Page 117 - Intro to Space Sciences Spacecraft Applications

CHAPTER 5

Communications

One of the first practical uses for systems permanently stationed in space
was proposed by science fiction writer Arthur C. Clarke (most commonly
known for writing the story behind 2001: A Space Odyssey) over ten years
before the first crude satellites achieved orbit. Having a knowledge of
orbital mechanics, Clarke realized that a satellite in a geostationary orbit
could connect, in many ways, any two points that might fall within the large
field of view. In the early days of telephone, it might not have seemed too
unusual to imagine a switchboard in the sky allowing easy, instantaneous
communications between such locations. Though devoid of operators, hun-
dreds of such switchboards exist today and connect millions of places
around the globe from the geostationary band above the earth's equator.

COMMUNICATIONS THEORY

Figure 5-1 reviews some of the terms that describe the characteristics
of electromagnetic waves as discussed in Chapter 4.

wavelength
amplitude

phase

A = wavelength (distance [m,pmn
m = meters p = microns = 1 O*m
f = frequency (cycles per sec [Hz])
= CIA c = 3 x 1 08m/s
Figure 5-1. Electromagnetic wave. The characteristics of an electromagnetic
wave are used to carry information for communications.

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