digital selective calling

What is Digital Selective Calling (DSC) ?

Digital Selective calling (DSC) is an initial means of alerting device for any Distress / Urgency / Safety communications. In other words this device is used to bring the attention of OOW before transmitting the Distress / Urgency / Safety Message using other communication methods such as Radiotelephony (R/T) or Telex (NBDP).

Purpose of DSC

Digital Selective calling (DSC) provides automated access to coast stations and ships. The alert information is stored in the DSC receiver and can be displayed or printed out after receipt. A DSC alert always includes the identity of the transmitting station and an indication for the purpose of the call.

The system is used on the VHF, MF and HF frequencies. GMDSS regulations require the use of DSC for Distress, Urgency and Safety Alerting. Also for everyday use, DSC is a reliable and easy way to establish contact between two maritime mobile parties and can be compared to a pager.

In Terrestrial Distress communications, DSC is the initial means of attracting attention. After the initial DSC alert, the subsequent follow up communications will be conducted using Radiotelephony on VHF and either by Radiotelephony (R/T) or Telex (NBDP) on MF or HF.

DSC is based on modern Digital communication techniques. DSC calls are SELECTIVE, because they can be directed to "All Stations", "Group of Stations", "Geographical Areas" or to an "Individual Station"

There are four levels of priority available in a DSC call

DSC Priority (Highest to lowest in sequence)

  1. Distress / Distress Relay
  2. Urgency
  3. Safety
  4. Routine

DSC forms an integral part of Distress alerting, relaying and acknowledgement. DSC may also be used to make Urgency and Safety message announcements and for everyday Routine calls.

Watch-keeping by Ships

Ships, while at sea, shall maintain a continuous DSC watch appropriate to the sea area in which the ship is sailing:

In Sea Area A1: VHF DSC Ch. 70 (156.525 Mhz)

In Sea Area A2: VHF DSC Ch. 70 (156.525 Mhz)

    MF DSC 2187.5 Khz

In Sea Area A3/A4: VHF DSC Ch. 70 (156.525 Mhz)

    MF DSC 2187.5 Khz

   HF DSC 8414.5 Khz

  One other HF DSC Distress frequency depending on ship's position & time of day. (This watch is kept by means of a DSC Scanning Receiver)

(Ships should also maintain a watch on VHF Ch. 13 for Safety of Navigation and until further notice on VHF Ch. 16)

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A continuous watch for broadcast of maritime safety information shall also be kept, for the area in which the ship is sailing by: Navtex (518 Khz), Inmarsat-C or EGC SafetyNet receiver and HF MSI

DSC Distress Alert

A DSC "Distress Alert" transmitted by a ship station will automatically be address to "ALL SHIPS" and will be received by only ships and coast stations equipped with DSC and within propagation range of the DSC frequency used.

Two types of DSC Distress Alerts can be sent: Undesignated or Designated

Undesignated Distress Alert Contain following minimum information:

  1. Distress ship's ID Identification (9 digit MMSI No.)
  2. Distress ship's position and time of position update

Such alerts are sent in a distress situation when there is little or no time to format a Distress Alert. All DSC equipment must be capable of sending at least the minimum information when it is activated by simply pressing a clearly marked Distress Button or Buttons.

Designated Distress Alert Contains following Information:

  1. Distress ship's identification (9 Digit MMSI No.)
  2. Distress ship's position and time of position update
  3. Nature of Distress (selected from a Menu)
  4. Class of emission for the subsequent communication (selected from Menu)

MMSI - Maritime Mobile Service Identity number are of three types:

  1. 419643000 (Ship Station MMSI Numbers, using two or three trailing zeros)
  2. 41984365 (Group MMSI Number starts with zero - Group of ships of same Nationality only)
  3. 004194642 (Shore CRS MMSI Number)

419 is the MID - (Maritime Identification Digits) identifies the country of registration. 232 and 233 MID belongs to England, while 419 belongs to India.

Such alerts are preferred and should be sent in a Distress situation when time permits.

DSC Distress Call format

Format Specifier DISTRESS (Automatically included)
Self Identity 9 digit MMSI of the transmitting station
Nature of Distress Can choose from Menu:

  1. Fire, Explosion
  2. Flooding
  3. Collision
  4. Grounding
  5. Listing & in danger of Capsizing
  6. Sinking
  7. Disabled & adrift
  8. Abandoning ship
  9. Man Overboard
  10. Undesignated distress. This is used as "default" information
  11. EPIRB emission (Used only fro VHF DSC EPIRBs).
Position Ship's position automatically included if connected to navigation interface to position fixing equipment which constantly updates the information prior to the emergency
Time This is the time the distress position is valid.

(Note: "Piracy" and "Armed Robbery" are also considered Distress situations)

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Distress Alert Attempt

The DSC sequence transmits etc single character twice with an overall message check at the end. The DSC Distress call is sent 5  times without a break. This is done to increase the possibility that it will be received at the first attempt and it is essentially this feature, together with its unique format which makes it a "Distress Alert" and not just a Distress call.

The transmission speed of a DSC call is 100 bauds on MF/HF and 1200 bauds on VHF. Thus the duration of a DSC call is 6.2 to 7.2 seconds on MF/HF but only 0.45 to 0.63 seconds on VHF. Unlike all other calls, the distress alert is repeated 5 times in a single burst. Therefore an MF/HF Distress alert attempt lasts for about 35 seconds while a VHF Distress alert attempt lasts for about 3 seconds.

In order to increase the probability of a DSC distress alert attempt being received, it is repeated automatically between 3.5 to 4.5 minutes interval. This automatic repetition can be stopped either by manually canceling the distress or by the reception of DSC acknowledgement.

Distress Alert Repetitions

If no acknowledgement is received in response to a distress alert, the ship station operator may manually repeat the distress alert attempt (on different frequencies if desired) after a delay of approximately 4 minutes. This delay allows time for any acknowledgements to be received.

Testing the DSC Equipment

There are two  types of test calls:

  1. Self / Internal Test: done daily on VHF as well as MF/HF DSC equipment without radiation.
  2. Live / External Test: done weekly only on MF/HF DSC equipment with radiation calling an authorised CRS. The live test is done using Safety Priority on any distress frequency.

There is no live testing on VHF Ch. 70

However, weekly or at the first available opportunity a test call using the inbuilt protocol should be made on 2187.5 kHz to suitably equipped shore stations. Shore Stations are programmed to check and automatically respond to DSC test calls. After the shore station acknowledges, there is no further communication. The result of the test should be logged. In the case of long voyages test can be carried out on any of the HS DSC Distress frequencies. If no station is available for testing, same to be logged and the test than carried out at the next available opportunity.

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