The Electronic Age - The Science of Navigation - page 206

atmosphere gives rise to refraction of radio waves, which we can under-
stand by considering how the atmosphere appears (to a radio engineer) to
be layered.
The bottom 12 km of the earth's atmosphere is called the troposphere . All
our weather—cloud formation, pressure and temperature variations, pre-
cipitation levels and humidity fluctuations—happens within this layer.
These meteorological e√ects influence the attenuation of radio waves and
thus the range of radio communication. Electrical storms within this layer
interfere with radio transmission and reception. Above the troposphere,
extending from 12 km up to an altitude of 50 km, is the stratosphere .
Here, the air density is pretty constant, and water levels are low. Radio
waves travel in straight lines through this layer, and apart from a slight
attenuation—a small reduction of wave intensity with each kilometer the
waves pass through—the stratosphere is like the vacuum of space so far as
radio-wave propagation is concerned.
Above the stratosphere lies the ionosphere . This layer is thick (extending
upward from 50 km for hundreds of kilometers to the edge of space), but
the air in it is very thin. Despite its low density, the ionosphere has a much
greater influence on radio wave propagation than the much more substan-
tial layers beneath it. Solar radiation ionizes the molecules of air in the
ionosphere, altering the atmospheric electrical conductivity and causing
radio waves that reach it at a shallow angle (say, those that were trans-
mitted horizontally from a ground station) to refract. These waves follow a
curved path that returns them to the lower layers of the atmosphere. Such
waves may reflect o√ the earth's surface and bounce between surface and
ionosphere, or they may refract o√ the lower levels of the ionosphere, pass
through the stratosphere, and refract again o√ the ionosphere. In short, the
surface and the ionosphere act like a waveguide, channeling radio waves
along the gap between them, as illustrated in figure 8.3. This phenomenon
leads to OTH radio; such trapped waves can bounce several times o√ the
ionosphere and travel a long way around the world. 7
The radio waves that follow the curvature of the earth's surface because
of ionospheric refraction are called ground waves . Many of the earliest radio
and RDF applications used ground waves. These waves must have long
wavelengths because shorter-wavelength radio waves are attenuated more
strongly by the troposphere and do not refract so markedly through the
7. OTH radar also exists: long-wave radar signals are bounced o√ the ionosphere to
detect targets that are over the horizon and out of sight.