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Module 1 Batteries
Module 2 Wiring
Module 3 Engines
Module 4 Lightning
Module 5 DC Systems
Module 6 Electronics

Module 8 Water
Module 9 HVAC
Module 10 AC Power

Radios-Marine - GMDSS and Marine Communications

Radios-Marine - Communications and GMDSS

At the completion of this Module you will understand the principles of the Global Maritime Distress and Safety System (GMDSS), and how system components operate (Digital Selective Calling (DSC) VHF radio works, MF/HF radios-marine, INMARSAT C, NAVTEX, EPIRBS, SART’s), How to use VHF and SSB Radio, INMARSAT, EMAIL, Weatherfax, Cellular Phones, and how to perform basic maintenance and troubleshooting on all of the systems

What is Global Maritime Distress and Safety System (GMDSS)

The Global Maritime Distress and Safety System (GMDSS) has now been fully implemented. All commercial shipping over 300grt are now on a fully automated and satellite based communications and safety system. The large safety problem is that for yachts, motorboats, trawler yachts and fishing trawlers are now unplugged from this safety system and radios-marine issues are now real. The main elements of GMDSS are INMARSAT C, Navtex, VHF and SSB/HF Telex and Digital Selective Calling (DSC), EPIRBS and SARTS. Try calling an oncoming merchant ship these days, you rarely get a response. For the latest GMDSS and DSC radios go to Westmarine, I bought a Standard radio unit for my yacht.

What is Digital Selective Calling (DSC)

Digital Selective Calling (DSC) is at the heart of GMDSS communications. DSC has a number of important features. It utilizes a single VHF channel, specifically channel 70. A digitally encoded message is transmitted instead of a voice one. For many the obtaining of a new DSC VHF is a vexed and expensive one, however it need not be complicated. A GPS is interfaced to provide the transmitted position data for transmission. There are a number of new GMDSS DSC VHF units available specifically for yachts and smaller vessel, and I have a Standard Horizon radios-marine from West Marine. See full details in my book The Marine Electrical and Electronics Bible. Radios-marine, consider your communications options.


NAVTEX is controlled by the various navigation authorities within each of the 16 Navareas who participate in the provision of meteorological and Maritime Safety Information (MSI) navigational safety information for transmission. NAVTEX is the broadcasting of weather and navigational information by teleprinter although these days flat screen displays also do this without the paper. It is an integral part of GMDSS. Broadcast frequency for NAVTEX is 518kHz and uses a dedicated receiver, permanently tuned to the frequency and an integral printer is used. In normal use, the unit is left on permanently, and can be programmed to receive specific messages. Ranges are typically around a maximum of 250nm. There are some excellent smaller units around such as those from ICS Electronics and I have one of these on my yacht, and as a second step towards GMDSS it is a sensible purchase. If you are buying a used yacht then DSC may already be installed, however make sure you get your MMSI number checked and changed. Weatherfax is still functioning and will be of use to those with HF/SSB radio. Radios-marine, consider your communications options.


Under the GMDSS the world is divided into areas for search and rescue and these areas are based on the NAVAREA zones. The 121.5/243 MHz and 406 MHz EPIRB units are COSPAS-SARSAT compatible. This is a world-wide satellite assisted SAR system for location of distress transmissions emitted by EPIRB's. The 406 MHz beacons can have a unique identification code in the distress message so that precise vessel data can be included. Search & Rescue Transponders (SART’s) are devices that are designed for use in search and rescue. The EPIRB is the best boating-safety electronics on board and will put potential rescue vessels in the area, but the SART transponder will accurately localize your position. The EPIRB is generally the first GMDSS compatible item purchased for the yacht, and if the budget allows and you go deep sea then a 406MHz unit is the way to go.


For the blue water yacht the whole question of long range communications and boating-safety has become a very difficult one. Already the major radio shore stations are closed for VHF radio and HF/SSB operations. Whilst Amateur or HAM radio is now proving its value and worth to the primarily USA based cruisers who use it, and there are some useful commercial E-mail services on HF/SSB are now available, the whole question is still a hard one to address. Iridium is one option and INMARSAT services are somewhat expensive for cruisers, although a range of cheaper options are now becoming available. Radios-marine, consider your communications options. Space Weather and Marine Radio Start leaning about marine-radio and the effects of space weather. More on Space Weather and Marine Radio More about marine-radios and the effects of space weather

Important Radio Resources

If your cruise in the Pacific then you should subscribe to Penta Comstat (VZX). Penta Comstat provides a continuous automatic HF radio gateway to the Internet for email messages. SailMail is an email over HF radio service provided for members of the SailMail Association.

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