Page 70 - Communications Satellites Global Change Agents

46 IPPOLITO AND PELTON

cellular and PCS mobile telephone systems. This is to create separate spot beams
that can form a pattern of cells that allow the satellite system to reuse the same fre-
quencies again and again. Elimination of the interference is accomplished by cre-
ating highly focused spot beams. Each of these beams is separated from each
other and aimed at different locations to prevent radio frequency interference by
spatial isolation. Another way to eliminate interference is to use polarization tech-
niques. These isolation techniques minimize radio frequency interference be-
tween various beams. The circular or orthogonal polarization isolation process is
much like using Polaroid sunglasses to screen out the sun's rays in one direction,
but allowing natural daylight into your eyes. The net effect of more frequency al-
locations, frequency reuse via spot beam cells and spatial separation, as well as
frequency reuse via cross-polarization is that there is now more than 100 times
more spectrum to use for satellite communications than in the early years. Further,
new digital modulation and efficient multiplexing systems also increased spec-
trum efficiency as well.
• Bigger and more reliable rockets have been developed that can launch much
bigger satellites; there are also better thruster-jet systems onboard the spacecraft
to keep the satellites pointed precisely where they need to be. These new rockets
can either launch several smaller satellites at a time or use a single launch for plac-
ing the largest satellites into orbit. The rockets have become more reliable, but
their costs have not greatly decreased. Thus, any innovation that allows a satel-
lite's antennas or the power system to be less massive helps reduce the cost by
lowering launch costs.
• The use of carbon-epoxy elements that are stronger and lighter than metal
have created cost savings. The design of satellite systems has become more effi-
cient by using lower mass but stronger materials. We have also seen lower mass
antenna and communications systems with longer life. This has created less de-
mand for rocket launches. (This means that sometimes design gains made on one
technical or economic front can result in an adverse impact on the other side of the
business. This is why systems analysis, which includes the user terminal, the
TTC&M network, the satellite design, and the launch cost, must be undertaken to
show the most cost-effective economic solution.)
• Reliable and higher performance microelectronics and solid-state equip-
ment on board allows satellites to be less massive, more reliable, and to more eas-
ily convert to new and more efficient digital communications systems. The new
electronic subsystems in satellites have continued to shrink in size and resemble
electronic computers with specialized communications-oriented software. A
modern satellite is thus very much like a mainframe computer in the sky with soft-
ware that enables it to relay TV, data, or telephone calls through its central proc-
essing unit (CPU).
• New ground antenna systems called very small aperture terminals (VSA Ts)
also use microelectronics to work with the high-powered satellites. These VSATs