GMDSS for Dummies

GMDSS – The Idiot’s Guide

Er, what is it?

Global Maritime Distress and Safety System – a system of terrestrial and satellite communications to enable fast response to Distress situations at sea from shore-based resources.

Why? And what is its history?

Radio was first installed on ships around the turn of the 20th century. At the time, radio (or “wireless”, as it was known) was used primarily for transmission and reception of passenger telegrams. Radio watch keeping hours were not standardised, and there was no regulatory requirement for carriage of radio by ships. Indeed, there was a general lack of any sort of regulation of the radio spectrum. Amateur/experimental stations often interfered with commercial stations and vice-versa. But all that changed one cold clear night in April 1912…

The most modern passenger liner of the time, the RMS Titanic, sank on her maiden voyage after a collision with an iceberg. Some 1,500 people were killed in the disaster. Fortunately, 700 odd people were saved, thanks mainly to the efforts of the Titanic’s two Radio Officers, who managed to summon help from nearby vessels.

However, the vessel closest to the disaster (the Leyland liner Californian) could not be summoned, as her Radio Officer had just gone off watch after 12 hours on duty. The Californian managed to establish communications with other searching vessels after the Titanic had sunk. By then, it was too late – 1,500 people, including the cream of American and European society, had frozen to death in the icy waters of the North Atlantic.

The Titanic disaster brought about a number of fundamental changes to Marine Radio:

* carriage requirements and radio watchkeeping hours were standardised;

* message priorities were standardised – ie: distress and safety traffic always has priority over commercial traffic;

* distress frequencies were standardised; and

* radio silence periods were introduced.

The Titanic disaster also served as the catalyst for the introduction of the International Convention for the Safety of Life At Sea (the SOLAS Convention). The introduction of the first SOLAS Convention was delayed by WW1 – it came into force in the 1920’s.

Fast forward to the 1970s: after some 80-odd years of development, marine distress alerting still relied on a human being sitting in front of a receiver. Ship’s Radio Officers sent distress messages using Morse Code (or radiotelephone) in the hope that another ship or shore station would hear the call and respond.

The stage was set for some significant change….

Pre-GMDSS Marine Radio equipment was required to provide operation over a minimum specified range of 150 nautical miles. This was based on the (not unreasonable) assumption that ships usually travelled well-used routes and that there were sufficient ships at sea and shore stations dispersed about the world to receive distress calls.

Without radio, there can be no SAR (Search and Rescue)…

However, if a ship was outside the normal shipping lanes, or was rapidly overwhelmed by the forces of nature, her distress alert might go unheard… many ships have gone to the bottom without any distress signal being sent – they have, to use the common parlance, “sunk without trace”.

The pre-GMDSS systems were, in reality, based on pre-WWII technology. So the 1979 IMO Assembly decided that a new global distress and safety system should be established in conjunction with a coordinated SAR infrastructure to improve safety of life at sea.

The system would take advantage of the latest technological developments. And so was born the Global Maritime Distress and Safety System (GMDSS).

The GMDSS is specifically designed to automate a ship’s radio distress alerting function, and, as a consequence, removes the requirement for manual (ie: human) watch keeping on distress channels.

What does it do?

The basic concept of the new system is that Search and Rescue (SAR) authorities ashore, as well as shipping in the immediate vicinity of the ship or persons in distress will be rapidly alerted so that they can assist in a coordinated SAR operation with the minimum of delay.

The new system moves the emphasis from ship-ship alerting to ship-shore alerting. GMDSS has introduced technology which has completely transformed maritime communications.

The system also provides for urgency and safety alerting, and also for the broadcast of Maritime Safety Information (MSI – weather reports and navigation warnings).

A fundamental of the GMDSS is to take the Search out of Search and Rescue

To whom does it apply, and when does it come into force?


My yacht does not weigh over 300 tonnes and does not carry passengers – why should I care?

Fact: every rescue performed at sea for HK based yachts in recent years has been a co-ordinated response from the HK Marine Rescue Co-Ordination Centre and has been carried out by commercial ships.

You need to be able to talk to the commercial ships.


There is no 2182 kHz listening watch – you can shout ‘Mayday’ all day long and no one will care or hear. For ‘they’, read the MRCCs, Coast Radio Stations and all commercial ships.

If you call the MRCC on a satellite phone to their published telephone number they do not know before they answer the phone if it is a distress call coming in – it could be the pizza man! You MUST use the GMDSS methods of contacting the MRCCs to get fast efficient help!

OK OK OK! I am listening – what, as far as a yacht is concerned, does it consist of?

In the GMDSS, watch on the Distress frequencies is kept by means of automatic DSC (Digital Selective Calling) equipment. Using digital techniques, attention can be attracted automatically without the need for aural watch, and over an increased range. DSC equipment is Selective because the attention can be attracted of All Ships, Individual Ship, Group of Ships or ships within a defined geographical area. If a DSC Distress Alert is sent, it rings an alarm at the receiving end and gets them to switch on their voice equipment so you can communicate.

Equipment we need to be concerned about therefore is:

VHF Radio with Class D DSC. All ships need to have at least one permanently installed VHF radio with full DSC support. In addition, they will be required to carry at least one handheld VHF radio. Although commercial vessels are still required to keep an aural watch on VHF Channel 16 you need to be aware that for many ships this is no longer done. VHF DSC IS A PRIMARY ALERTING AND COMMUNICATION DEVICE.

MF/HF Radio with Class E DSC. All ships which operate more than 150 miles from land (though properly this should be described in GMDSS sea area A3, which in some countries is right inshore) are required to carry two separate methods of communicating to shore or have one method and the ability to fix the communications device while under way. No one does that, so almost all commercial ships have an Inmarsat C system AND an HF DSC radio to fulfill this requirement. Therefore, if you have the same, you can alert them to your distress situation and they will get in contact. THIS IS A PRIMARY ALERTING AND COMMUNICATION DEVICE.

Most yachtsmen call an HF radio an SSB radio, though that refers to the method of transmitting voice, as opposed to the frequency at which it transmits.

NAVTEX/SAFETYNET. NAVTEX transmissions are made by multiple countries and ports round the world. They consist of regular weather reports and navigational warnings as well as meteorological warnings as and when required. NAVTEX is primarily a coastal weather warning system (up to 150 miles offshore).

SAFETYNET information is broadcast via EGC on Inmarsat C and carries similar information, though primarily for the high seas.

INMARSAT – the satellite system designated by the IMO to carry this distress and safety information. Distress and safety communications are centred on Inmarsat C, a text messaging service capable of broadcast text messaging via satellite. Inmarsat C and Mini-C terminals have a distress button which will generate a message to be sent to the MRCC automatically. THIS IS A PRIMARY ALERTING AND COMMUNICATION DEVICE.

Note that this is NOT a voice telephone system. It may be a good idea to have such a system as well. Commercial systems which include voice and support Inmarsat C are readily available but are, of course, relatively expensive.

Other GMDSS-supported Inmarsat systems are Fleet 77 and Inmarsat B. Both of these are likely to be both too bulky and too expensive for the individual yachtsman.

AIS – Automatic ship Identification System. All commercial vessels transmit a signal at frequent intervals which can be picked up by an AIS receiver and displayed on a screen such that the course and speed of all vessels within range can be easily seen. Yachts can optionally carry a transmitter which will send the same information out to anyone listening. Also broadcast is the Ship’s Maritime Mobile Service Identity. This looks just like a phone number, and once you know a ship’s MMSI you can call it up on your DSC radio by punching in its number. How do you know a ship’s MMSI? Either they have told you already, or you read it on the screen from its AIS transmissions. AIS is not properly part of the GMDSS and is more of a collision avoidance system, but is nonetheless a low-cost system of very high importance to leisure marine users.

EPIRB – Emergency Position Indicating Radio Beacon. Once activated it transmits a signal to a satellite, while floating in the water, giving its position and thereby indicating that a vessel is in distress. This also uses the MMSI number as a unique way of identifying the ship. GPS enabled versions of EPIRBs (commonly called G-PIRBs) communicate with geostationary satellites, meaning that your signal gets down to land much faster – typically 10 minutes rather than 90 minutes. NOTE: the EPIRB is considered a SECONDARY alerting device. It normally indicates to the SAR authorities that you have abandoned ship and can therefore no longer be communicated with except at close range using a handheld VHF radio.

PLB – Personal Locator Beacon. The same technology as an EPIRB except that they are required to be able to transmit for 24 hours rather than 48 hours duration (ie smaller batteries) and they are not designed to transmit from a floating position, but instead are meant to be hand held. So, the same electronics, but shorter battery life, and therefore small enough to put in a pocket. In most large countries there is a central register as of course these are best registered to an individual. In Hong Kong, there is also now a register for serialized HK encoded PLBs (note that means you cannot buy them from overseas as it must have a HK serial no). Please remember though that a PLB is NOT an EPIRB – it will not transmit automatically when immersed in sea water, and cannot be registered as the primary EPIRB for your boat.

SART – Search and Rescue Transponder – designed, when activated, to provide a track back to the unit on a searching vessels radar screen. All commercial ships are required to have two or more on board. Prime usage is in a liferaft. It will tell the people in the raft when a radar is detected so that they know when it might be appropriate to fire flares. It makes X-band ship radars (mandatory on ships) to radio direction finding devices so they see your position

All that? How do I get to know about this and understand how to use it correctly?

The RHKYC now runs regular non-SOLAS vessel GMDSS courses – commonly called the Long Range Certificate or LRC course. The RHKYC is an approved Radio Training School recognized by the Office of the Communications Authority (OFCA), the regulatory body in Hong Kong.