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WHAT IS DIGITAL SELECTIVE CALLING?

Digital Selective Calling (DSC) is an automated marine radio technology used by ship and coast stations for sending and relaying distress alerts and for other urgency and safety traffic. It can also be used for routine calls and offers the ability to send and receive GPS positions, since all VHF DSC radios can be connected to a GPS receiver.

DSC is a paging system that uses data signals to automate the transmission and reception of calls. A DSC message is a brief burst of data, transmitted on VHF marine channel 70 from one station to alert another station or stations, which automatically indicate the identity of the calling station and the priority or purpose of the call.

Rather than just indicate that the sending station is in distress, the DSC system allows a great deal more information to be transmitted, including:

• The priority of the call;
• The address, i.e. all ships or a single ship/station;
• The identification of the ship in distress;
• The position of the ship in distress; and
• The nature of the distress.

Following an alert by DSC message, communications are established by radiotelephone on a different channel to that used for the DSC call (voice calls are not permitted on Channel 70).

DSC equipment is fitted with a dedicated red distress or emergency button, which requires two separate and independent actions, such as lifting a hinged flap and holding the button down for a short period, to activate a distress call.

DSC MESSAGE INFORMATION

The DSC system supports a number of call categories which mirror the standard maritime prioritisation of message traffic, i.e.

1. DISTRESS
2. URGENCY
3. SAFETY
4. ROUTINE

Distress alerts are automatically addressed to all stations. Urgency, safety and routine calls can be addressed to all stations, an individual station, or a group of stations.

The DSC system provides the following types of call:

Call Type	Sent by	Addressed to	Message contents
Distress alert	Distressed vessel	ALL stations by default	MMSI; Position and time of call; Nature of distress.
Distress alert acknowledgement	Coast radio stations	Distressed vessel
Distress alert relay	Coast radio stations	ALL stations
All stations	Particular ship or station	ALL stations	URGENT or SAFETY priority calls; Frequency or channel for subsequent communications.
Single ship (or station)	Particular ship or station	Particular ship or station	URGENT or SAFETY priority calls
Routine	Particular ship or station	Particular ship(s) or station(s)	ROUTINE calls

Maritime Mobile Service Identity (MMSI)

All stations fitted with DSC are allocated a unique 9-digit identification number, known as a Maritime Mobile Service Identity (MMSI). This unique number is used to call other radios in the same fashion as a mobile phone, but unlike mobile phones, DSC allows an operator to call multiple radios in range at the same time.

The MMSI, which is allocated internationally, is permanently programmed into the DSC equipment and is sent automatically with each transmission. Government Search and Rescue (SAR) authorities maintain a database of MMSIs, which include details such as a physical description of the vessel in which the equipment is fitted and contact details for next of kin.

Should a DSC alert be received, this vessel and contact information is invaluable for SAR planning.

Position and time of call

DSC distress alerts automatically include GPS-derived position and time. If a GPS is not connected or its position output is corrupted, the DSC radio will automatically substitute the position information with the digit 9 (sent 10 times) and the time with the digit 8 (sent 4 times) in a distress message.

Some DSC radios will output a position received in a distress alert to a connected GPS or chart plotter.

Nature of distress

DSC controllers offer the operator a menu of possible nature of distress situations from which to choose:

• Fire, explosion
• Flooding
• Collision
• Grounding
• Listing
• Sinking
• Disabled and adrift
• Undesignated distress
• Abandoning ship
• Piracy/armed robbery attack
• Man Overboard

If no nature of distress information is selected, the DSC radio will send the default setting of "Undesignated". The call will conclude with the advice that subsequent communications are to be carried out on radiotelephone, which, though not specified in the message, is always on VHF marine channel 16.

Message repetition

VHF DSC messages are sent at a relatively fast speed (1200 baud). Each distress alert is actually sent five consecutive times, which takes approximately 5 seconds.

If an operator initiates a DSC distress alert, and does not manually cancel it, the alert will repeat until the radio receives a DSC distress acknowledgement message from another ship or shore station.

Distress alerts are automatically repeated with a random delay of between three and a half and four and a half minutes. This allows acknowledgements arriving randomly to be received without being blocked by retransmission.

Incoming message alarms

DSC equipment is fitted with specific aural and visual alarms to indicate reception of DISTRESS or URGENCY priority messages.

Alarms indicating DISTRESS or URGENCY messages are required to be reset manually - they are not self-cancelling.

DSC distress message alarms are loud enough to be easily heard throughout the bridge of a merchant ship, or throughout the cabin of a yacht or power boat.