comparatively high-power transmitters and very-high-gain antennas may be used. On the
contrary, from satellite to earth the link is power-limited for two reasons: (1) in bands
shared with terrestrial services such as the popular 4-GHz band to ensure noninterference
with those services, and (2) in the satellite itself, which can derive power only from solar
cells. It takes a great number of solar cells to produce the RF power necessary; thus
the downlink, from satellite to earth, is critical, and received signal levels will be much
lower than on comparative radiolinks, as low as
-150 dBW. A third problem is crowding.
The equatorial orbit is filling with geostationary satellites. Radio-frequency interference
from one satellite system to another is increasing. This is particularly true for systems
employing smaller antennas at earth stations with their inherent wider beamwidths. It all
boils down to a frequency congestion of emitters.
It should be noted that by the year 2000, we can expect to see several low earth-orbit
(LEO) satellite systems in operation. These satellites typically orbit some 500 km above
Frequency Bands: Desirable and Available
The most desirable frequency bands for commercial satellite communication are in the
spectrum 100010,000 MHz. These bands are:
37004200 MHz (satellite-to-earth or downlink)
59256425 MHz (earth-to-satellite or uplink)
72507750 MHz (downlink)
79008400 MHz (uplink)
These bands are preferred by design engineers for the following primary reasons:
Less atmospheric absorption than higher frequencies
Rainfall loss not a concern
Less noise, both galactic and man-made
Less free-space loss compared with the higher frequencies
There are two factors contraindicating application of these bands and pushing for the use
of higher frequencies:
1. The bands are shared with terrestrial services.
2. There is orbital crowding (discussed earlier).
Higher-frequency bands for commercial satellite service are:
10.9511.2 GHz (downlink)
11.4512.2 GHz (downlink)
14.014.5 GHz (uplink)
17.720.2 GHz (downlink)
27.530.0 GHz (uplink)
These two bands are intended mainly for military application.