The following Marine Electronics excerpt on GMDSS for small vessel safety and is extracted from my best selling books The Motorboat Electrical and Electronics Manual and the Marine Electrical and Electronics Bible. You can order direct from Amazon, click on the book image
The Global Maritime Distress and Safety System 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 crusing and sailing yachts, motorboats, trawler yachts are now virtually unplugged from this safety system although its changing.
The main elements 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 on VHF 16, you rarely get a response.
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 item purchased for the yacht.
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-communications 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.
Digital Selective Calling (DSC). 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 DSC VHF units available. Remember licensing requirements are different, and a new course and certificate is also required. See full details in my books
For the offshore 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 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. If you spend time cruising around Key West then you may only need the short range DSC radio, if you go further down into the Caribbean something better is required. All About GMDSS