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GMDSS is the abbreviation for Global Maritime Distress and Safety System.

Original text from "What is GMDSS" WUN 0112 by Ary Boender. Used with permission


The old maritime distress and safety system needed to be improved, that's why the International Maritime Organisation decided that a new, improved system for distress, safety, radiocommunications and procedures, should be established.

The basic concept for GMDSS is that search and rescue authorities ashore, as well as shipping in the immediate vicinity of the ship / persons in distress, will be rapidly alerted to a distress incident so that they can assist in a coordinated SAR operation with the minimum delay. The system also provides for urgency and safety comms and the promulgation of maritime safety info as meteo- and navigational warnings. GMDSS uses both satellite and terrestrial stations. Like most new systems, GMDSS needs specific high- tech equipment. A typical GMDSS ship-station includes a message terminal with keyboard, monitor and printer. Further a remote alarm for distress transmissions and an Inmarsat-C transceiver. Other nice gadgets that may connected are navigators as the MK9 and MK10 for GPS, DGPS, Decca and Loran readout.


A ship must also have a COSPAS/SARSAT EPIRB or an INMARSAT EPIRB onboard (EPIRB = Emergency Position Indicating Radio Beacon). The SARSAT (Search and Rescue Satellite Aided Tracking) satellite monitors 121.5, 406 and 243 MHz and the COSPAS (Russian for SARSAT) satellite monitors 121.5 and 406 MHz, while INMARSAT monitors 1.6 GHz. EPIRBs transmit the callsign of the ship and trigger the various satellites. EPIRB relays can be found in the range 1.544-1.545 and 1.6455-1.6465 GHz. Ships must fit equipment based on their areas of trading, irrespective of their size. The following four areas have been defined:

  • A1: within range of a VHF coast station with DSC
  • A2: within range of an MF coast station with DSC
  • A3: within cover of International Marine Satellite system (INMARSAT)
  • A4: all areas outside A1, A2 and A3

Digital Selective Calling

Digital Selective Calling (DSC) is an integral part of the GMDSS and is used for transmitting distress alerts from ships and for transmitting the associated acknowledgements from coast stations. DSC is basically a calling system. Each call consists of a packet of digitised info of one of four priorities: Distress, Safety, Routine or Urgency. Messages can be routed to 'all stations' or to selected stations by using their selcal code. Distress messages are automatically broadcast to 'all stations'. Terrestially there are a number of channels allocated, one on MF (2187.5 kHz), five on the maritime HF bands (4207.5, 6312, 8414.5, 12577, 16804.5 kHz) and 1 on VHF (Ch.70 - 156.525 Mhz). All these channels are simplex.

DSC is further used for establishing ship-shore communication. A number of paired (duplex) channels are allocated to permit suitably equipped ship or shore stations to alert the other when the originator wishes to establish communication giving the type of communication and the proposed frequencies for that communication. These are on MF/HF. For VHF Ch.70 is again used in simplex.

The information that can be passed by a DSC call includes:

  • The caller's Maritime Mobile Service Identity or MMSI (a number that uniquely identifies the caller, similar to a phone number).
  • The MMSI of the unit being called. This can be a specific unit or a group of units (for example all Coast Guard units).
  • The caller's location. This can be derived directly from an interface to a GPS/Loran/SatNav receiver, or it can be entered manually.
  • The requested working frequency & mode. This is the frequency and mode of emissions that will be used to transact business. DSC is only used for call set-up. Once the call is established the two parties change to a working frequency and mode to continue the call.
  • The priority of call (Distress, Urgent, Safety, Routine). DSC can be used for anything from distress calls, to setting up a routine phone patch through a commercial coast station.
  • For distress calls, the DSC call can even indicate the type of emergency (fire, taking on water, etc).

DSC calls are SELECTIVE because unlike traditional voice radio calls, DSC calls can be addressed to a certain user or set of users. For example, DSC calls can be addressed to:

  • A specific vessel or shore station
  • Receivers in a specific geographic area
  • A specific set of DSC receivers (for example all Coast Guard receivers).

When a DSC receiver "hears" a DSC call it looks at the address, geographic information, etc. It alerts an operator only if the call is intended for that unit. This eliminates the need for a watchstander to constantly monitor the receiver.

As stated before, DSC is primarily designed to establish a radio call. When a DSC call is received, the receiving station will use the DSC protocol to acknowledge the call. At this point the two parties will move to a working frequency and a different mode (voice, fax, RTTY) to complete the call.

  • Format: The "packet" comprises a short burst of information giving the format specifier, the called party identity, the category, the originator identity, a telecommand, addition information such as ship's position, receive frequency, transmit frequency. The information is transmitted with forward error correction (FEC), and the packet contains a checksum, all intended to ensure data integrity.
  • Signal: The F1B (or J2B) packet is transmitted on MF/HF at 100 baud 170 Hz and lasts about 6-7 seconds. On VHF the baud rate is 1200 and the packet lasts approximately 0.5 seconds.
  • Identity: The originating/called station identities are achieved by individual Maritime Mobile Selective-call Identify (MMSI) numbers. These are nine digits long. Provision is also made for calling "All ships" and "All ships in a specified geographical area".

Related Links

Original text from WUN 0112 by Ary Boender. Used with permission