Page 30 - NAVAL FORCES 03/2017

C5I

Features

Alix Valenti
Something Old and

Something New

High frequency (HF) radio, one of the most utilised bands of com-
munications for navies around the world, was briefly overshado-
wed by the arrival and development of satellite communications
(SATCOM). However, SATCOM’s own limitations, combi ned with
the efforts of the industry to overcome HF’s shortcomings, have
brought back to the fore the key advantages of HF communi-
cations.

Understanding HF radio
Navies around the world make extensive use of HF bands (ranging
from 3 to 30 megahertz (MHz)) for their communications, whether
ship-to-ship or ship-to-shore, because they can travel across a
wide variety of distances. A paper published by the Australian
Government Bureau of Meteorology, ‘IPS Radio and Space
Services’, indicates that there are three types of HF propaga-
tion: ground waves, for short distances of up to 100km over
land and 300km over sea; direct or line-of-sight wave;
and, sky wave, which can travel all distances and works
through reflection in the ionosphere.
The ionosphere is the upper part of the atmos-
phere, which extends from about 50km to 2000km
above the Earth’s surface. Within the ionosphere,
radiation from the sun, such as X-rays, cause gas
molecules to release electrons, which are nega-
tively charged and are called ‘free electrons’,
whereas the molecules that lost the elec- United Launch Alliance (ULA)
Atlas V rocket carrying the
trons are called ‘ions’ and are positively third Mobile User Objective
charged. It is the presence of free elec- System satellite for the
trons and ions in the ionosphere that U.S. Navy creates a light trail
as it lifts off Tuesday, Jan. 20, 2015
affects HF radiation moving upward from Space Launch Complex-41
by making them bend back toward at 8:04 p.m. EST.

28 NAVAL FORCES III/2017