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The Global Maritime Distress and Safety System (GMDSS) is an international system relating to all vessels over 300 gross tonnes and all passenger vessels engaged on international voyages, irrespective of tonnage. It provides comprehensive communications for distress, urgency and safety operations in the terrestrial and satellite services. It specifies methods to be used to enable vessels requiring assistance to transmit specific alerting signals to indicate they require help.

There are nine vital communication  functions  which all vessels complying with SOLAS regulations must be able to fulfil, namely:

•    Transmitting   ship-to-shore   distress  alerts  by  at  least  two separate and independent means, each using a different radio communication service

•    Transmitting and receiving ship-to-ship distress alerts

•    Receiving shore-to-ship distress alerts

•   Transmitting and receiving search and rescue co-ordinating communications

•    Transmitting and receiving on-scene communications

•    Transmission and reception of location signals

•    Reception of maritime safety information

•    Transmitting and receiving general radio communications to and from shore based radio systems or networks.

•    Transmitting and receiving bridge to bridge communications.

One of the features of GMDSS enables watchkeeping duties to be performed by automatic means both ashore and on ships. It is unlikely that a manual radio watch will be carried out on the RT distress frequencies in any particular band, therefore it is important to precede any communications with an appropriate alert. There are four levels of priority given to such alerts:

•    Distress: When the vessel or person(s) on board are in grave and imminent danger and require immediate assistance

•    Urgency:  When  the  safety  of  the  vessel  or  person(s)  is threatened and they require assistance. Examples include; not under command and require a tow; vessel overdue; person(s) require medical assistance

•    Safety: These are reserved for meteorological and navigational warnings

•    Routine: Normal alerts to attract the attention of coast stations or other ship stations

It is in the interest of safety that the watchkeepers  are aware of which sea area the ship is in at any time. There are four sea areas within GMDSS. The Admiralty List of Radio Signals Volume 5 provides comprehensive details.

A1 Area

This is an area within radiotelephone range of at least one VHF coast station at which continuous  DSC alerting is available, as defined by a contracting government.

A2 Area

This area excludes area A1 and is within radiotelephone range of at least one MF coast station at which continuous DSC alerting is available, as defined by a contracting government.

A3 Area

This area excludes areas A1 and A2, but is within the coverage range of the

Inmarsat satellite system, between latitudes 70º North and 70º South.

A4  Area

This area covers any sea areas not covered by areas A1, A2 and A3, ie, the polar regions.

Distress Alerting

The primary function of a distress alert is to inform a coast station and/or a Marine Rescue and Co-ordination Centre (MRCC) of the ship’s situation. On receipt of a distress alert, an MRCC will co-ordinate the rescue and will relay details to other ships in the area. If the ship is in distress, the main objective should always be to send the distress alert ashore by any appropriate means. However,  personnel  may  also  consider  alerting  vessels  in  the  vicinity  by sending a distress alert using Digital Selective Calling (DSC) equipment on VHF Channel 70 (for vessels within approximately 20 miles) or MF on 2187.5 kHz (for vessels within approximately 150 to 200 miles).

The distress communication procedure should always be as follows:

a)     Send a distress alert on an appropriate band according to the sea area as listed below. This is a very important action as it attracts the attention of radio personnel enabling them to listen to your distress message.

b)     Expect an acknowledgement from a shore station either by DSC or telephony.

c)     Send a distress call and message on the Radio Telephony (RT) distress frequency  in the same band as the distress alert and follow the instructions given by the MRCC/controlling station.

GMDSS Distress, Urgency and Safety Frequencies in Terrestrial Radio Bands

Sea Area         Band          DSC Alerting Frequency   RT Communications

A1                    VHF                         Channel 70                          Channel 16

A2                    MF                           2,187.5                                 2182

A3/A4              HF*                          4,207.5                                 4,125

A3/A4              HF*                          6,312                                    6,215

A3/A4              HF*                          8,414.5                                 8,291

A3/A4              HF*                          12,577                                  12,290

A3/A4              HF*                          16,804.5                               16,420 (Frequencies are quoted in kHz)

* Select an HF frequency band according to the distance from the nearest HF shore station and the time of day. Generally  speaking,  the higher the band the greater the range. At night, a lower band will achieve greater distances. If unsure, use 8 MHz. (Inmarsat distress procedures are described later.)

Example of Distress Transmission Procedure in Area A1

Systems To Use For Distress Alerting


Procedure on Receiving a DSC Distress Alert

Example of distress call and message by RT:




21° 34” North, 68° 15” West

On Fire

Require immediate assistance


Urgency Alerts

For messages concerning the safety of the vessel or person(s) on the vessel, use the following procedure on any appropriate radio band according to the sea area:

a)     Send a DSC urgency alert.

b)     Send an urgency call and message.

Example procedure of how to request medical assistance from Area A3:

a)     Send  a  DSC  urgency  alert  on  8414.5  kHz,  indicating  the intended RT transmission frequency (8291 kHz) in the call. Do NOT expect to receive an acknowledgement.

b)     Transmit  an  urgency  call  and  message  on  8291  kHz  as follows:

PAN PAN, PAN PAN, PAN PAN, All stations, all stations, all stations,

This is XYZ, XYZ, XYZ

I have crew with severe injuries and require medical assistance, My position is 22° 30” North, 79° 27” West,


Safety Alerts

If  it  is  necessary  to  send  a  meteorological  or  navigational  warning  use the following procedure on any appropriate radio band according to the circumstances:

a)     Send a DSC safety alert.

b)     Send a safety call and message.

Example procedure of how to advise vessels in the vicinity of a danger to navigation and at the same time inform shore stations in Area A1:

a)     Send a DSC safety alert on VHF channel  70, indicating  the intended RT transmission channel in the call. Do NOT expect to receive an acknowledgement.

b)     Transmit the safety call and message on VHF channel 16 (or 13).

SECURITAY, SECURITAY, SECURITAY, All stations, all stations, all stations,

This is XYZ, XYZ, XYZ

Large floating container sighted in position 30° 20” North, 64° 55” West,

Danger to navigation keep a sharp lookout, OVER.

Procedure on the Receipt of a DSC Distress Alert

See diagram above

Procedure on the Receipt of a DSC Urgency or Safety Alert

On receipt of a DSC urgency or safety alert, tune the RT to the frequency indicated in the received alert and await reception of the call and message. Do NOT attempt to acknowledge the urgency or safety alert.

Procedures for Sending Alerts via Inmarsat

Inmarsat-C Distress Alerts

Inmarsat-C is an ideal system for distress alerting and messaging. It can be used from sea areas A1/A2 and A3, but NOT area A4. Inmarsat-C does NOT support voice communications, so all messages appear as text. Inmarsat-C is a store and forward system. There are no live links between the ship and shore authorities, therefore expect a short delay before any response from ashore.

Inmarsat-C Distress Transmission Procedure

a)     Send a distress alert (either designated or undesignated).

b)     Expect a response from an MRCC within 2/3 minutes.

c)     Compose a distress message on the Inmarsat-C editor using the following format:


18° 35” North 77° 58” West

On fire

Require immediate assistance

28 persons on board

d)     Using distress priority and ideally selecting the nearest land earth station (LES) to the ship’s position, send the distress message. If an LES is not selected here it will default to the last used LES. Stand by for further communications from the MRCC.

Urgency or Safety Alerts via Inmarsat-C

If  required  to  send  urgency  or  safety  priority  messages  via  Inmarsat-C, compose the message using the edit facilities. Leave the message on the screen as text, then:

a)     Go to ‘TRANSMIT’ mode.

b)     Select routine priority.

c)     Select the appropriate LES.

d)     Select the special code from the following:

32 to request medical advice

38 to request medical assistance

39 to request maritime assistance

42 to provide weather danger and navigational warnings

e)     Send the message as text.

GMDSS Radio Watchkeeping

At sea, the vessel shall maintain a continuous radio watch on the following:

Frequency/ChPurpose of Watch
VHF Ch. 16 *RT distress/urgency/safety and route call/reply
VHF Ch. 13International bridge-to-bridge safety of navigation
VHF Ch. 70Short range DSC distress/urgency/safety and routine alerts
MF 2187.5 kHzMedium range DSC distress/urgency and safety alerts
HF 8414.5 kHz **Long range DSC distress/urgency and safety alerts
518 kHzReception of NAVTEX MSI
Inmarsat-CReception of EGC MSI including shore-to-ship distress alerts
Inmarsat-BReception of shore-to-ship distress alerts

*   Vessels are required to monitor VHF channel 16 .

** Plus  at least  one other  HF frequency  from  4,207.5,  6,312,  12,577  and 16,804 kHz.

As the vessel has Inmarsat-C,  there is no requirement  to monitor HF DSC frequencies for A3 distress alerts. MF/HF DSC equipment can be configured to watch the 2187.5 kHz frequency only.

General Rules for Communications

All Stations are Forbidden to Carry Out

•    Unnecessary communications

•    The transmission of profane language

•    The transmission of signals without identification

Avoid Interference

All stations are forbidden to carry out the following:

•    The transmission of superfluous signals and correspondence

•    The transmission of false or misleading signals

All stations shall radiate the minimum power necessary to ensure satisfactory service.

Secrecy of Communications

All administrations bind themselves to take the necessary measures to prohibit and prevent the following:

•   The unauthorised interception of radio communications not intended for the general use of the public

•    The  divulgence  of  the  contents,  simple  disclosure  of  the existence, publication or any use whatsoever, without authorisation, of information of any nature obtained by the interception of radio communications

Radio Log Keeping

All vessels are required to keep a radio log on the navigating bridge convenient to the radio installation. It should be available for inspection by any authorised representative of any administration.

The log contains details of the ship’s name, call sign, MMSI number, etc, details of persons qualified to operate the radio equipment and the daily diary of operation of the radio equipment. Entries in this latter part should contain the following:

•    Details  of  communications  relating  to  distress,  urgency  and safety including times and details of ships involved and their positions

•    A record of important incidents such as breakdown or malfunction of equipment, adverse propagation and interference

•    Serious breaches of radio procedures by other stations

•    The position of the ship at least once per day

•    Details  of  the  tests  carried  out  on  radio  equipment  as  in paragraph 5 below

Note: Any messages received as hard copies, such as NAVTEX, EGC, etc, can be appended in date order at the rear of the logbook and  an indication of the time and frequency received can be noted in the log.

Testing of GMDSS Radio Equipment


•    The proper function of the DSC facilities shall be tested at least once per day without radiation of signals, by use of the means provided by the equipment

•    Battery  voltage  should  be checked  once  per  day  and  where necessary brought up to fully charged condition

Weekly Tests

•    Proper operation of the MF DSC facilities shall be tested weekly by means of a test call with a coast station. When out of range of an MF coast station for longer than one week the ship should make a test call on the first opportunity when the ship comes into range of such a coast station

Note: Live tests should NOT be made on VHF DSC equipment.

Monthly Tests

•    Each EPIRB shall be examined monthly by operating its test facility (if this is in line with the owners standing orders) and ensuring that it is able to float free. It should be inspected for security and any signs of damage

•    Each  SART  should  be  tested  by  means  provided  and  by observing rings on nearby 3cm radar

•    Each  survival  craft VHF  shall  be tested  on  a channel  other than channel 16, using a rechargeable  battery not the lithium battery

•    The  radio  battery  compartment  should  be inspected  and  the security  and condition  of all batteries  providing  a source  of energy for any part of the radio installation should be checked

•    Printers should be checked daily to ensure an adequate supply of paper •    The condition of all aerials and insulators should be checked monthly

Brief Description of GMDSS Equipment

Search and Rescue Transponder (SART)

The purpose of a SART is to indicate the position of survival craft or survivors during search and rescue operations. It operates in the 3cm radar band only. When activated, a SART sweeps the 3cm radar band and on receipt of radar pulses from a search and rescue craft it transmits coded signals. This results in a series of dashes appearing on the rescue craft radar display; similar to those of a RACON. The echo nearest to the rescue craft’s own position represents the position of the SART. The minimum range of a SART is 5 nautical miles. In order to achieve this, the SART should be mounted at least 1 metre above sea level in a vertical aspect. If lying in the sea, the range may be limited to approximately 1 mile.

Emergency Position Indicating Radio Beacon (EPIRB)

An EPIRB is a secondary means of transmitting a distress alert ashore – usually from a survival craft. It can be activated manually, but may also be released automatically  by a hydrostatic release mechanism if the vessel sinks. Three types of EPIRB can be used within GMDSS:

•    COSPAS/SARSAT  satellite EPIRB giving coverage of all sea areas

•    Inmarsat-E EPIRB giving coverage in sea areas A1/A2 and A3

•    VHF DSC EPIRB giving coverage in sea area A1 only

All EPIRBs must be capable of indicating the vessel’s ID and position. Vessel ID information  is encoded into the EPIRB by the equipment manufacturer. Positional information can be determined automatically by the COSPAS/ SARSAT satellites from measuring the Doppler effect; by having an in-built GPS receiver or by manually inserting the position via a keypad on the EPIRB. For  COSPAS/SARSAT  EPIRBs,  there  may  be a maximum  of 90  minutes before the alert is received ashore.

Inmarsat-E EPIRBs provide almost instantaneous alerting.

VHF EPIRBs work on VHF channel 70 and send a designated DSC alert to coast stations and vessels within an A1 area. They have an in-built SART for determining position.

Digital Selective Calling (DSC)

DSC is an automated watchkeeping and alerting system operating in the VHF, MF and HF bands. It permits unmanned watchkeeping for distress/urgency/ safety and routine calls in the terrestrial  radio service by having dedicated watchkeeping receivers listening out continuously.

VHFChannel 70Distress/urgency/safety and routine alerts
MF2187.5 kHzDistress/urgency/safety alerts
MF2177.0 kHzRoutine shore-to-ship alerts
MF2177.0 kHzRoutine ship-to-ship alerts
MF2189.5 kHzRoutine ship-to-shore alerts
HF4207.5; 6312; 8414.5; 12577; 16804.5 kHzDistress/urgency and safety alerts
HF4, 6, 8, 12, 16, 18, 22 and 25 MHz bandsPaired  DSC  frequencies  are  available  for routine alerts. Details in ALRS Volume 1

Note: Frequencies shown in red should be monitored continuously by DSC watchkeeping receivers whilst at sea. To receive routine DSC alerts in MF and HF bands an additional scanning receiver must be fitted.

Maritime Mobile Station Identity (MMSI) System

Each mobile station (ship) and shore station having DSC equipment is issued with a unique MMSI number. This number is programmed into all DSC equipment on installation. Self-identification is always automatically included in any DSC transmission. The MMSI system also permits individual stations or groups  of stations  to be called. The allocation  of MMSI  numbers  is as follows:

Ships Stations

9 digits, the first three being the country MID: eg, 232123456.

Shore Stations

9 digits, the first two being 00, then country MID: eg, 002321234.

Group of Stations

9 digits, the first being a single 0, then country MID: eg, 023212345.

Reception of Maritime Safety Information (MSI)

GMDSS provides facilities for the reception of meteorological warnings, navigational  warnings  and shore-to-ship  distress  alerts. SOLAS  regulations require ships to monitor the appropriate frequencies in order to receive MSI in their area.

Short Range MSI

NAVTEX – operating on the following frequencies:

•    518 kHz for English language broadcasts

•    490 kHz for second language, or supplementary broadcasts

•    4209.5 kHz in tropical zones to overcome the effects of MF static

518 kHz has to be included in a NAVTEX receiver. The other frequencies may or may not be fitted according to vessel requirements.

Long Range MSI

•    Enhanced Group Call (EGC): Operating via Inmarsat-C

•    HF  NAVTEX:  Operating  in  areas  where  MF  NAVTEX  and EGC are not available

Details of these systems providing  worldwide  coverage  are to be found in Admiralty List of Radio Signals Volumes 3 and 5.

Facilities on NAVTEX and EGC receivers allow operators to programme reception  of  messages  from different  areas.  EGC  receivers  automatically restrict the reception of messages to the NAVAREA that the vessel is in by awareness  of the vessel’s position via GPS input. The world is divided up into 21 ‘NAVAREAS’,  each having its own provision. Additionally,  choice can be made over the type of warning available for reception. In order not to receive unwanted information, navigators should programme MSI equipment accordingly.

Types of message which can be programmed:

A:  Navigational warning*

B:  Meteorological warning*

C:  Ice report

D:  SAR info (distress alerts relays etc)*

E:  Meteorological forecasts

F:  Pilotage messages

G:  Decca warnings

H:  Loran-C warnings

I:   Omega warnings

J:   Satnav warnings

K:  Other navaid warnings

L:  Navigational warnings additional to letter A* V, W, X, Y:  Special services – trial allocation

Z:  No messages on hand

Note: Message types marked * cannot be disabled.

Satellite Communication Systems

Inmarsat,  the  International   Mobile  Satellite  organisation   provides  high quality voice, telex, data and facsimile circuits to suitably equipped vessels. The system comprises of four geostationary satellites in orbit approximately

36,000kms above the equator. Each satellite provides coverage for a particular ocean region, as below. Communication, via these satellites, at latitudes greater than approximately 70° are unreliable due to the satellites being out of line-of- sight when so far north or south.

The four satellites cover the main ocean regions and are named accordingly:

•   AOR-W             Atlantic Ocean Region West

•   POR                   Pacific Ocean Region

•   IOR                    Indian Ocean Region

•   AOR-E              Atlantic Ocean Region East

Network Co-ordination Station (NCS)

Each ocean region has its own Network Co-ordination Station (NCS) which controls the allocation of channels to MESs and LESs within its region. When a call is initiated, the NCS connects the MES to the LES.

Land Earth Station (LES)

Within each of the satellite ocean regions there are a number of Land Earth Stations (LES). The function of the LES is to provide a connection between the Inmarsat system and national and international  telecommunications  systems worldwide. An LES may also be referred to as a Coast Earth Station (CES).

Mobile Earth Stations (MES)

Each vessel equipped with suitable Inmarsat equipment is known as a Mobile Earth Station  (MES).  Each MES is issued  with a unique  Inmarsat  Mobile Number (IMN). If a user has more than one MES, each will have its own Inmarsat Mobile Number  (IMN).

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