The challenge of communication in planetary exploration has been
unusual. The guidance and control of spacecraft depend on reliable
communication. Scientific data returned to earth are irreplaceable, or
replaceable only at the cost of another mission. In deep space,
communications propagation is good, relative to terrestrial
communications, and there is an opportunity to press toward the
mathematical limit of microwave communication. Yet the limits must be
approached warily, with reliability as well as channel capacity in mind.
Further, the effects of small changes in the earth's atmosphere and the
interplanetary plasma have small but important effects on propagation
time and hence on the measurement of distance. Advances are almost
incredible. Communication capability measured in 18 bits per second at a
given range rose by a factor of 10 in the 19 years from Explorer I of
1958 to Voyager of 1977. This improvement was attained through ingenious
design based on the sort of penetrating analysis set forth in this book
by engineers who took part in a highly detailed and amazingly successful
pro- gram. Careful observation and analysis have told us much about
limitations on the accurate measurement of distance. It is not easy to
get busy people to tell others clearly and in detail how they have
solved important problems. Joseph H. Yuen and the other contribu- tors
to this book are to be commended for the time and care they have devoted
to explicating one vital aspect of a great adventure of mankind.