VHF – DIGITAL SELECTIVE CALLING (DSC)
Marine Safety Radio System
VHF DSC is used by ship and coast stations with particular focus on the automatic transmission and relaying of distress alerts and for other urgency and safety traffic. It can also be used for routine calls.
By way of background, it should be noted that the GMDSS [Global Maritime Distress & Safety System] was introduced in February 1999, with specific focus on improving the S&R (Search and Rescue) functionality. The overriding rationale for the GMDSS being to provide one internationally standardised marine radio system. This was achieved by employing the technology known as Digital Selective Calling (DSC).
Whilst satellite-based systems do play a part in the GMDSS, the normal terrestrial radio system continues as the primary system for ship-ship and ship-shore communications. To this end, DSC makes use of VHF marine radio channel 70 for transmission of its automated messages.
However, instead of voice, the channel is used primarily for the transmission of pre-packaged digitised information from one station to alert another station or group of stations within reception rage.
Note that VHF marine channel 70 is dedicated to DSC operation. Radiotelephone calls are thus prohibited on Channel 70.
What does VHF DSC do?
In times of distress, DSC enables a single-button push alerting of distress together with information valuable to the SAR authority. The transmitted DSC message contains vital information pertinent to the sending station such as the senders’ GPS position, the identity of the calling station, the purpose and priority of the call and other optional information.
To send a distress alert, one need only push the red DISTRESS button on the radio and proceed with attending to the emergency or depart the vessel. The plight of the vessel will continue to be automatically signaled to all until the radio ceases to function or the boat sinks.
It alerts all DSC equipped stations to your emergency but provides only basic information. Mariners are accordingly advised to make the traditional MAYDAY call in addition to hitting the distress button. That is, it does not necessarily replace a voice call on the radio.
To enable this functionality, every DSC radio is allocated a unique number (the MMSI number).
Following an alert by DSC message, communications are then established between the transmitting station and the receiving station(s) by radiotelephone on a different channel to that used for the DSC call i.e. different to channel 70.
That is, DSC is used as a means of establishing initial contact between stations with further communication conducted on an agreed upon channel different to channel 70.
DSC is also used by ship and coast stations for relaying distress alerts as well as for transmitting safety and other urgent messages.
Making use of the MMSI number, one is able to call other radios in a manner similar to using a mobile phone i.e. the conversation remains private between and limited to the parties. It is also possible to set up a PAL’s network conceptually similar to a WhatsApp group.
However, in times of distress (pushing the red button), it acts very unlike mobile phones in that the DSC radio calls (alerts) every other [DSC] radio within range at the same time. That is, the call is very public and heard by every other (DSC) station within range.
Provided it is connected to the GPS/ Chart plotter (standard on high end radios), it is also possible to set up a “buddy tracking” system that enables one to track the position of a friend’s location on a chart plotter. This is useful when cruising in company or even for tracking vessels during long distance races. However it only works if the set is programmed to reply automatically to position polls from anyone on your MMSI contact list.
In all other respects, the DSC radio works exactly as the existing (normal) VHF radio.
The new technical standard has eliminated many unnecessary functions and enhanced ease of use. This makes the implementation of the DSC functionality a relatively painless process. In its simplest form, all that is required is the insertion of the MMSI number during radio installation.
The Internet contains many online documentary and video tutorials on programming and using the VHF-DSC radio system.
Every DSC call contains the following information:
- the identity of the calling station;
- the priority of the call – DISTRESS, URGENT, SAFETY or ROUTINE;
- the station(s) being called (a specific station or ALL stations); and
- the channel on which subsequent communications are to be carried out by radiotelephone (apart from distress calls, which always default to channel 16).
What are the advantages of VHF DSC?
The advantages of VHF DSC are listed below.
- Activation of asingle button automatically sends a distress call to all DSC equipped ships, boats and shore stations in range. The call automatically includes:
Your identity, your position (if a GPS receiver is connected), and the nature of your distress
VHF DSC equipment is fitted with a dedicated red DISTRESS or emergency button. The button requires ‘two separate and independent’ actions to activate a distress call which usually involves lifting the hinged flap over the button and holding the button down for a short period. This prevents inadvertent calls.
- There is no need for you to work out your position for transmission over the radio. The current position of the vessel is automatically transmitted by the DSC.
- The call will beautomatically repeated until stopped by an acknowledgment message.
DSC is particularly useful if you have to abandon ship quickly – just push the red DISTRESS button and leave – your plight will be automatically signaled to all until the vessel sinks.
- There is no need to constantly monitor the radio to establish whether another station may have heard your distress call. This frees up an additional crew member to assist with the emergency.
- The DSC system also provides a feature known as position polling. This allows another station (shore or ship) to automatically request your DSC radio to send its GPS position to be displayed on the receiver’s chart plotter, as explained above.
This is particularly useful for marine rescue stations/ race support boats to enable the base to automatically poll the position of the rescue craft thereby allowing the boat crew to get on with the job, rather than having to regularly report their position over the radio.
- DSC automates the radio monitoring process. There is no need to constantly monitor Channel 16, just in case someone is calling you.
Also, if someone wants to talk to you, they simply dial your DSC number, just like a mobile telephone. Your DSC radio will ring (like a telephone) when someone calls you.
- The DSC system supports a number of call categories. These categories mirror the standard maritime prioritisation of message traffic, namely, DISTRESS, URGENCY, SAFETY, 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.
- DSC equipment is fitted with specific aural and visual alarmsto indicate reception of a DISTRESS or URGENCY priority messages
- Alarms indicating DISTRESS or URGENCY messages are required to be reset manually. They are not self-canceling. That is, the unit demands human intervention before the alarm sound can be cancelled.
- The technology permits a longer transmission range for the digital signal than is possible for voice.
Maritime Mobile Service Identity (MMSI)
All stations fitted with DSC are allocated a unique 9 digit ID number, known as a Maritime Mobile Service Identity (MMSI).
This MMSI is permanently programmed into the DSC equipment (at installation) and is transmitted automatically with each transmission.
MMSI are allocated internationally, with the first 3 digits, known as the Maritime Identification Digits (MID), representing the station’s country of origin.
The South African MID is 601. The full international MID list may be found on the web page: http://www.itu.int/online/mms/glad/cga_mids.sh?lng=E
The MMSI number obtained from ICASA is entered into the SAR database together with other details pertaining to the vessel and its owner. This data forms the base for information gathered by search and rescue coordinators and which significantly improves the chances of rescue.
DSC equipment classes
In terms of purchasing equipment, the DSC technical standard defines different classes of VHF DSC equipment.
Class A equipment, which includes all the facilities defined in the international standard to comply with the GMDSS requirements, is designed for merchant ships. Class-A DSC equipment uses two antennas – with one dedicated to the DSC receiver.
Class D equipment, is specifically designed for recreational vessels. It provides VHF DSC distress, urgency and safety as well as routine calling and position polling.
Class D equipment includes a dedicated channel 70 DSC receiver. This obviates the need for additional installation set-up to be performed while ensuring that no DSC calls are missed for lack of monitoring the radio.
Note: When purchasing a DSC radio, ensure that the unit supports position polling as many class D radios do not provide for this.
SC101 equipment, which was developed in the US some time ago, and was intended to be used in the US domestic market only. It provides a very limited feature set and omits some very useful options, such as polling. It has been replaced by the Class D standard.
SC101 equipment has been banned from sale in the US as of March 2011.
When purchasing a VHF DSC radio, check very carefully that it is NOT an SC101 model.
While higher end DSC radios come with a built-in GPS, the less expensive ones have a facility to connect to an external GPS receiver.
The cost differential between purchasing a low-end radio without a built in GPS receiver and a higher-end one is easily justified when one takes into account the following.
- The inconvenience and cost associated with attaching to an external GPS source.
- The added benefit of having access to an additional GPS unit in the vessel.
SAMSA and DSC
SAMSA Notice 4 of 2016 indicates the new anticipated implementation date for DSC enabled radios to be 1 January 2017. It further states that:
“In effect this still means that at the first safety survey after 1 January 2016, of Category D, C and B small craft as stipulated in the NSVSR 2007 as amended, a 29 MHz will no longer be appropriate as part of the safety equipment stipulated in the annex to the NSVSR 2007 regulations.
However the requirement for DSC functionality is temporarily put on hold until 1 January 2017 to enable the Coastal Radio Service to upgrade the complete structure to VHF DSC on channel 70.”
Point #2 of the same publication points out that the Restricted Radiotelephone Operators Certificate (RROC) are valid for voice communication only and recommends obtaining the Short Range Certificate (SRC) by way of examination. It reads as follows:
” To prevent unauthorized uses of DSC Enabled Marine VHF radios on the anticipated date of 1 January 2017 and taking into consideration the large number of holders of these certificates [the RROC] , the examination for SRC and SRC Conversions should be under-taken urgently.
These courses and examinations are available at a number of accredited institutions.”
Finally, it indicates that MMSI numbers may be applied for at ICASA, Cape Town, and “… for these numbers to be issued for DSC enabled VHF Marine Radios, a SRC or Interim SRC should be submitted “.
This article was prepared at the request of the Commodore (RNYC) as a first look into DSC enabled radios. While all attempts have been made to ensure the accuracy of the contents, it remains the responsibility of owners and skippers to ensure compliance and application.
The author has no links with any training institution or business in the yachting arena.
- SAMSA Notice 2 of 2011
- SAMSA Notice 4 of 2016