Page 69 - Communications Satellites Global Change Agents

2. EVOLUTION OF SATELLITE TECHNOLOGY 45

The general pattern of development in the satellite industry is actually
straightforward. The "big picture" does not require a degree in rocket propul-
sion or electrical engineering to understand. The big trends that reflect how sat-
ellite communications technology has evolved and expanded over the last 40
years can be summarized as follows:

• Satellites have expanded the performance of their power systems; instead of
producing less than 100 watts, these spacecraft now can produce 2 to 15 kilowatts
depending on the type of service provided. This power increase has been accom-
plished in several ways: (a) by making the solar cell arrays larger; (b) by using
higher performance solar cells that convert the sun's energy more efficiently; (c)
by creating new ways of pointing the solar arrays so that they are always facing
the sun; and (d) by developing and using larger, high-performance batteries with a
longer operating life to sustain satellite operations during solar eclipses.
• Satellites now have more rf spectrum to use. There are more radio frequen-
cies that have been allocated for operational use. Newer satellites also have ways
to "reuse " the frequencies many times over without causing interference. This
primarily involves the use of a number of narrow and highly focused "spot
beams. " Frequency reuse is largely the result of new high gain, multibeam space
antennas. Altogether the usable spectrum for satellite communications has, by a
variety of means, increased nearly a hundred times. This expansion of the avail-
able frequencies has been done in several ways. New frequency bands, typically
in ever-higher frequency bands, have been allocated for satellite use. These spec-
trum allocations for commercial services start with frequencies as low as 137
MHz and 400 MHz. In the early days of satellite service, the most important spec-
trum was in the 4,000 MHz to 6,000 MHz bands (known as C-Band). This alloca-
tion has limited bandwidth (i.e., 500 MHz), and throughput capability is not too
large, but there is a limited problem with rain fade. Next comes the 12,000 MHz
to 14,000 MHz (or Ku-band), which has more spectrum (and thus greater through-
put capability), but increasing problems with rain fade. Today we are moving into
the highest of the commercially available spectrum known as the Ka-band. These
frequencies in the 18,000 MHz to 30,000 MHz bands are transitioning from ex-
perimental systems into the first commercial services via SES Astra (in Europe)
and Eutelsat with Spaceway, Wild Blue, and others to follow soon. These new
Extremely High Frequency (EHF) bands are difficult to use because radio waves
at these extremely demanding frequencies and small bandwidth are "bent" by
heavy rainfall. This creates what is called precipitation attenuation, and this cre-
ates problems when transmitting and receiving signals during rainstorms.
In addition to this expansion of the frequency bands by new allocations made
by the International Telecommunications Union (ITU) and their assignment un-
der national licenses by governments around the world, there are a variety of tech-
niques to expand available spectrum for commercial satellite systems. The most
important of these procedures follows a process similar to that used in terrestrial