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VHS - DSS The Facts
Digital Selective Calling
Digital Selective Calling (DSC) is upon us and there is little reason to buy a new VHF set if it is not at least DSC compatible, even if you don't buy a linked controller to give you DSC facilities right now. Within five years we will all - if we wish to carry VHF on board - have had to pay for a new examination and operator's certificate plus the cost of a new VHF set and DSC controller. It's all part and parcel of the Global Maritime Distress and Safety System (GMDSS) that is already a legal requirement for shipping and will finally end the use of Channel 16 as a calling channel in 2005.

For a DSC control unit to work properly it needs to be linked with a compatible VHF set and a GPS. Position and time are drawn from the GPS, while the DSC acts like a pager, alerting a potential rescuer to a distress situation and setting the VHF to Ch16 for voice operation.

We used the ICS VHF3/DSC3 combination for this feature - fairly typical of present yacht equipment. Future units will differ in detail, but not by much in 'how to' operating terms, because they must all meet the same system requirements.

Firing up
When the controller is first turned on, it draws down information from the connected GPS regarding position and time and these will be displayed on the controller's screen. This means, of course, that the GPS must also be up and running.

Once the position information has appeared on the screen and the VHF is tuned in to monitor Channel 16, you are ready both to receive and to transmit distress or other messages. The following sections will take you through the steps necessary to make Distress, Urgency and Routine calls - the commonest radio calls made by yachtsmen - though not, happily, in that order.

Ready and waiting

The opening screen on your DSC waiting for you to select the kind of call you want to make. The call type may be labelled Routine or Individual according the manufacturer
DSC - Digital Selective Calling
A DSC distress call is known as a Distress Alert and what it actually does is to produce a message on the DSC screens of other stations and set off audible alarms on those sets. The on-screen message gives your MMSI, which is a positive identification of your vessel, the nature of your distress (if you had time to include it), your position, the time and how further communication will be carried out, which for a yacht will be by voice on Channel 16. The Distress Alert is transmitted automatically every four minutes until it is acknowledged by the Coastguard. While this is going on, your DSC screen shows a message that the call has been sent and a reply is awaited.

The controller showing the protective cover swung open to reveal and make available the Distress alert button. The cover is important in preventing unintentional transmission of distress calls

Once that Distress Alert is received, a return message is automatically sent by the recipient that tells you to transmit a Mayday message by voice on Channel 16 and gives the MMSI of the Coastguard station making the acknowledgement. (All Coastguard stations have MMSIs prefixed with two zeros.)

With that message in front of you, you know that if you are not able to do anything else, the basic information of who you are and where you are has been received. However, if you are able, it is time to use Channel 16 VHF, to which your set will automatically tune, and make a normal voice Mayday transmission, which must include your MMSI to ensure a tie-up with the DSC transmission.

And that's what the DSC does for you in a Distress situation. It's easy to operate and has the advantage of giving all the basic information in one initial, fast, automatic transmission. But remember, your MMSI identifies you uniquely, so when choosing your DSC controller, make sure that the Distress button is obvious, yet protected against accidental use.

At the shore station, when your Distress Alert is received, an alarm will sound and this screen will pop up on their DSC equipment showing your MMSI, the nature of your distress, your position and the time of that position, the date and time of receipt of the alert and the method of communication to be used (Channel 16, voice). Also, it tells the operator that his VHF has been tuned ready for his response

Digital Selective Calling

These are sometimes referred to as Pan Pan calls and are used in non-life threatening situations where there is trouble and help is needed, but there is no 'grave and imminent danger' requiring the use of a Mayday call instead. They include requests for urgent medical advice (Pan Pan Medico).

When you choose an Urgency message the DSC assumes it is to go to All Ships on Channel 16 and prepares everything accordingly

You press Enter or Accept to agree with the plan

Using DSC, an urgency alert is sent either to 'All stations' or to an individual Coastguard station identified by its MMSI. When the alert has been sent, your DSC controller retunes your VHF set to Channel 16 and prompts you to send an appropriate voice message, which will need to include your position, since, for some obscure legislative reason, the DSC Urgency alert does not include it.

The screen now changes, requiring you to instruct the DSC to make the call or not

Digital Selective Calling

Once you've selected the Routine option you must enter the MMSI of the station you are calling, whether it is a Coastguard shore station or another vessel. The number can be entered manually using the keypad or called up from a directory of numbers you've previously installed. It also suggests a VHF channel for the traffic to be exchanged on - in this case 06. When you're ready, you press Enter to send the Routine call alert

There are two ways of making a routine call, depending upon whether the station you are calling is equipped with DSC. If you want to call up a Coastguard station you do so with your DSC equipment, which always broadcasts its initial call on Channel 70, specifying which station you are trying to contact by use of its MMSI as the 'address'. The acknowledgement from the Coastguard station that will appear on your controller's screen will nominate which channel to use for a voice exchange - probably Channel 67 - and your VHF will automatically retune to it. On the other hand, if you want to call a marina to ask for an overnight berth, you ignore the DSC controller and tune the VHF to Channel 80 (or occasionally 37) and call in the conventional manner.

Once you have sent the Routine call alert, the screen on your controller changes and tells you it is waiting for a reply

A safety of navigation call, for example reporting a sighting of a drifting buoy or a half-submerged container, is made to the Coastguard in the same way, using the DSC and then a voice transmission on the channel nominated by the Coastguard station in their acknowledgement.

At the station being called, the DSC screen shows the message that the station with the MMSI shown (yours!) is calling and requesting a conversation on Ch 06. The station operator presses Acknowledge, which makes your DSC retune your VHF to Ch 06, and then calls you up by voice. You then reply and proceed as normal

With DSC, VHF and GPS all up and running.
  • Uncover red DISTRESS button
  • Press the button

Screen changes to show:

  • Position
  • Time
  • Undesignated

  • If there is time, use soft key to select the designated type of distress situation from the menu - sinking, abandoning, fire, flooding, grounding, man overboard etc.
  • Hold down DISTRESS button during countdown and beeping alarm. (The transmission can be aborted during countdown.)Screen shows: Distress call sent. Waiting for reply.
  • When distress is acknowledged, the radio will automatically retune to Channel 16, prompting you to send conventional voice Mayday.

(Press and hold talk button throughout message)
BOAT INFORMATION: size, type (sailing boat, powerboat, etc), hull colour, anything distinctive
(Release talk button)
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