Home' RTCA Documents for Review : DO-224D Contents Appendix A
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A criterion sometimes specified for design of LOS radio links is to obtain first
Fresnel zone clearance; i.e., to let no point on the earth's surface lie within the
first Fresnel zone around the LOS between transmitter and receiver. The first
Fresnel zone is defined as the locus of all points where the total path length
between transmitter and receiver is up to one-half wavelength greater than the
distance along the direct path. Generally, when first Fresnel zone clearance is
obtained, earth reflections do not increase path loss much beyond the free space
path loss for the same distance. The trouble with using this criterion for air-
ground links in the 117.975-137 MHz frequency band is the fact that first Fresnel
zone clearance cannot be obtained at low elevation angles (long ranges) with
practical ground antenna heights. As a result, a long-range communication link
design must take into account fading due to reflection from the earth's surface.
Diffuse reflection from the surface of the earth also causes signal fading. This
phenomenon can be particularly strong in urban areas, where vertical buildings
provide significant reflections over a range of azimuth and elevation angles.
Because diffuse reflections add noncoherently (with random phase) at the
communications receiver, as contrasted to the coherent (in-phase) addition of
specular reflections, the total amplitude of diffusely reflected signals is almost
always smaller than the amplitude of the direct LOS signal. Thus, the effect of
diffuse reflection is to cause fluctuations in the amplitude and phase of the
resultant received signal, but seldom to cancel the direct LOS signal and produce
a deep fade. Fade depth is usually less than 10 dB. Because reflected signal path
lengths can be many wavelengths longer than that of the direct LOS signal, the
amplitude and phase of the reflected signal both change as an aircraft moves
along its flight path. This causes moderately fast frequency selective fading due
to diffuse reflection from the earth's surface, with approximate worst-case delay
spreads up to 200 microseconds and approximate worst-case Doppler spreads up
to 100 Hz.
At some ground sites, foliage attenuation of the VHF radio signal may occur at
low elevation angles. Attenuation of the direct LOS signal, which can cause
either shallow or deep fades, depends upon the path length through the foliage.
Unlike microwave frequencies, radio waves in the 117.975-137 MHz band
penetrate foliage to some extent. Fading due to foliage attenuation varies slowly
with time, is fairly flat across the 117.975-137 MHz frequency band, and has
very small delay and Doppler spreads.
Variations in the decrease in index of refraction with height in the troposphere as
weather changes cause variations in atmospheric refraction (ray bending). Thus,
an aircraft which is just within the radio horizon for a given degree of downward
ray bending may be over the horizon for a slightly lesser degree of downward ray
bending, leading to a deep slow fade when the weather changes. Such fades are
flat across the 117.975-137 MHz band and have very little delay or Doppler
spread. For air-ground paths entirely within the troposphere, one can interpret
changes in atmospheric refraction as changes in the ratio K of effective earth
radius to true earth radius from its nominal value of 4/3. In the United States, a
typical variation in K is from 1.1 to 1.5. For an aircraft at 30,000 ft, for example,
the distance to the radio horizon from a ground antenna at 100 ft height changes
from 204.5 nmi at K=1.1 to 238.8 nmi at K=1.5.
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