Radio Vhf

Radio Vhf

Digital Selective Calling (DSC) on Marine VHF - What Is It and Why Do I Need It?

Trying to call someone on your VHF radio has always been a challenge.  For those unfamiliar with the process here's a little analogy to help draw the picture.  Imagine a large totally dark room, which we'll name the "Call Room" (ch16/9), surrounded by many smaller rooms, called "Talking Rooms".  When you need to talk to someone you go to the "Call Room", shout out their name and then listen carefully for them to answer back.  If you don't hear them answer after a couple of tries you begin to wonder.  Are they out there?  Are they there but just didn't hear you?  Better try again.

Everyone knows that when you're in the "Call Room" you're not supposed to be carrying on conversations.  That's what the "Talking Rooms" are for.  However, there's always a few that just can't follow the rules and insist upon chit chatting in the "Call Room", making it near impossible for others around them to hear someone calling.  Finally you hear the person you called answer back.  You quickly yell out the number of one of the "Talking Rooms" and tell them to meet you there.  You wait for confirmation that they heard you, but there's nothing.  Surely they heard so you head to the designated "Talking Room".  When you open the door there's already two other people there but no sign of the person you called.  Should you wait longer for them to show up or head back to the Call Room and try again?  Pretty confusing isn't it?

Consider what could happen if there's an emergency.  You're in trouble, need help immediately, and only have one opportunity to call for assistance.  You desperately go to the Call Room and make your plea.  You tell anyone listening who you are, that you have an emergency, and quickly describe where you are.  That's all the time you have.  Did anyone hear you?  Is help coming?  What was previously an inconvenience is now potentially a crisis.  There has to be a better way.  There is.

Digital Selective Calling, or DSC, was developed as part of an international effort to address and improve the issues of safety and security at sea.  A universal standard was adopted that would provide for a reliable, fast, and automated method for establishing communication and exchanging critical information. 

Today, most VHF radios are DSC capable and easily recognized by the red button cover with the word "Distress" on it.  To distinguish one vessel, or radio, from another, a unique identification number is assigned and must be programmed into the VHF radio.  A MMSI, Maritime Mobile Service Identity number, is like a nine (9) digit telephone number for your radio.  Details on obtaining an MMSI number as well as additional information on licensing and VHF channel designations maybe found at http://www.navshack.com/toolsandinfo. 

The DSC system has a designated channel (70) that all DSC equipped radios automatically monitor continuously as well as use to call other radios.  Regardless of what channel you manually select on your radio, the DSC channel is always operating in the background without any operator input.  When your radio sends out a call for another DSC unit it is instantly received and processed.  No more guessing and confusion like there is in the big dark "call room".  You can even design a "talk channel" to use and the radio you called will recognize the request and upon confirmation of receipt, automatically switch to the appropriate channel so you can conduct your voice communication. 

The US Coast Guard is currently completing modifications to their old VHF radio system to provide access via DSC to the boating public.  The new automated system is called Rescue 21.  Coverage maps of the new system are available for viewing and download at  www.wherearethemaps.  When a boater activates the distress button on their radio within range of the Rescue 21 system the Coast Guard will receive the call and respond accordingly.  Connecting your GPS unit to your DSC VHF provides the Coast Guard with your current latitude and longitude in order for them to know your exact location and expedite emergency operations.  Mandatory equipped vessels are required to interface their GPS with the DSC radio and all other vessels are strongly encouraged to do so. 

When the CS, Controlling Station (Coast Guard), receives an emergency DSC call they will acknowledge receipt.  An acknowledge message is received by the calling station and both radios automatically switch to channel 16 to conduct emergency voice communications.  All other DSC VHF radios within range will also receive the distress call and therefore serve as a backup in case the Coast Guard is unable to receive the call.  Procedures on how to respond to a DSC distress call are available at www.needtoaddthem.com.  It is a good idea to print and post these procedures for future reference should the situation arise.

As a side benefit to the emergency system, individual and groups of boaters may find that utilizing DSC will greatly enhance their ability to communicate with each other.  Nonemergency calls may be placed to a single boat or even a group of vessels at the same time.  Positions may also be exchanged via your VHF radio that is interfaced with a GPS.  However, the operator must give consent to a position request by specific entries to their radio.  No one is able to "track" your position without your consent other than when you have sent an emergency call via the "distress" button.

Digital Selective Calling (DSC) is one of the greatest advances we've seen for enhancing distress calls and responding to an emergency situation at sea.  Unfortunately, nonemergency calling via VHF DSC is one of the most underutilized operational functions of the modern marine radio.  Take time to read your radio operator manual and learn how to utilize this incredible feature that's just waiting to be discovered inside your radio.   

About the Author

Dave has spent almost 40 years in the marine electronics industry as a technician, manager, and business owner. He earned a 1st Class FCC Radiotelephone License with Ship Radar Endorsement in 1972 and has received numerous accreditations from manufacturers, and trade associations.

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