Digital Selective Calling

From January 2001 all radios sold through a retailer (except handheld units) have had to feature digital selective calling (DSC), so what is DSC, why will it change the way we communicate and how is it used?

What is DSC?
DSC uses digital data, rather than voice, to transfer information between radios. Using this mechanism provides several important benefits including increased volume and accuracy of data and the ability to direct the information to specific addressees (hence selective call).

The effective range of communications when sending digital signals is also improved. DSC is primarily intended to initiate ship-to-ship, ship-to-shore, and shore-to-ship radiotelephone and MF/HF radio-telex calls.

DSC calls can also be made to individual ships or groups of ships. DSC distress alerts, which consist of a preformatted distress message, are used to initiate emergency communications with ships and rescue coordination centres.

When fully implemented, DSC will eliminate the need for persons on a ship’s bridge, or onshore, to continuously monitor radio receivers on voice radio channels, including VHF Ch16 (156.8MHz) and 2182kHz. The 2182kHz listening watch aboard GMDSS-equipped SOLAS ships ended on 1 February 1999. The VHF Ch16 watch ended on 1 February 2005 and the coastguard no longer have a dedicated headset watch.

What is a DSC controller?
A DSC controller is a unit that encodes and interprets DSC messages. In some radios this is built-in and in others it is an external ‘add-on’. DSC controllers have the facility to alert another vessel or rescue centre directly and exclusively to the fact that you want to talk, before opening voice communications in the normal manner, on the normal channels. In this respect, the system works rather like a pager with the recipient being alerted by an audio alarm, which has different sounds for routine and distress calls.

Most importantly, before putting out an urgent safety message you can digitally alert ‘all ship’'. Or before broadcasting a Mayday, in the time-honoured format, you can transmit a distress alert at the press of a red button, which will set off an alarm in ship and shore stations within radio range.

What is an MMSI?
Maritime Mobile Service Identity (MMSI) numbers are a series of nine digits, which are used to uniquely identify a radio station or group of radio stations. When sending a DSC message the MMSI is automatically included, and you can address messages to specific vessels or groups using their MMSIs. The MMSI has a standard format (just like a telephone number) and it identifies the type of station, country of registration and vessel identity. In the UK MMSIs are assigned as a part of the ship’s radio licensing procedure.

What is GMDSS?
The SOLAS (Safety Of Life At Sea) international convention lays down a wide range of measures designed to improve and maintain the safety of shipping. It applies to a wide range of commercial and passenger vessels.

The GMDSS (Global Maritime Distress and Safety System) is an integral part of SOLAS and is a radio-based communications system designed to support the implementation of search and rescue (SAR) plans. The GMDSS provides for automatic distress alerting and locating in cases where a radio operator doesn’t have time to send an SOS call, and, for the first time, requires ships to receive broadcasts of maritime safety information that could prevent a distress from happening in the first place.

The GMDSS consists of several systems, some of which are new, but many of which have been in operation for many years. The system will be able to reliably perform the following functions:

• alerting (including position determination of the unit in distress);
• search and rescue co-ordination; locating (homing);
• maritime safety information broadcasts;
• general communications;
• bridge-to-bridge communications.

Specific radio carriage requirements depend upon the ship's area of operation, rather than its tonnage. The system also provides backup means of distress alerting and emergency sources of power.

How DSC will affect you?
From 1 February 1999 SOLAS vessels are no longer required to monitor the 2182kHz MF distress frequency.
From 1 February 2005 they will no longer need to monitor the Ch16 VHF distress channel. On the same date, Coastguard rescue centres will cease to keep a dedicated headset watch on Ch16, and rescue centres in other countries may stop listening on the channel altogether.

Thereafter, you may find that the only way to communicate with SOLAS ships is with a DSC equipped radio.

The Case for DSC
There is no question that GMDSS has a big part to play in improving safety at sea.

Firstly a vessel’s position and the time are automatically included in distress and urgency alerts if a DSC radio set is interfaced with a GPS navigator (if it isn’t, the position can be manually input). Secondly, a digital alert is more certain of reaching a maritime rescue centre than voice transmission, having a 15 per cent greater range and not being prone to misunderstanding. Thirdly the increasing use of the digital broadcasting channel, Ch70, should free up Ch16 for use during an emergency which will still be available for making contact.

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