b. To meet this potential, the military system must not only include the fixed sites, which will be located in
selected politically stable areas, but must also have available transportable equipments to move rapidly to areas
where emergency demand might arise.
5. Capacity. The military services, for reasons explained previously, require more and more communications
channels. The need for increasing channel capacity is especially true when handing digital traffic.
6. Quality. The quality of the transmission required in the military system differs from that in commercial
practice. The expected increase in digital traffic will place increased demands on the military systems in terms
of digital quality. In order to achieve a high probability of error-free transmission, quality checks will be
necessary in most military digital channels.
LEARNING EVENT 3: LIMITATIONS OF PRESENT LONG-DISTANCE TRUNKING SYSTEMS
The facilities for long-distance trunking most generally used in present-day systems consist of HF, ultra high
frequency (UHF) ionospheric scatter, and microwave equipment. These facilities may also include tropospheric
scatter or multihop line-of-sight (LOS) installations, as well as military and commercial cables. All of these
trunking methods have inherent limitations which prevent them from meeting entirely the requirements for
which they were designed.
1. High frequency limitations.
a. The propagation of HF signals over long distances depends upon the refraction that occurs in the
ionosphere. The height of the ionized layer and the degree of ionization are subject to pronounced variations,
daily and seasonally. The degree of ionization is also influenced by sunspot activity-the effect of increased
sunspot activity being to increase the maximum usable frequency. Since there is an observed cycle of
approximately 11 years in the sunspot activity, the usable portion (for long-distance communication) of the HF
band also varies in approximately an 11-year cycle. The usable signal bandwidth may need to be reduced by
half its normal value during periods of most severe sunspot activity to maintain reliable communications.
b. Channel capacity in the 3-to 30-megahertz (MHz) HF band is limited most directly by the crowded
conditions of the band. This band must be shared by many users, foreign and domestic, and the portions
allocated to DCS add up to only a small fraction of the 27-MHz spectrum. The increasing use of single-
sideband suppressed carrier transmission for DCS circuits has permitted more efficient utilization of the
available bandwidth slots; but circuit requirements also have grown and the band is now more crowded than