If you already have a VHF Radio fitted to your boat you probably have an incredible safety feature on your boat and are not even be aware that it exists. 
 
This is the case for quite a few leisure vessels. Normally it’s not until buying a new radio and you start poking around to find out how the red ‘Distress’ button works that you find out this radio has a few hidden tricks up its sleeve. Once you read up on it you will discover there is a lot of capability built into DSC radios that might prove very handy, either for yourself, or to help another vessel that could be in distress. 
 
DSC stands for ‘Digital Selective Calling’. DSC radios have two receivers, the normal one you use when selecting a channel, and a second receiver that always listens to channel 70. All DSC messages are sent and received on channel 70, which is why that channel is not available for voice traffic. 
 
Before you can use DSC, you will need to obtain a unique MMSI (Maritime Mobile Service Identity) number for your boat. The MMSI is a nine-digit number that you can get for free at the following website: 
 
 
This website collects basic information about your boat, such as the length, type, and vessel name, etc. Once completed they will send you your MMSI Number, (Normally a few minutes), you must enter this MMSI number into your radio’s memory enabling your radio to broadcast your identity and recognise messages sent to and from you. NOTE: The MMSI number is hard-coded into the radio so make sure that you check you have done it correctly before pressing the enter key! 
 
On most DSC radios, the ‘Distress’ button is protected by a red cover so that you can’t accidently push it. DSC radios must also be connected to a GPS so that the radio ‘always knows’ where you are. Some radios have GPS units built in, whereas others may require you to connect to the NEMA output wires from your GPS to the NEMA input of the radio. 
 
If you do not connect your DSC radio to a GPS, or don’t bother to enter the MMSI number into it you may as well just use channel 16 to call for help as the system is pretty useless without it. 
 
All radios may differ slightly, so I will just describe how the one I use works. You can check the manual for your own radio to see any differences. Let’s walk through making a distress call in an emergency. 
Press the ‘Distress’ button once. 
You now have the opportunity to select what type of distress you are in. The list has items such as Fire, Flooding, Collision, Grounding, Capsizing and so on. If you are in too much of a rush to get out the nature of distress listed or simply leave it on the first option of ‘Undesignated’ if no time or the distress type isn’t listed. 
Press and hold the ‘Distress’ button again until the audible countdown alarm stops. The radio then sends a DCS message giving your position, your vessel’s MMSI identification number, and the nature of the distress, if selected. The main advantage of this is that it will switch all DSC radios in range to channel 16 automatically, ready to receive the distress (Mayday) call that you follow up with by voice. 
DSC distress messages are automatically repeated every four minutes until they have been acknowledged by a ship or Coastguard Station. 
When the Coast Guard acknowledges your distress your radio will automatically switch to channel 16 (if not already done so) and you are now live with the Coast Guard. 
 
The advantage of the DSC system is that they already know the most importing facts about your situation: The name and description of your vessel (from your MMSI), your exact location (from the GPS information transmitted in your DSC message), and the nature of your distress. For example, if you had to abandon ship, you won’t be there to answer the Coast Guard’s questions, but they will know the critical information! 
 
Note that the DSC message will go out even if someone is hogging channel 16 to discuss good fishing or other weighty topics. That is why DSC occupies a separate channel (70). The messages are in digital format, and short, so the channel can handle a lot of messages simultaneously. 
 
If someone else fires off a DSC distress call, your radio will pick it up. The message will give you the MMSI of the caller, the nature of their distress, and the GPS position of their vessel. On my radio I can even make this a waypoint and ‘go to’, allowing me to see the distance and direction to that vessel. This gives me a much greater chance of being able to find and help the vessel in distress compared trying to figure out where they are from listening to channel 16. 
 
If in the open ocean, it is always good to use your DSC to make an ‘All Ships Call-Distress’. This is like the ‘Distress’ button, except you are making it clear that you want another vessel (rather than the Coast Guard) to respond. When you send the message, you select a voice channel (usually 16) that you wish to be hailed on. The beauty of DSC is that it does not matter what channel the other vessels are monitoring, since DSC is always listening on channel 70, so they will get your ‘All Ships Call’ no matter what. 
 
You can also use DSC for non-emergency purposes. For example, let’s say you have a club cruise out and want to make sure you can raise the other members of the cruise no matter what VHF channel they are monitoring. Before the cruise you collect the MMSI numbers for the other boats and enter them into your radio. At any time you can send a DSC message either to a single boat or a group of boats if you have a Group MMSI Number. Your message tells them who called (you!) and what channel you want them to reply on. 
 
A very good use of DSC is if using in conjunction with AIS (Automatic Identification System). Let’s say that your AIS unit shows that you have a big freighter bearing down on you in the middle of the ocean. Their AIS broadcast includes their MMSI number, so you automatically have the one piece of information you need to connect with them via DCS. In my radio I just have to select the vessel and click a button. Again, they get the call no matter what channel they happen to be listening on, and know what channel to hail you on in response. They also get your position from the content of the DSC message, so they have no excuse for running you down! 
 
Some other useful tools within in the Call Menu are options such as: 
• Position Request – Providing you know the MMSI number of the vessel you want to know where she is you simply push, Call, Position Request, Select the vessel name and press send. The vessel will receive your request and simply needs to press reply and you now have their Lat & Long. 
• Position Send – Same as for above except you are sending the information instead of requesting it. 
• DSC Log – A very underestimated and useful tool. The DSC log records all DSC calls, distress or otherwise and stores them chronologically. So should you receive a distress DSC message and didn’t get the position from the follow up voice call you will find it in this menu option. 
• DSC Test – Again not used very often by the leisure boat user as they don’t know it’s there. As for testing the voice side of the radio you should also test the digital side of it too! 
 
Note that DSC messages on Class D radios are transmitted by VHF radio, so ranges can vary greatly, i.e. aerial height and atmospheric conditions. 
 
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