Digital Selective Calling

By Andrew Spaulding

If you have a newer VHF radio it probably has a big red button cover on it that says “Distress” or something similar. This button engages the Digital Selective Calling (DSC) system which is a core part of the Global Maritime Distress Safety System (GMDSS). DSC is a digital paging system that uses data signals to automate the transmission and reception of VHF radio calls.  

A DSC message is a brief burst of digitized information transmitted from one station to alert another VHF station or stations.  The DSC message automatically indicates the identity of the calling station and the priority/purpose of the call. Every DSC radio has a unique number, which you can use to call other stations. DSC allows you to call every other VHF radio in range at the same time if you are in trouble. Following an alert by DSC message, communications are established between the transmitting station and the receiving station on a different VHF channel than was used for the DSC call.

This feature of DSC is good for sending distress calls. The ability for your VHF radio to be connected to your GPS allows the DSC to send a distress call with your GPS position. The single act of pressing the DSC “Distress” button automatically sends a distress call to all DSC equipped ships, boats and shore stations within range. Distress calls automatically include (1) your identity; (2) your GPS position (if the GPS is connected); and (3) the nature of your distress. The call will be automatically repeated until battery power is lost.

For DSC to work properly you need to obtain and program your VHF radio with a unique 9 digit ID number known as your Maritime Mobile Service Identity (MMSI). You need to apply for one with a designated authority. See below for a list of contacts to apply for your MMSI. Once you have an MMSI and you give it to your fishing or cruising buddies, they can call you directly over the VHF, just like a phone call.

DSC supports a number of call categories. The categories are Distress, Urgency, Safety, and Routine. Distress calls are automatically sent to all stations within range. Urgency, Safety and Routine calls can be addressed to all stations, an individual station or a group of stations.  All of the DSC calls include the identity of the calling station, the priority of the call (Distress, Urgency, Safety, or Routine), the station(s) being called, and the channel on which subsequent communications are to be carried. Distress level calls default to channel 16.

MMSI Issuers

If your boat requires a station license, you must get your MMSI directly from the FCC http://www.fcc.gov/

BoatUS - http://boatus.com/mmsi/

SeaTow - http://www.seatow.com/boating-safety

Power Squadron - http://www.usps.org/php/mmsi/home.php

Thanks to Dunstan and Associates for content. Click here for their web site.