Digital Selective Calling

Purpose of DSC
Digital Selective Calling provides automated access to coast stations and ships. The message information is stored in the receiver and can be displayed or printed out following receipt.
There are four levels of priority �

  • distress
  • urgency
  • safety
  • routine They are available for DSC calls.
At the coast station, ship-to-shore distress calls receive priority handling and are routed to the nearest Rescue Co-ordination Centre (RCC). On board ship, DSC receivers sound an alarm when a distress call is received.

DSC Concept
DSC is a technique of transmitting digital codes which allow suitably equipped stations to:

(a) Transmit and receive distress alerts.
(b) Transmit and receive distress alert acknowledgments.
© Relay distress alerts.
(d) Announce urgency and safety calls.
(e) Initiate routine priority calls and set up working channels for subsequent general communications on R/T or telex.
The detailed DSC procedures are contained in ITU-R Recommendation 541 (always refer to the latest version).
DSC channels have been allocated in the 2, 4, 6, 8, 12 and 16 MHz MF/HF bands and on VHF channel 70.
DSC Call Format
All DSC calls automatically include phasing signals, error-checking signals and the identity (MMSI number) of the calling station. The protocol includes an initial dot pattern which is used to alert scanning receivers that a DSC call is imminent. Other information can be added, either manually or automatically. The actual information added is dependent upon the purpose of the call.
The DSC call is set up by entering information, using the command menu of the DSC controller that is attached to, or incorporated into, the transmitter.

MMSI Numbers
Each ship station has its own unique nine-digit Maritime Mobile Service Identity (MMSI) which is included automatically in each call. Included in the MMSI number are the Maritime Identification Digits (MID) which identify the country licensing/controlling the station.
Three types of MMSI numbers are in common use to identify individual ships, groups of ships and coast stations.
Examples, using the MID 232 (United Kingdom), are:

  (a) Ship stations  - 232001021
  (b) Group of ships  - 023201143 (1 leading zero)
  © Coast station1  - 002320018 (2 leading zeroes)
Other codes make provision for �All Ships� calls or calls to ships in particular geographical areas. The MMSI number is used to form the international subscriber number for Inmarsat B, -C and -M ship earth stations. Because of the way the MMSI is translated to a satellite terminal number, suitable MMSIs are limited to those with three trailing zeros. To avoid exhausting the supply of MID numbers too rapidly, MMSI numbers with three trailing zeroes should only be assigned to ships which expect to have automatic access to the public switched telephone network (PSTN) on a world-wide basis or which expect to use Inmarsat B, -C or -M. In practice, most Administrations assign MMSI numbers with three trailing zeroes only to vessels subject to the SOLAS Convention.
Other ships which require access to such networks on a national or regional level are assigned MMSIs with only one or two trailing zeros.

Operational Procedures
On the MF and HF bands, one frequency in each band is allocated for distress, urgency and safety purposes. These frequencies are programmed into the DSC equipment for selection when necessary; also in each band there are both national and international call and reply frequencies for routine calling. Whenever possible, national channels should be used for routine calls;

e.g., on MF, 2187.5 kHz: is used only for distress/safety.

2189.5 kHz: is used only for distress/safety.

2177 kHz: is used internationally for ship/ship calls and for shore/ship calls.

On the VHF band, channel 70 is used for distress/safety as well as routine calling. In the case of a routine �All Ships� call the power output should not exceed 1 W.
An example situation would be to contact a nearby ship. Some DSC installations have provision to reduce the power level from 25 W to 1 W automatically. The actual band chosen will depend upon the distance involved. A list of DSC frequencies is given in section 18 and is usually to be found in manufacturers� manuals.

General Calling
Procedures Routine calls should be made just once on a single frequency or channel. Coast stations may, however, transmit a second call within 45 seconds if no acknowledgement has been received. If the call is still not acknowledged, the coast station should normally wait at least another 30 minutes before repeating the call attempt on the same frequency, or 5 minutes on another calling frequency.

Ship stations should not repeat a call attempt to a coast station until after an interval of at least 5 minutes when using manual procedures. In the case of ship stations using the semi-automatic/automatic DSC option, the minimum interval for the first repeat call is 5 seconds on VHF or 25 seconds on MF/HF. Alternative frequencies may be used for the repeat call attempt, if appropriate. The ship station call may be repeated again, but not until at least a further 15 minutes have elapsed.

 To call a coast station or another ship by DSC in order to initiate routine/business communications:

(1) tune the transceiver to the appropriate DSC channel/ frequency for the call;
(2) set up the ship station DSC controller using the following general procedure (see manufacturers� manuals for specific guidance):
  • select the format for calling a specific station
  • enter address (identification) � MMSI of called station
  • select category of the call � routine or ship�s business
  • (ship station MMSI is entered automatically)
  • select type of subsequent communication � R/T or NBDP
  • enter proposed working channel information if calling another ship (no proposal    is normally made when calling a coast station since it is the responsibility of the called coast station to indicate a vacant working channel in its acknowledgement)
  • select end-of-sequence signal � usually the signal RQ, which means that a mandatory acknowledgement is re-quired, when calling a specific coast station or ship station (the called station terminates its acknowledgement with the signal BQ)
  • press <Call>
Routine DSC messages should be acknowledged using DSC either on the same frequency or on the appropriate paired frequency either manually (within 4½ minutes) or automatically. If it is not possible to use the working channels suggested in the call, this should be indicated in the acknowledgement. If no channel is mentioned, as would normally be the case when calling a coast station, suitable frequencies or channels should be proposed in the acknowledgement. Following agreement on, and transfer to, the working frequencies/channels the called station prepares to receive the traffic.

DSC DISTRESS ALERTING PROCEDURES

DSC Distress/Safety
Frequencies DSC alerts would normally be sent on the following frequencies or channels:
2187.5 kHz
8414.5 kHz
4207.5 kHz
12 577 kHz
6312 kHz
16 804.5 kHz
156.525 MHz VHFChannel 70

Note that channel 70 (156.525 MHz) may also be used for general calling, whereas the other DSC distress and safety frequencies may only be used for DSC distress calls and for  announcing DSC urgency and safety messages.

DSC Distress Alerting
On DSC, two types of distress alert call attempts exist:

  • A single-frequency call attempt; where the alert is sent on one frequency or channel.
  • A multi-frequency call attempt; where up to six consec-utive alert calls can be sent once on each of the six DSC frequencies on MF and HF, i.e., 2, 8, 4, 6, 12 and 16 MHz.
    • The DSC message format identifies the station in distress, provides its last recorded position and, if entered, the nature of the distress. Sea Area Considerations In sea area A1, channel 70 would be used in the first instance to transmit a distress alert. In area A2, 2187.5 kHz and channel 70 could be used. In areas A3 and A4, 8414.5 kHz would be used in the first instance or a multi-frequency call attempt should be used; 2187.5 kHz would then be used to alert ships in the vicinity.
     NB: Some makes of equipment cannot perform multi-frequency calls; instead, each frequency has to be selected manually in turn.

    The DSC Distress Alert Call
    The DSC distress alert contains the following information:

    • Format Specifier - DISTRESS (automatically included)
    • Self Identification � 9-digit MMSI (automatically in-cluded)
    • Nature of Distress � UNDESIGNATED DISTRESS is sent by default unless a choice is selected manually from a list of eight specific dangers, i.e., fire/explosion, flooding, collision, grounding, listing and in danger of capsizing, sinking, disabled and adrift, or abandoning ship.
    • Position   � LAT/LONG, automatically included if equipment is interfaced with, for example, GPS. The position can be entered manually at time of inci-dent. Otherwise default inform-ation, �no position information�, is sent or the last entered position (this should be updated manually at the beginning of each watch period).
    • Time    � TIME the position was valid. Auto-matic or manual entry, otherwise default information is transmitted.
    • Subsequent Communications � R/T or NBDP, indicates whether radiotelephony or narrow-band direct printing will be used for the subsequent distress commun-ications. The default information is �radiotelephony�.
    In a basic distress alert only the identification, position/time and the words �undesignated distress� are transmitted. The distress alert is automatically repeated every 3½ to 4½ minutes at random unless a DSC acknowledgement is received or the function is manually switched off.

    On receipt of a DSC distress acknowledgement, the ship in distress shall commence the distress traffic on the appropriate R/T or NBDP distress traffic frequency as follows:

      MAYDAY
      THIS IS  MMSI and CALL SIGN / IDENTIFICATION of ship in distress
      POSITION (if not included in the DSC distress alert)
      NATURE OF DISTRESS AND ASSISTANCE WANTED
      ANY OTHER USEFUL INFORMATION

    Watchkeeping on DSC
    Frequencies: Every ship at sea shall maintain a continuous DSC watch as appropriate to the sea area in which the ship is sailing. The DSC watchkeeping frequencies are:
      (a) channel 70
      (b) 2187.5 kHz
      © 8414.5 kHz and one other HF frequency, selected according to the time of day, the season of the year and the distance from appropriate coast stations.
    Those coast stations maintaining a continuous watch on the DSC distress and safety frequencies are listed in the GMDSS Master Plan, the ITU List of Coast Stations and in various national publications, e.g., the ALRS Vol 5.