GMDSS for Small Craft

by Alan Clemmetsen

96pp. 20 black and white photos, 20 diagrams


ISBN 1 898660 38 7

Table of Contents

1. A Brief History of the SOLAS Convention5. GMDSS Licensing Requirements
2. An Overview of GMDSS6. The Shore Station Infrastructure
3. GMDSS Communication Links7. Why not just use a Cellular Radio?
4. Recommended GMDSS Equipment Fitting for Small Craft8. The Future Development of GMDSS

Excerpt from Chapter Two "An Overiew of GMDSS"

It is a common misconception that GMDSS is simply a means of distress alerting. In reality, it is far more that that. The various elements of GMDSS are described below:

Distress Alerting
This can occur in three ways, ship to shore, ship to ship and shore to ship. The initial alert should be from ship to shore. If terrestrial (rather than satellite) radio links are used, nearby ships will also hear the alert, although it will normally be up to a shore-based Rescue Co-ordination Centre to respond with an acknowledgement. It must be emphasised that vessels at sea should not normally acknowledge receipt of an initial distress alert.

Distress Relay
Once an RCC has heard and acknowledged a distress, it may wish to alert other vessels in the area by means of a Distress Relay. This can be addressed to a precise geographic area so that vessels too far away to render assistance will not become involved unnecessarily. Any vessel receiving a distress alert directly, or a distress relay, must contact the appropriate RCC to offer assistance. It must be emphasised that vessels at sea should not normally send a distress relay themselves.

Search and Rescue
Up to this point, all communication links will have been one way only. Once the search and rescue phase is entered, all communication is two-way to co-ordinate the activities of ships and aircraft via whatever terrestrial and satellite communication links are available. Specific frequencies and radio channels are allocated for this purpose.

On-Scene Communication
Vessels and aircraft close to the casualty will communicate between themselves using short range terrestrial communication (VHF or MF). Again, specially-designated radio channels will be used.

Precise location of the casualty will be aided by the use of a Search and Rescue Radar Transponder (SART) or the 121.5 MHz section of a satellite EPIRB (Emergency Position Indicating Radio Beacon). Both of these can be carried in, or attached to, a liferaft.

Promulgation of Maritime Safety Information (MSI)
Information regarding potential navigation or meteorological hazards, weather forecasts and changes or malfunctions of aids to navigation such as GPS or Decca/Loran, as well as search and rescue information are promulgated via NAVTEX, International SafetyNET® (Inmarsat C EGC) and some NBDP (Radio Telex) services.

General Communications
General communications between vessels and between vessels and harbour authorities, pilots, coastguards etc. are also provided for within GMDSS. Calls of this nature can be set up to be fully automatic. Provision also exists for calling a group of vessels using a common, temporary MMSI (Maritime Mobile Service Identity) and for calling all vessels within a defineable geographic area.

Bridge to Bridge Communication
Communication between vessels at sea for purposes of safety and collision avoidance is conducted on VHF Channel 13 (see p. 45).

The overall operation of GMDSS is illustrated by the following series of diagrams.

They show the three main stages of effecting a rescue:

The intial alert can be sent in a number of ways:

All these methods give the vessel's identity (MMSI) as well as its location. A DSC alert is the only category that can be picked up by another vessel. All categories can, however, be detected by suitably equipped coast stations and RCCs.

Other vessels in the area can then be alerted:

On-scene communication takes place either via satellite or terrestrial radio links, with final homing-in on casualty being achieved with the aid of a SART (Search and Rescue Radar Transponder) or 121.5 MHz EPIRB. 406 MHz and 121.5 MHz EPIRBs are frequently combined in one unit.

Under all circumstances, a shore-based Rescue Co-ordination Centre takes charge of the operation. Such is the quality of modern communication, that this may be located as much as a hemisphere away from the actual casualty.

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Last updated: 2. Januar 2001