Technical information

quencies are likely to be the most effective. The closer a frequency is
to the MUF, the better it is likely to be.
The effect of solar disturbances including solar flares and magnetic
storms is to change the particle density in the ionosphere. Therefore,
the “window” of useable frequencies may begin to close, with radio
waves of frequencies in the lower range dropping out first as they are
absorbed by the ionosphere.
Next, the radio waves of upper frequencies in the useable “window”
may start to penetrate the ionosphere and go into outer space. It is
even possible for the entire “window” to close, particularly if you are
flying in a polar region in latitudes above 60 degrees north or 60
degrees south. Solar disturbances have the most negative effects on
HF communications in these regions.
If you are flying in polar regions and are having difficulty raising any
ground station located in the same region, remember this: even
though the “window” of useable frequencies may have closed in the
polar regions, another “window” may be open in regions closer to the
equator which are less affected by solar disturbances. Try calling a
station closer to the equator in latitudes lower than 60 degrees north
or 60 degrees south, and use a higher frequency. If you can raise a
station in these areas, that station may be able to relay your mes-
sage.
Description
1-5
KHF 950/990 Pilots Guide
Rev. 0
Dec/96
G
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M
IN
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CRITICAL ANGLE
AIRCRAFT
DE
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AIRCRAFT
EARTH
IONOSPHERE
REFRACTION
Figure 1-1 Effects Of Different Skywave Paths