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6.5.1.5 Summary
As MF and HF automatic link establishment radios evolve, their design is constantly being
updated with new features and functions that increase the level of automation and
increase the radio’s ability to adapt to changing propagation conditions. A fully adaptive
MF/HF system typically operates under microprocessor control, incorporates automation
for most functions, and is capable of a variety of diversity schemes with a set of channel
intelligence functions for automatically establishing and maintaining links in an adaptive
manner in response to time-varying channel propagation, external noise, and
electromagnetic-compatibility (EMC) conditions.
6.5.2 E-mail and Data Services
6.5.2.1 Introduction
Telex over Radio (TOR), generally referred to as Narrow Band Direct Printing (NBDP) due
to its use of relatively narrow bandwidth in the HF spectrum (channels are spaced 500 Hz
apart) and its direct printing capability has been an important element in maritime
communications for many years. It was widely used up to the introduction of the GMDSS
in 1992. Before the introduction of telex services utilising the Inmarsat space segment,
initially through Inmarsat A and latterly via Inmarsat-C, TOR was the primary method of
telex communications for ships at sea. ITU standards were developed and remain in force
today.
Inmarsat based systems gradually replaced NBDP because they were faster and more
efficient, required less operator skills and were available continuously.
A number of systems have been or are being developed. Norway has been testing an HF
system capable of data communications including e-mail. Such a system might provide
public correspondence and part of the GMDSS in sea areas A3 and A4. Because of the
promising results distress communications may also be considered.
6.5.2.2 GMDSS issues
Subsequent to GMDSS implementation NBDP has been in rapid decline principally
because most GMDSS installations include an Inmarsat-C capability that offers a user
friendly always available telex option. NBDP uses low data rates by today’s standards (50
Baud) and relied on operator intervention, an expensive commodity in an era when Radio
Officers were gradually being phased out for routine communications and shore based
telex systems were also in decline and closing around the world. Furthermore the modern
electronic data communications world was already arriving on board ship with PCs and
networking.
Although NBDP was gradually disappearing, new methods of text communications based
on terrestrial radio (HF) were being developed and growing in popularity. Norway
proposed within the IMO that the mandatory requirements for MF/HF radio equipment to
be fitted with NBDP for ships operating in sea areas A3 and A4 should be reconsidered,
as the system was outdated. At that time COMSAR within IMO determined that NBDP
should be retained until a viable alternative was found that would perform all the present
functions of the NBDP. It was generally accepted that new mobile-satellite and MF/HF email
systems were currently used at sea and could be considered as an alternative at a
later stage, but this would need to be approved and adopted by the IMO for use within the
GMDSS.
Norway has again raised the issue at the IMO with the objective of considering HF E-mail
as an alternative system to HF NBDP and the need to develop to develop performance
standards accordingly. In addition at WRC-03 the ITU modified Appendix 17 of the Radio
Regulations to permit maritime HF frequencies to be used for data transmissions.
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