BendixKing KHF 990 - Manual (Page 161)

be considered generally equivalent to the more well known
“Greenwich Mean Time” (GMT).
The most frequent sounds heard on WWV and WWVH are the puls-
es that mark the seconds of each minute, except for the 29th and
59th seconds pulses which are omitted completely.
GEOPHYSICAL ALERTS ON WWV
WWV broadcasts geophysical alert announcements at 18 minutes
past each hour. (A similar recorded message can be heard by calling
commercial number (303) 497-3235.) These broadcasts contain
information on solar activity which can cause disturbances in the
earth’s magnetic field which, in turn, can reduce the effectiveness of
HF propagation in the ionosphere. However, this geophysical alert
data is in a raw form and must be interpreted by the pilot if he wants
to determine what this information means for HF propagation condi-
tions. These broadcasts do not make HF propagation forecasts which
predict the best frequencies to use.
However, several values reported on these broadcasts provide useful
clues as to whether HF propagation conditions are likely to be good
or bad. The following discussion may aid in determining the meaning
of the geophysical alert announcement.
The disturbance of the earth’s geomagnetic field is caused by solar
particle radiation. The K index measures how disturbed the earth’s
geomagnetic field is. It is taken every three hours. Generally speak-
ing, the higher the K index, the poorer HF propagation will be.
Possible K index values run from 1 to 9. K values of 5 or 6 indicate
you may have difficulty communicating on HF. This is particularly true
if you are flying north of 60 degrees North or south of 60 degrees
South because geomagnetic disturbances cause HF communications
to deteriorate more in polar regions than in middle latitudes or around
the equator. Geomagnetic disturbances can even improve the avail-
ability of higher frequencies in equatorial latitudes.
If K index values are 7, 8 or 9, HF communications may be impossi-
ble in polar regions. If you are flying above 60 degrees North you
might try calling a ground station closer to the equator in a region less
affected by magnetic disturbances. If you are flying south of 60
degrees South, also call a station toward the equator.
As HF communications deteriorate in polar regions, you are likely to
lose the use of the lower frequencies in the window of usable fre-
quencies first. Then the higher frequencies may start to go as well.
Services
8-3 KHF 950/990 Pilots Guide
Rev. 0
Dec/96