Hemisphere GPS MBX-4, User Manual

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6
1: Introduction
absorb the power of the transmission more than higher conductivity
surfaces, such as sea water or arable land.
A radiobeacon transmission has three components: a direct line of sight
wave, a ground wave, and a sky wave. The line of sight wave is not
significant beyond visual range of the transmitting tower, and does not
have a substantial impact upon signal reception.
The ground wave portion of the signal propagates along the surface of
the earth, losing strength due to spreading loss, atmospheric refraction
and diffraction, and attenuation by the surface over which it travels
(dependent upon conductivity).
The portion of the beacon signal broadcast skywards is known as the
sky wave. Depending on its reflectance, the sky wave may bounce off
the ionosphere and back to Earth causing reception of the ground wave
to fade. Fading occurs when the ground and sky waves interfere with
each other. The effect of fading is that reception may fade in and out.
However, this problem usually occurs in the evening when the
ionosphere becomes more reflective and usually on the edge of
coverage areas. Fading is not usually an issue with overlapping
coverage areas of beacons and their large overall range.
Atmospheric attenuation plays a minor part in signal transmission
range, as it absorbs and scatters the signal. This type of loss is the least
significant of those described.