VHF DSC Radios: Distress Calls with no MMSI

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
Posts: 11862
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula

VHF DSC Radios: Distress Calls with no MMSI

Postby jimh » Sun May 19, 2019 11:30 am

I was recently asked the following question in a private inquiry:

Can a VHF Marine Band DSC radio with an internal GNSS receiver or connected to an external GNSS receiver send position info when the radio's distress button is pressed WITHOUT an MMSI entered into the radio?

Since I much prefer publicly exchange of information, I have replied here.

The answer depends on the age of the radio. If the radio is a current CLASS D DSC radio it cannot make ANY DSC CALLS until a maritime mobile service identity (MMSI) has been entered into the radio.

The applicable portion of the recommendation ITU-R M.493-13 which is the current required behavior for radios in the USA is excerpted below:

ITU-R M.493-13 wrote:12.4 MMSI

DSC equipment should not transmit any DSC call until own ship’s MMSI allocated to the ship by the relevant administration has been configured and stored in the DSC equipment.

Once stored, it should not be possible for the user to change the MMSI without advice from the manufacturer. The DSC equipment should display own ship’s MMSI on start-up unless the MMSI has not been configured.

If the MMSI has not been configured, the equipment will display a warning that the unit will not transmit any DSC calls until own ship’s MMSI is entered. The equipment should stay in this state until the operator confirms he has read the display and input own ship’s MMSI. The MMSI should be readily displayed on the HMI when the DSC equipment is on.

If the radio is an older DSC radio qualified only to the RTCM SC-101 recommendation, it can make a DSC call without an MMSI or a position being available.

I don't have the full text of the RTCM SC-101 recommendation because I have not invested the necessary costs to purchase the document; it is not available publicly. (And it is obsolete.) I presume it must not have been clear on the topic of the radio refusing to perform DSC functions if it had no MMSI. That assumption is made on the basis of the USCG reporting that an alarming percentage of the DSC DISTRESS ALERT calls they received did not contain any MMSI information. (For more see my separate article on the September 2011 USCG Safety Alert.)

A follow-on question was:

How old would a VHF Marine Band DSC radio be if it were approved under the RTCM SC-101 rating?

A VHF Marine Band DSC radio qualified to RTCM SC-101 could be as old as c.1995, which is the date of the recommendation. DSC radios qualified only to RTCM SC-101 were available for sale in the USA in c.2003. I bought such a radio, an ICOM IC-M402 in c.2003. It was only years later that I realized that most of the DSC functionality of the radio was not working due to the default setting. For more details about the IC M-402 radio see a separate article about rehabilitation of the IC M-402.

The Federal Communications Commission (FCC) announced a sunset date for DSC radios that met only RTCM SC-101 behaviot, prohibiting their manufacture, importation, sale, or installation after March 25, 2011. However, a later clarification stated that such radios already installed could continue to be used. See




for details.

For more about the problems associated with VHF Marine Band radios that are qualified only the RTCM SC-101 requirement, see

DSC Distress Alert Message Test: Receiving Capabilities

There are many shortcomings with DSC radios only qualified to RTCM SC-101, including the inability to send the position with high resolution. For more information see part one of my four part series of articles in which I tested DSC radios:

DSC Distress Alert Message Test
http://continuouswave.com/whaler/refere ... sTest.html

As I have previously mentioned, with a new DSC Class-D VHF Marine Band radio available for only $140, continuing to use an obsolete radio that only qualifies to RTCM SC-101 makes little sense to me.