideband signal transmission without aliasing. 2. CONTROL OF COVERAGE Interference between neighbouring satellites and neighbouring Earth stations must be avoided, as must any unplanned interaction between satellite technology and terrestrial technology. This suggests tight beams, which implies large antenna gains. If the antennas are to be of a reasonable size, then the frequency must be at least in the microwave region. A range of antennas would thus be available for world-wide coverage, and spot coverage at a reasonable cost in terms of money and volume. Note that Laser (especially blue green laser) and visible light might be used. However, both suffer from considerable atmospheric attenuation, horrendous weather loss and need precise alignment if they are to operate optimally. Outside of Earth's atmosphere, Lasers offer a much more viable wide-band option. World coverage would need very special transmission systems indeed. Conclusion: Use a frequency higher than VHF would allow the production of tight beams using reasonable structures. 3. CONSIDERATION OF ATMOSPHERIC ABSORPTION The atmosphere absorbs very little signal energy below 1GHz, and a little in the range 1GHz to 10GHz. However, above 10GHz atmospheric constituents (mainly H2O and O2 absorb significant amounts of signal depending on frequency. Figure 1 shows two graphs, the lowest is the so-called microwave window (1 - 10GHz) where absorption is almost insignificant. The other curves are for frequencies above 10GHz. Note the atmospheric windows around 35GHz and 95GHz, and a major non-window around 20GHz. As can be seen from the figure the level of atmospheric absorption increases (generally) significantly with frequency.
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