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Feature 5 | COMMUNICATIONS SYSTEMS


Sea changes in maritime communication


George Marsh looks at how the introduction of new technology has spurred the dramatic changes in communication trends within the maritime industry over recent years.


morse lamps and lines of sight with signal stations ashore. First, radio made communications over large distances possible by utilising the electromagnetic spectrum. Wireless transmission of morse code, then voice and now voice plus data has been the evolutionary progression. Another sea change came about


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when orbital satellites made it possible to overcome the obstacle of the earth’s curvature more reliably than ducting long radio waves around the earth’s surface. Satellites have since made global maritime communication routine. Digitising the signals has greatly increased capacity and latterly, with huge increases in processing power, we have seen the entry of the Internet and broadband into the marine communications mix. The IMO-backed Global Maritime


Distress and Safety System (GMDSS) has wrapped up several elements into an essential communications core for vessels having to meet Safety of Life at Sea (SOLAS) requirements, and we have witnessed the improvement in distress and safety alerting wrought by the Digital Selective Calling (DSC) element.


ommunication at sea has come a long way since it depended on signal flags, semaphore,


MF/HF communications For many years, medium and high-frequency (from 500kHz to 30MHz) radio was the inter-ship and ship-to- shore mainstay and, in a large number of vessels, it still is. Marine HF radios are also known as SSB (single-sideband) this being the now-standard refinement of amplitude modulation (AM) that makes more economical use of electrical power and bandwidth than the previous dual sideband modulation. Long-wave signals can, to an extent, propagate round the earth’s surface so that ranges of hundreds or thousands of miles are possible with HF, although much depends on the prevailing atmospheric and solar conditions, which can vary widely. At one time successful radio operation was such a ‘black art’ that ships needed dedicated radio operators. Nowadays control has been largely automated and selecting pre-programmed frequencies best suited to current conditions, along with the appropriate transmit power levels, is much easier. Transmit power typically available is in the 100-150W range, but can be up to 500W for some potent HF-SSB sets. MF/HF radio telephony (RT) has its


own characteristics and certain advantages over satellite communication. For instance, calls are basically free, although shoreside


stations may charge for making link calls. Unlike satcom, the system does not rely on a fallible man-made infrastructure. Acquisition and commissioning costs can be lower than for satcom. Users can tune into MF/HF broadcast stations such as the BBC World Service, some coastguard broadcasts and amateur radio. They can receive international time signals, an advantage for traditional astro-navigation. Multi-party conversation is possible; thus users can talk to groups of individuals - hence the phenomenon of MF/HF-SSB ‘nets’. According to one user, “marine SSB is more like a chat room than a phone call”. Single-button digital selective calling for safety and distress purposes has far greater range and chance of being heard with HF than with VHF, the range of which is basically limited to line-of-sight.


Technology converges Today, radio and information technologies have converged to deliver vastly enhanced communication capability. For instance, HF-SSB sets can be used with modems (modulator/demodulators) and tablet or laptop computers to provide digital data-based communication including e-mail messaging - albeit without large-file attachments due to limited data speed, typically 2.4kHz. A radio linked up in this way can send and receive e-mails using services from such commercial providers as Ship Com and SailaMail, as well as items such as weather fax, notices to mariners and even charts. Disadvantages include variable


ICOM IC-7200 high-frequency single side-band receiver. 48


propagation conditions already mentioned, and the need for operators to have a long-range radio licence before they can legally use SSB in transmit mode. Antenna optimisation is crucial while professional planning and installation are necessary for best results.


Ship & Boat International November/December 2008


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