A vital, often predominant function in every space mission is that
of communications. From the moment of launch, the only connection
between spacecraft and earth is the communications system. This
system is responsible for sending scientific data back to earth in
the specified quality and quantity together with engineering data
reporting the condition of the spacecraft. The communications
system also provides the capability of tracking the spacecraft and
commanding it to take certain actions. Without an effective
communications system a successful mission would not be possible.
To appreciate the challenge that one faces in designing such
systems for planetary exploration, one must consider the enormous
distances that are involved. Voyager spacecraft, for example, are
now more than one billion miles from earth, tens of thousands of
times farther than the most distant communications satellite, and
continue to transmit data and respond to commands. The necessity of
minimizing spacecraft weight presents a major problem to
communications systems designers. The far-reaching implications of
spacecraft weight become apparent as the designer considers the
problems of providing power supply, antennas, and other necessary
devices and supporting elements. Another important challenge is the
extreme reliability required of the communications system on the
spacecraft. Once the spacecraft is launched, on-board failures can
no longer be repaired except by use of redundant systems. System
degradation due to aging, imperfect antenna pointing, or imperfect
trajectories can be expected; and the designer must know how much
degradation to expect from each case and must design the equipment,
the operations, and the procedures of data analysis accordingly.
The telecommunications engineer works with the most precise and
advanced techniques of the engineering world. Since the launch in
1958 of Explorer I, the first free-world satellite, there has been
substantial progress in improving communications capability. Even
though substantial progress has been made in the last 25 years,
space exploration is still in its infancy. There has been no
exploration beyond the solar system. There are numerous galaxies
and billions of stars to investigate. Bigger and tougher challenges
are still ahead; more exciting times are yet to come. These
challenges will undoubtedly call for more advanced
telecommunications systems to transmit information to and from deep
space. Telecommunications technology is still in its infancy.
Through the years, a number of telecommunications design
techniques, procedures, and analyses contributing to the success of
deep space exploration missions have been developed and applied.
The purpose of this book is to provide descriptive and analytical
information useful for the optimum design, specification, and
performance evaluation of deep space telecommunications systems.
The book emphasizes system performance information. Long, tedious
derivations are not included. The book should serve to acquaint new
telecommunications engineers with the techniques available to them
and should summarize for the experienced engineers the analyses and
information necessary for their work. It also provides a background
for understanding the interface between the Deep Space Network and
the spacecraft and is intended to facilitate the conceptual designs
and analyses for the enhancement of telecommunications performance
and assurance of compatibility between spacecraft and ground system
capabilities.
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