2. Signal quality. The quality of the received signal determines the usefulness of any communications link.
Signal quality, in turn, is determined by the S/N of the communications link. Methods of improving the S/N of
a satellite communications system require consideration of several basic factors.
a. Reduction of manmade noise. Manmade noise is reduced either by removing the noise source from the
vicinity of the receiving antenna or by placing the receiving antenna in an area remote from noise sources.
Careful shielding of circuit components also helps to reduce noise.
b. Reduction of atmospheric noises. Atmospheric noise is nearly impossible to remove from the signal
because the atmosphere is included in part of the radio transmission path. However, atmospheric noise is less
noticeable in the VHF and UHF than in the lower frequency ranges. Generally, the higher the frequency of
transmission, the less the effect of atmospheric noise on the signal.
c. Thermal noise. Thermal noise is related to temperature. Ground terminals of satellite communications
systems are designed to minimize this source of noise. Radio receiving sets in the ground terminals reflect the
finest and latest circuit techniques to minimize the effect of thermal noise. Parametric amplifiers using
refrigerated circuit elements further reduce the effect of temperature rise on internal noise development. Three
major sources of natural thermal noise are Earth temperature, sun, and certain galaxies (galactic noise). Noise
from these sources is reduced by correct antenna orientation at specified times.
d. Directional antennas. The use of highly directional antennas at the ground terminals is an absolute
necessity in satellite communications. Moreover, the ground terminals must orient their directional antennas
precisely toward the satellite in orbit. Even a slight inaccuracy of antenna pointing by either ground terminal
reduces the signal level sharply and raises the noise level.
e. Type of signal. The type of radiated signal has a bearing on the S/N. Frequency modulation (FM) has
the advantage of peak noise reduction over amplitude modulation (AM), and is favored at present. FM,
however, requires a relatively large bandwidth in the radio receiving and transmitting equipment. Generally, in
radio receivers, the wider the bandwidth the less gain per stage is available in the RF and IF amplifiers. Also,
the wider bandwidth allows low-level noise to enter the received signal. High-level noise is normally removed
by limiter circuits. Further refinement of design may someday improve the noise reduction in AM circuitry.
When that becomes practical, a great many more channels of communications can be accommodated in the
same frequency spectrum occupied by the FM signal in present-day design. Furthermore, narrowing each
channel of communications also improves the S/N.
f. Power considerations. The amount of power in the intelligence-bearing portion of any composite radio
signal is related to the number of channels in use. The fewer the channels, the more power can be concentrated
in each one, thus improving the S/N.
SS0031
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