DSC Distress Alert Message Test: Receiving Capabilities

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
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DSC Distress Alert Message Test: Receiving Capabilities

Postby jimh » Sat Jan 27, 2018 10:50 am

About four years ago in April 2014 I began testing VHF Marine Band radios with digital selective calling (DSC) features, including an older radio rated only to meet the the Radio Technical Committee for Maritime Service (RTCM) Special Committee 101 (SC-101) recommendation (RTCM SC-101) and a newer radio that met the more comprehensive and now mandatory DSC CLASS-D ratings. I discovered a very serious shortcoming in the RTCM SC-101 radio that I tested: it was only TRANSMITTING its position to the nearest one-minute resolution for latitude and longitude. This is described in detail in the first article in what became a four-part series. See

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

I recently revisited the testing of the older RTCM SC-101 rated radio to observe its behavior when RECEIVING position information from other DSC radios. The results were again very surprising. I will present them here:


A distress alert was sent from a CLASS-D radio and received on an RTCM SC-101 radio. The display of the receiving radio failed to indicate position information. The radio's display only presented a message that a distress alert had been received. A careful reading of the receiving radio instruction manual did not provide any method to find the position information that was transmitted. The instruction manual explicitly failed to mention any notion of the the position of the vessel originating the distress alert. Here is an except from the manual:

     Receiving DSC calls
    --Receiving a distress call
    While monitoring Ch 70 and a distress
    call is received:
      --Emergency alarm sounds for 2 minutes.
      --Push any switch to stop the alarm.
      --“DSC” appears and “RCV Distress” scrolls in the display,
      then Ch 16 is automatically selected.
      --Continue monitoring Channel 16 as a coast station may require


A position report was sent from a CLASS-D radio and received on an RTCM SC-101 radio. The display of the receiving radio only indicated the position information to one minute of resolution. This outcome is actually most surprising in that the radio displayed the position of the other vessel on a non-emergency position report, when it had failed to display the position of the vessel when sent in a distress alert.


A further problem with the DSC radio under test that conformed only to RTCM SC-101 recommendations was a complete lack of any NMEA-0183 data output from the radio. This means the radio cannot be interfaced in any way to an electronic chart plotter display. More modern radios qualified to DSC CLASS-D typically provide a NMEA-0183 (or a NMEA-2000) communication port. When those radios receive DSC calls they typically produce a corresponding NMEA protocol output; the data can be connected to and displayed on a electronic chart plotter. In the case of a distress alert message, the ability to be able to visualize the position of the vessel in distress on an electronic chart in relation to your own vessel's position provides extremely useful information. You can immediately know the distance and bearing to another vessel from the navigation computer in the chart plotter, and that information is critical if being able to offer assistance in a distress situation.


The two behaviors of an RTCM SC-101 radio when interoperating with a CLASS-D DSC radio reveal further shortcomings of the RTCM SC-101 radio. The omission of the position information received from a DISTRESS ALERT call is the most serious deficiency in the radio. The loss of the position information sent by the vessel in distress is unforgivable. It is hard to understand how the receiving radio could not provide this information, particularly when the radio was able to provide position information received in a routine, non-distress, position report from another vessel.

The lack of any NMEA communication port on the RTCM SC-101 radio further degrades its usefulness in providing information about the position of other vessels received via DSC position reports.

The testing has reveal further and very serious deficiencies in the behavior of RTCM SC-101 radios. The decision of the FCC to ban the manufacture, import, sale, or installation of these older radios is justified by the poor behavior of these radios, particularly in reception of DISTRESS ALERT calls and the failure to display position information.

Although the Coast Guard has clarified that continued use of DSC radios with only RTCM SC-101 radios already installed is permitted, the risk to safety (of your own boat and to other boats) seems significant. Considering that a modern, new VHF Marine Band radio qualified to CLASS-D DSC features can be purchased for less than $150, the cost to eliminate the risk to safety posed by continued use of older DSC radios is very modest compared to the risk of failed rescue that those radios create.

The RTCM SC-101 radio tested was an ICOM IC M-402. The CLASS-D radio used to originate the test transmissions was a Standard-Horizon GX1500.

The information on my recent tested has been added to the initial article in the series on DSC DISTRESS ALERT testing.

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Re: DSC Distress Alert Message Test: Receiving Capabilities

Postby jimh » Mon Jan 29, 2018 9:20 am

[The following text is an excerpt from the Federal Register that mentions changes in DSC radio equipment related to the RTCM SC-101. The document is archived electronically and available from

https://www.federalregister.gov/documen ... unications ]

20. In determining to adopt more stringent requirements for DSC radio equipment, we carefully considered the impact of such action on small entities that manufacture or use such equipment. We ultimately concluded that we should not exempt any entities from compliance with the new DSC technical standards because indefinite reliance on equipment meeting the old standards could jeopardize the safety not only of passengers and crew on vessels using such equipment but also passengers and crew on other vessels. [Emphasis added]

This same document also goes on to say:

...With respect to grandfathering protection...we are grandfathering indefinitely the use of any DSC equipment that was properly certified under the SC101 standard and placed in service prior to the expiration of the applicable three-year or seven-year grandfathering period; such equipment, therefore, may continue to be used until the end of its useful life.

Note that the effective data of the new DSC regulations was March 25, 2008. The "three-year" grandfathering period was thus until March 25, 2011 for non-portable radios and the "seven-year: grandfathering period was thus until March 25, 2015. This is confirmed by a USCG announcement in August 2010, which stated:

Beginning on March 25, 2011, the Federal Communications Commission will prohibit the manufacture, importation, sale and installation of fixed mounted (non-portable) digital selective calling (DSC) equipped marine radios that do not meet the requirements of International Telecommunications Union (ITU-R) Recommendation M.493–11 or higher, and in the case of Class D VHF DSC equipment only, International Electrotechnical Commission (IEC) International Standard 62238

as well as also stating:

On March 25, 2015 the FCC will prohibit the manufacture, importation, and sale of portable DSC-equipped radios that do not comply with these standards.

Finally, the USCG, although not the agency that can regulate radios, indirectly referred to the FCC's intentions as described above from the Federal Register notification regarding grandfathering, when they said (in that same announcement):

Therefore, after March 25, 2011, radios built to RTCM Standard SC-101 can no longer be manufactured, imported, sold or installed; however, previously-installed radios meeting the older standard may continue to be used. [Emphasis in original.]

I had always thought it rather odd that the USCG would be able to institute such an exception, but with the recently discovered FCC Federal Register announcement (cited above), it is clear that the FCC's intent to permit grandfathering was clearly stated when the new regulations were announced by the FCC. However, the USCG announcement is not quite verbatim with the FCC's stated intentions on what is considered "previously-installed radios." In the case of recreational boaters who have voluntarily equipped their boats with a non-portable VHF Marine Band radio with DSC, a DSC radio rated to RTCM SC-101 can continue to be used as long as it was initialled installed before March 25, 2011.

The FCC also mentions in their PART 80 rules at § 80.225 Requirements for selective calling equipment:

(6) Approved DSC equipment that has been manufactured, sold, and installed in conformity with the requirements of this section may be used indefinitely.

Since RTCM SC-101 radios were in conformity at the time they were sold and installed, they qualify for the permission for indefinite use.

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Re: DSC Distress Alert Message Test: Receiving Capabilities

Postby jimh » Mon Jan 29, 2018 2:40 pm

There is another glaring, significant, and possibly dangerous behavior in older DSC radios: if an older DSC radio makes a DISTRESS ALERT transmission, when finished the radio receiver remains tuned to Channel 70. This means that if another boater or a rescue agency attempts to make voice contact with the vessel in distress by calling them on Channel 16, the distress communication channel, the radio will not receive those transmissions because it is now tuned to Channel 70.

It is possible to get the radio to tune to Channel 16 following a DISTRESS ALERT broadcast, but this interrupts the re-transmission timer that was going to resend the distress alert at a four minute interval. That means the radio will not automatically re-transmit another distress alert.

Class-D radios are required to have a dedicated separate receiver for the DSC channel, and there is no reason the voice communication receiver would ever need to be tuned to Channel 70. The only radios that can make a voice transmission on Channel 70 are very old--very old--VHF Marine Band radios that were made long before Channel 70 was reallocated to the DSC function. I have not researched exactly when that occurred, but it must have been well before c.2000.