Global Positioning System Reference
In-Depth Information
transmitter and receiver, one cannot guarantee a good signal. There must also be
sufficient clearance at all places along the path due to the interferences within
what is known as the “Fresnel zone.”
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Figure 6.8
Mobile field strength effects.
The signals arriving at a moving antenna may be coming from many
directions due to reflections and are known as multipath signals. Standing waves
can occur where signal reflections add and subtract so the effect, if moving, is a
regular deep fade in signal at the rate at which standing waves are passed. The
most dominant reflection is that between the direct signal and a version that has
reflected off the ground. These are shown at the bottom of the diagram and can be
many tens of decibels deep in level. Fading and all the other variables in the
propagation environment makes it very difficult to measure or predict exactly
what signal strength will be at any place. The best estimate that can be made will
be as a statistic. For example, a mean-level prediction may be made within a
square, the sides of which may be many hundreds of wavelengths long but with a
variance that is perhaps 30dB. A spot measurement on one day will almost
certainly be different when taken at exactly the same place on another day.
Another factor moving the levels around is the motion of the antennae especially
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The Fresnel zone is a region of space between a transmitter and receiver that takes the form of a solid
ellipse. If the edge of any object obstructs the zone, it can cause very significant attenuation of signals.
The zone's geometry depends on the frequency of transmissions and the spacing of the end nodes. In
the center of the zone, at least 60% of the zone must be clear for a useful transmission.
