Page 119 - Intro to Space Sciences Spacecraft Applications

106 Introduction to Space Sciences and Spacecrafi Applications
given names like VHF and UHF. Through international agreements, cer-
tain frequencies have been assigned to particular uses, such as AM broad-
cast and shortwave. These uses have come about mainly due to the prop-
agation properties of the frequencies and their usefulness in carrying
certain types of information. These points will be further explained in the
following sections.
Figure 5-3 shows how electromagnetic waves of certain frequencies
propagate differently. These propagation characteristics and the reasons
behind each are explained as follows:

Direct Waves. Essentially any frequency can be transmitted between two
stations with a direct, unimpeded path between. This is known as line of
sight (LOS) communications and is used in many practical applications
using frequencies in the radio, microwave, and even visible and other
electromagnetic bands.

Ground Waves. At frequencies less than a few megahertz, electromagnet-
ic energy can interact with the material in the earth and tend to follow the
contour of the earth’s surface. This property allows stations to communi-
cate over the horizon (OTH) in longer ranges than direct path stations. As
we will see, however, these lower frequencies are limited in how much
information they can transmit. Frequencies associated with ground waves
are commonly used for public amplitude modulated (AM) broadcasts. Fre-
quency modulated (FM) radio stations broadcast on higher frequencies that
require direct paths between the transmitter and receiver. This is the reason
why AM stations can be picked up generally farther away than FM sta-
tions. (Note: Modulation techniques are presented later in this chapter.)

Sky Waves. Frequencies higher than AM frequencies lose the ability to
follow the surface contour but, up to a certain point, can have an interac-
tion with the ionosphere which allows longer range propagation.

Ionospheric Interaction. The ionosphere was described in Chapter 4 as a
region of electrically-charged (ionized) atmospheric constituents. Electro-
magnetic waves, upon encountering these electrical charges, tend to
change their direction of propagation through refraction. If enough
charges are encountered, the wave can be turned back downward toward
the surface, appearing to be “reflected by the ionosphere at a certain alti-
tude. Depending on the wave frequency and the electron concentration of