BendixKing KHF 990 - Manual (Page 17)

There are even times when solar disturbances improve the usability
of higher frequencies in the HF band, particularly in equatorial
regions. Another phenomenon which occurs during solar distur-
bances may allow you to communicate with a station even though the
“window” is closed. This is known as scatter propagation, in which a
radio wave is broken up in the ionosphere and scatters in various
directions. Refer to the discussion of geophysical alerts in
Appendix
A for information on broadcasts which announce solar disturbance
phenomena, and how to interpret these broadcasts.
Because frequency propagation cannot be predicted with total accu-
racy, ground stations responsible for aircraft HF communications will
typically operate on several different frequencies within the HF band.
The pilot is then able to choose the optimum communication frequen-
cy for the existing ionospheric conditions.
One feature that will be particularly useful when a trial and error
method is used to find an HF frequency which is working well. This is
the system’s capability to be programmed by the pilot with 99 chan-
nels (using the KCU 951 Control Display Unit), 100 channels (using
the KCU1051 control display unit) or 19 (using the KFS 594 miniature
control display unit). Rather than having to select the four to six digits
each time you want to try another frequency, you can preprogram the
frequencies you need to contact a particular ground station. Then if
you call and fail to get through, you just change to another channel.
(Automatic channel selection for optimum communications reliability
is simplified with the addition of Automatic Link Establishment (ALE),
available on the KCU 1051 Control Display Unit.
NOTE: It is advisable to program at least three frequencies for each
station you plan to contact, in case one frequency suddenly becomes
unusable. During times of solar disturbances, a useable frequency
can fade out in less than a minute. And the “window” of useable fre-
quencies can shift rapidly during solar disturbances or during sunset
and sunrise when the level of ionization in the ionosphere is chang-
ing rapidly.
Tables 1-1 and 1-2 show typical pro pagation distances after one
reflection from the ionosphere for various frequencies during different
hours of the day and for different seasons of the year. It may prove
helpful in selecting the optimum HF frequency for the communica-
tions distance your operation requires.
Description
1-6 KHF 950/990 Pilots Guide
Rev. 0
Dec/96