Generator of DSC, Inmarsat and Navtex messages

  • Generator of DSC, Inmarsat and Navtex messages
  • Sending a DSC call
  • Digital selective calling
  • Contents of a DSC call
  • MMSI
  • Communication channel
  • Types of calls
  • Distress types
  • Coordinates and time
  • Procedure
  • Marine VHF radio
  • VHF transmission range
  • VHF channels
  • Priority channel 16
  • Channel 70
  • Transmission power
  • Rules of use
  • Communication with other ships
  • Communication with coast stations
  • Distress communication
  • Types of VHF radio sets
  • Sending an Inmarsat message
  • Sending a Navtex message
  • References
  • Generator of DSC, Inmarsat and Navtex messages

    The purpose of this software is to generate DSC, Inmarsat and Navtex messages and send them to targeted equipment. This software

  • keeps list of ship radio stations
  • sends and receives DSC calls to/from ship stations
  • sends and receives Inmarsat C calls to/from ship stations
  • sends and receives Navtex calls to ship stations
  • simulates functions of coast radio stations (e.g. sends distress acknowledgements to ships)
  • The listbox in the upper half of the window contains a list of registered ships with working radio eqipment. For standalone (1 computer) mode it will be a single ship. This list is dynamic; in networked mode, if a new student starts a radio simulator on his computer, this ship station is registered and automatically added to the list; if a student quits his simulator, or switches to standalone mode, this radio station disappears from the list. Anyway, the list contains ship stations to whom the user of this program (actually an instructor) can send messages, whose parameters are specified at the lower part of the application window.

    To send a message to a ship station, do the following :

  • Click at a line of the list to select a ship. The text marked by "Send to MMSI..." will show the MMSI of the target ship; while the text "Send to group MMSI..." will display a target group MMSI used while sending group DSC calls
  • Click "Message" menu item to select a type of message
  • Use the corresponding dialog appeared to send the message
  • Sending a DSC call

  • Specify the sender type in "Send from..", either ship or coast radio station, and select the sender MMSI from the two corresponding combo boxes
  • Select a DSC call type, such as
  • Routine (sent to a single ship with the selected MMSI)
  • Group (sent to a flotilla designated by the selected group MMSI)
  • Urgency (sent to all ships accessible from the current sender position)
  • Safety (sent to all ships accessible from the current sender position)
  • Routine acknowledgement (sent to the sender of most recent individual call received)
  • Distress acknowledgement (sent by a coast station only to the sender of most recent distress call)
  • Press the [Send] button
  • Digital selective calling

    Digital selective calling (DSC) is part of GMDSS (Global Maritime Distress and Safety System) intended to initiate voice communications between ships, ships to coast and coast radio stations to ships. Specifically, it is designed to eliminate the need of constant radio watch on distress channel 16. Initially, DSC controllers were mostly separate devices connected to a separate radio. Nowadays, the radio and DSC controller are combined in a single device. In addition, they are usually connected to a GPS receiver to always have available current coordinates of the ship, which are included into a DSC call, what is especially important for distress alerts. VHF DSC calls are transmitted on VHF channel 70.

    Contents of a DSC call

    The information is transferred by radio as binary data containing a set of predefined parameters, like MMSI, coordinates, nature of distress etc.


    MMSI is short for Maritime Mobile Service Identity, the 9-digit number used for identification of a separate ship, group of ships or a coast station. Ship MMSI starts from non-leading zero, the first three digits identifying its country by its MID (Maritime Identification Digits) in the form MIDXXXXXX Groups of ships (fleets) have an MMSI for a group (group MMSI), starting from one leading zero : 0MIDXXXXX DSC calls addressed to a group MMSI are received by all ships in the group. Coast station MMSI starts from two leading zeroes : 00MIDXXXX

    Examples are

    - Dover 002320010
    - Solent 002320011
    - Portland 002320012
    - Brixham 002320013
    - Falmouth 002320014

    Communication channel

    This is channel number suggested for voice communication. For ship-to-ship communcations (routine and group calls) it is selectable by the caller from the four : 6, 8, 72 or 77 which are specially reserved for ship-to-ship communications. Safety, Urgency and Distress calls offer channel 16 for communcation.

    Types of calls

    Most common DSC types of calls are the following :

    - routine
    - all ships safety
    - all ships urgency
    - group
    - distress alert

    Distress types

    Distress type specified in a DSC messages can be

    - undefined
    - abandoning
    - piracy
    - MOB (Man-Over-Board)
    - fire
    - flooding
    - collision
    - grounding
    - listing
    - sinking
    - adrift

    They are selectable when a distress alert is being sent.

    Coordinates and time

    Coordinates and time are usually set by a GPS receiver connected to a DSC controller. If not, they can be set manually.


    The caller selects call type and specifies all data relevant to the type of call selected. When the caller sends a DSC message, its VHF radio automatically switches to the specified channel to wait for a reply (acknowledgment). On a receipt of a call, the receiver displays the contents of the message on the screen and once the call is acknowledged, automatically switches its radio to the channel suggested for communication. This begins voice communication. Distress alerts are acknowledged by coast radio stations.

    Marine VHF radio

    Marine VHF radio is used for a wide variety of purposes, such as

    - rescue operations
    - communicating with harbours and marinas
    - collision avoidance

    Use of VHF radio for collision avoidance is strongly discouraged by the UK. The reason is a possibly wrong identification of a ship with which a voice communication is established in case of more than two ships in vicinity. It is required to use Collision Regulations uppermost.

    VHF transmission range

    VHF frequency signals propagate in straight lines and not reflected back off the ionosphere, so their range is limited by the straight visibility lines. This is why, because of the Earth surface curvature, the range quickly increases with the height of antenna. A typical range for two yachts with antennas mounted at the top of mast (approx. 10m height) is 12 NM. In case of communication from a ship to a shore station with very high antenna the range increases to approx. 30 NM.

    VHF channels

    Marine VHF frequencies range from 156 to 174 MHz. They grouped as VHF channels, each having a pair of frequencies, one for transmission (TX frequency) and one for reception (RX frequency). If the two frequencies are equal, the channel is called simplex, otherwise it is called duplex. Marine VHF mostly uses simplex transmissions, where communication can only take place in one direction at a time. A transmit (push-to-talk or PTT) button on the set or microphone is used to start transmission and released to return to normal RX state. The duplex channels are designed to enable a communication which take place in both directions simultaneously and only can be used for communications from ships to shores. Please note that two ships cannot communicate to each other on the same duplex channel, as their RX and TX frequencies do not coincide.

    Priority channel 16

    Channel 16 (156.8 MHz) is the international calling and distress channel. This can be used to transmit distress messages only. Any routine communication on this channel is prohibited. All radio sets have a special button to terminate all other operation and quickly switch to channel 16 and restore transmission power to higher 25W.

    Channel 70

    Channel 70 is allocated by GMDSS (Global Maritime Distress and Safety System) for transmission of DSC (Digital Selective Calling) calls. These are not voice, but transmission of binary data by radio, including ship identification by its MMSI (Maritime Mobile Service Identity) number. Use of channel 70 for transmission of voice messages is prohibited.

    Transmission power

    Radio sets have special buttons to lower transmission power from standard 25W to lower 1W to lower power consumption. The choice of power output depends on the distance of transmission and transmitting conditions. Iternational regulations state to use the minimum power possible for satisfactory communication.

    Rules of use (excerpts from [2])

    The use of VHF radios is ruled by the International Telecommunications Union's (ITU) Radio Regulations. The main rules are the following : - Channel 16 may only be used for distress, urgency and very brief safety communications and for calling to establish other communications which should then be concluded on a suitable working channel. - Channel 70 may only be used for Digital Selective Calling not oral communication. - On VHF channels allocated to port operations or ship movement services such as VTS, the only messages permitted are restricted to those relating to operational handling, the movement and the safety of ships and to the safety of persons. - All signals must be preceded by an identification, for example the vessel's name or callsign.

    Communication with other ships (excerpts from [2])

    For routine ship-to-ship communications, channels 6, 8, 72 and 77 are available in UK waters. Channel 13 is used on a worldwide basis as a navigation safety communication channel, primarily for intership navigation safety communications. It may also be used for the ship movement and port services. The ship called may indicate another working channel on which further transmissions should take place. The calling ship should acknowledge acceptance before changing channels. When possible, the lowest transmitter power necessary for satisfactory communication should be used. When addressing another party, the corresponding ship names should be used, for example,

    "Seaship, this is Port Radar, Port Radar, do you have a pilot?"
    "Port Radar, this is Seaship, I do have a pilot."

    Communication with coast stations (excerpts from [2])

    On VHF channels allocated to port operations service, the only messages permitted are restricted to those relating to the operational handling, the movement and safety of ships and, in emergency, to the safety of persons. Communications should be carried out on the channel indicated by the shore station. When a change of channel is requested, this should be acknowledged by the ship.

    Distress communication (with excerpts from [2])

    Distress calls/messages have absolute priority over all other communications. When heard, all other transmissions should cease and a listening watch should be kept. Any distress call/message should be recorded in the ship's log and passed to the master. On receipt of a distress message, if in the vicinity, immediately acknowledge receipt. Channel 16 may only be used for distress, urgency and very brief safety communications and for calling to establish other communications which should then be conducted on a suitable working channel. Whenever possible, a working frequency should be used for calling. If a working frequency is not available, Channel 16 may be used, provided it is not occupied by a distress call/message. To send a distress message, the following procedure should be followed.

    (1) Set VHF channel 16

    (2) Press and hold PTT button and say :

    Mayday Mayday Mayday
    This is (ship name or callsign followed)
    Mayday (ship name or callsign followed)
    Position (say position coordinates)
    Nature of distress (say)
    Aid required (say what is required)
    Some additional information about the ship (number of persons onboard)

    (3) Listen for a reply and if none heard repeat the above procedure.

    Types of VHF radio sets

    They can be fixed or portable. Fixed sets are mounted on ships with reliable power supply, high antenna, bigger display and higher transmitting power. Portable radio sets are carried to a lifeboat.

    Sending an Inmarsat message

  • Type the message text in the edit box
  • Specify type of the call. In case of EGC SafetyNET call, this message will be sent to all ships, otherwise to a ship selected in the list before starting the dialog
  • Press the [Send] button
  • Sending a Navtex message

    The message structure must be formatted as a Navtex message. A NAVTEX message consists of

  • letters ZCZC which mark the start of message
  • a space
  • four characters B1, B2, B3 and B4
  • carriage return and line feed (to start a new line)
  • the text of message
  • the letters NNNN to identify the end of message
  • where B1 is a character identifying the station, B2 is a message category. B3 and B4 - message serial number. Message categories :
  • A Navigational warnings
  • B Meteorological warnings
  • C Ice reports
  • D Search & rescue information, and pirate warnings
  • E Meteorological forecasts
  • F Pilot service messages
  • G AIS messages
  • H LORAN messages
  • I Not used
  • J SATNAV messages (i.e. GPS or GLONASS)
  • K Other electronic navaid messages
  • L Navigational warnings - additional to letter A (Should not be rejected by the receiver)
  • T Test transmissions (UK only - not official)
  • V Notice to fishermen (U.S. only - currently not used)
  • W Environmental (U.S. only - currently not used)
  • X Special services - allocation by IMO NAVTEX Panel
  • Y Special services - allocation by IMO NAVTEX Panel
  • Z No message on hand
  • References

    1. Wikipedia.
    2. Maritime and Coastguard Agency. Marine Guidance Node MGN 324 (M+F). Radio: Operational Guidance on the Use Of VHF Radio and Automatic Identification Systems (AIS) at Sea.