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Digital Selective Calling
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DIGITAL SELECTIVE CALLING ISSUES

The single largest operational problem of the U.S. Coast Guard concerning DSC had been responding to the large number of MF/HF DSC distress relays being sent by ships ITU regulations require each relay to be individually acknowledged.  The Coast Guard treats each distress alert relay as if it were a separate distress Worse, certain radios insert the identity of a ship sending a relay, rather than relaying the identity of the ship in distress. The USCG requested that vessels not relay any DSC distress message which has already been acknowledged.  If you do relay a distress message, make sure the identity of the vessel in distress is correct, and send the relay to a USCG radio station using an identity such as 003669999, rather than sending it to all ships.

Since this problem was identified, radio operators have cooperated to reduce the number of relays transmitted.  Consequently, this is far less of a problem now.

DSC problems and plans to correct them.

Continuing DSC problems include:

  • The biggest problem is the lack of follow-up voice comms after transmission of a DSC call, particularly a distress call
  • Unnecessary and frequent alarms
  • Distress alerts without accurate location information
  • Distress alerts with unregistered MMSI identification
  • Limited use of DSC for routine communications
  • Inconsistent and illogical software menu defaults
  • Alarms disrupting ongoing radiocommunications

In 2001 the International Telecommunications Union, in addressing these problems, began a major update to their DSC standard Rec ITU-R M.493 to address these problems. Changes were adopted in and published in early 2004. The Safety of Life at Sea Convention now requires radios be interconnected to electronic position fixing devices (e.g. GPS receivers). Radios meeting these new requirements should show significant improvement over earlier models, and many problems listed above should no longer occur.  Of course, new radios will be designed to be fully interoperable with older radios.

IMO Flowcharts

The International Maritime Organization Communications and Search & Rescue Subcommittee released COMSAR Circular 45 of 04 February 2009, which includes simplified flowcharts on the actions a person on a ship should perform on receipt of a distress alert using DSC-equipped radios.  The documents is in Acrobat PDF format.