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DSC Distress Alert Message Test

by James W. Hebert

An older digital selective calling (DSC) radio qualified to RTCM SC-101 is tested for sending a distress message.

This article is the first article in a series of four article I have written in which the behavior of DSC radios is tested and examined. The index below provides access to all four articles:

DSC Distress Alert Message

The most basic concept of a digital selective calling (DSC) radio is the ability to send a digital Distress Alert message. Any DSC radio that has been programmed with a marine mobile service identity (MMSI) can send a digital Distress Alert message. The minimal Distress Alert message sent will simply be a broadcast of distress alert along with your vessel MMSI. By sending your vessel MMSI, if the message is received by the Coast Guard, they can discover a great deal of information, such as the vessel name, vessel description, owner name, and owner contact information, all by looking up the MMSI registration information. A much more useful message can also be sent by including the latitude and longitude of your vessel.

An expedient way to allow the DSC radio to send position information is to provide the data electronically to the radio via a NMEA port. Interfacing a GPS receiver to a DSC radio using the NMEA-0183 protocol and port-to-port wiring has turned out to be harder than perhaps was anticipated by regulatory authorities, and many DSC distress calls, about 90-percent according to the Coast Guard of the USA, lack position information. Indeed, in setting up this test, getting GPS receiver position data to the radio turned out to be harder than anticipated. (I will explain more below.)

Older DSC radios sold in the USA were often certified to a limited standard of compliance with DSC, based on the recommendation of the Radio Technical Commission for Maritime Services (RTCM) Special Committee (SC) 101 on Digital Selective Calling. Federal regulation 47 CFR 80.225 says, in part:

The requirements for DSC equipment voluntarily installed in....ship stations are as follows:...Prior to March 25, 2009, DSC equipment must meet the requirements of....RTCM Paper 56-95/SC101-STD, "RTCM Recommended Minimum Standards for Digital Selective Calling (DSC) Equipment Providing Minimum Distress and Safety Capability," Version 1.0, August 10, 1995....

Adoption of this recommendation by the FCC was done possibly to allow marine radio manufacturers to produce DSC radios with lower cost as a way to get DSC radios into use by recreational boaters, rather than forcing compliance with more rigourous classes of DSC radio. As a result, in the USA, DSC radios conforming to RTCM SC-101 recommendations began to appear in c.2001. By 2012, the state of the art had progressed, and DSC radios compliant with a more comprehensive standard, Class-D DSC, were readily available at very modest costs. In the USA, the manufacture, importation, sale, or installation of a non-Class-D DSC radio was prohibited by the FCC after March 2011. However, for about a decade many radios were sold with DSC to RTCM SC-101 recommendations, and that is the type of radio tested in this inquiry.

Radios of DSC type RTCM SC-101 lacked facilities for making a DSC test call. Unless one tested their radio by making an actual on-the-air DSC call, perhaps with another boater, it was very difficult to know if the DSC calling feature would work, particularly the DSC emergency calling. I suspect that most boaters never conducted any sort of test of the DSC distress call button; I never did in the ten years or so that I owned the RTCM SC-101 radio prior to this test.

My test revealed several very unexpected results. If you have never tried to send a distress message with your DSC radio, I think you may be surprised by what I found.

Test Set-Up

The test involved four devices:

The devices were interconnected using their NMEA-0183 ports. There were three connections:

The interfacing was accomplished with my own NMEA-0183 Universal Method. (A guide to my method is available in a separate article, and examples of implementation of the method using a very advantageous connector arrangement are shown in a second article.)

In this test configuration, the GPS receiver is actually contained in the chart plotter, so there are only three physical devices. The GPS receiver/chart plotter and the Class-D DSC radio had been previously connected together and used extensively. There was great confidence in the NMEA interconnections between them. The SC-101 DSC radio had not be previously used with this connection arrangement, and was being interfaced with this method for the first time.

A new cable was fabricated to connect the SC-101 radio to the NMEA Universal interface. Having made many such cables for a wide variety of devices, I was confident the cable was correctly made. The SC-101 radio has the simplest possible NMEA-0183 interface. It has only a LISTENER port, and the port is single-ended.

Photo: Test bench with Lowrance HDS-8, Standard Horizon GX1500S, and ICOM IC-M402.
The test bench set-up. All the devices are powered by connection to a common 12-Volt battery.

Photo: Close up view of Universal NMEA-0183 interface connectors and backplane wiring devices.
The NMEA signals are interfaced between devices using my Universal NMEA-0183 method.
Two TALKER/LISTENER single-ended ports from the GPS and chart plotter device
are connected on the left side. The Class-D radio TALKER/LISTENER differential ports
connect on the upper right, and the SC-101 radio single-ended LISTENER port connects
on the lower right.

Problems with the SC-101 Radio

When the GPS receiver and SC-101 radio were connected, the radio appeared to have no data received from the GPS receiver. This was unanticipated, and caused a considerable delay in the test while this problem was remedied. However, it is not unlike the real world situation, and many boaters have apparently run into problems of interfacing devices like this. There could be many causes of the NMEA-0183 interface between the two devices failing to transport any data:

After much checking of settings and wiring, and a thorough review of the literature from the manufacturer of the radio, I finally found the cause of the communication loss: the wrong data was being sent to the radio from the GPS receiver/chart plotter. I discovered this particular SC-101 radio was very fussy about the NMEA sentence it wanted to receive. In the radio owner's manual, there was actually no mention at all of any particular NMEA sentence, only a vague instruction to connect "an optional GPS receiver" to the radio. I searched on-line for more information, finally coming across an article at an on-line knowledge base provided by the radio manufacturer.

According to an article at the radio manufacturer's on-line knowledge base, the NMEA data sent to this radio's NMEA-0183 LISTENER port must be in one of the following NMEA sentences:

I had configured the GPS receiver to send $GPRMC, which should have been sufficient to convey position data from the GPS receiver, but the radio was still not indicating any position data was being received. Experimenting, I changed the GPS receiver output sentences to include $GPGGA. Suddenly the SC-101 radio recognized it was getting GPS position information. On some further testing, it was established that this radio really wanted to see only $GPGGA as the data sent to it; other sentences were ignored. Information about what sentence to send was completely omitted from the printed owner's manual. The on-line knowledge base article only mentioned $GPGGA as one of several sentences that could be sent. I could not find any clear information from the radio manufacturer that informed the user that only the $GPGGA sentence would work. This was only discovered by experimentation. Had I not persisted in working on this problem, I probably would not have uncovered this peculiarity of the radio.

It had taken several hours of experimenting, including a partial disassembly of the radio chassis to investigate for any loose connections, to find a remedy to the problem of no NMEA data. I was almost at the point of throwing the radio into the trash bin, figuring that its entire digital system had failed. (The radio is about 13-years-old, and it had not been used on my boat in many years. It was my bench radio and used mostly for listening to NOAA weather broadcasts.) Now, with this problem finally fixed, I could continue with the actual test I had intended to perform.

Distress Test Call

I connected the SC-101 radio to a 50-Ohm dummy load with a short length of RG-213/U transmission line. I connected a quarter-wave whip antenna to the Class-D radio, and placed the antenna a few feet from the dummy load. Using 1-Watt power on the SC-101 radio, I made a test voice transmission into the dummy load. The Class-D radio was able to receive this transmission without any problem. Since the test distress call would be made at 25-Watt power, I was certain the Class-D radio would receive it. (I hoped the nearest USCG RESCUE21 radio tower would not receive the test; the USCG RESCUE 21 tower is about 25-miles away, and my test position is located in the shore segment of the RESCUE21 antenna pattern. I doubt the transmission would be received at any distance from my test location. Just to insure the signal did not propagate far, I set the dummy load on the floor.)

With everything set up and checked, I tried my first test distress call. I lifted the red cover over the distress button, and for the first time in the history of this RTCM SC-101 radio, pushed the distress button.

The radio emitted a loud beep. I released the button. I waited. Nothing happened. Then I located and read the instruction manual. On this topic the manual did have something to say. I found the instructions for making a distress call require the button to be pushed and held for a least five seconds.

I repeated my attempt to make a distress call, this time holding the distress button down as instructed. The radio began to emit a series of loud beeps at one second intervals, counting down to the actual transmission of the distress call. After six beeps, a longer duration beep occurred, and the radio appeared to transmit briefly. The channel indicator changed to Channel 70. After transmitting, the radio continued to make beeps, sounding at one second intervals with a staccato double beep. (I believe it will continue to transmit periodically if left in this state, repeating the distress call in three to five minutes.).

A few seconds after the SC-101 radio sent the distress call, the Class-D radio began to emit a very loud warbled tone resembling a French police siren. The Class-D radio has a large alpha-numeric display, and it announced a distress call had been received.

The chart plotter display announced an alarm: "Alarm - Vessel message. From: [MMSI] Undesignated distress." It marked the position with a waypoint icon.

To stop the SC-101 radio from beeping, I just turned it off. Upon more experimenting, I later found the radio remained set to Channel 70 (the DSC channel), and would continue the one-second double-beeps, counting down to the next transmission of a distress message. However, if the radio channel were changed to Channel 16, the beeping would stop. The radio would no longer automatically repeat the distress transmission.

The Class-D radio that received the distress transmission via DSC could be stopped from beeping by hitting the CLR button. I then found that I could scroll through the distress message using the main large rotary knob of the radio. The distress message payload was shown, consisting of:

Photo: Display on Class-D DSC radio after receiving distress call via DSC.
The Class-D DSC radio received the test distress call and displayed this information about it.
Note that the latitude and longitude of the position data are truncated to whole minutes.

The chart plotter was also showing the position of the vessel in distress; it had automatically created a new WAYPOINT at that position. The waypoint was automatically named with the MMSI of the sending vessel and the suffix DSCnn, where nn was the next available number in the DSC call log. At this point, things seemed very odd to me.

Since the chart plotter was getting its position from the same GPS receiver as the SC-101 radio sending the distress message was getting its position, I expected the location of the vessel sending the distress message would be the same as the location of the chart plotter itself. But that was not the case. The distress vessel was located to the South and East of the chart plotter's position, about 0.65-miles away.

Screen capture: Chart plotter display after receiving distress call informtion via NMEA-0183 from an attched DSC radio.
This detail of the chart plotter display shows the offset between the current position
of the vessel receiving the message (the black arrowhead icon) and the transmitted position
of the vessel sending the distress message (at cross hairs); they're 0.65-miles apart!

Next I looked more closely at the position sent in the distress message (by viewing it on the LCD display of the Class-D radio.) The distress position sent was not the same position as the chart plotter's position by GPS. Initially I though, well, perhaps the GPS receiver was just reporting a slightly different position the moment I hit the distress call button. But that was not the case. The distress call had been sent with the position data truncated to degrees and whole minutes of latitude and longitude. Even though the GPS receiver was sending the position to three decimal places (or more) of resolution in minutes, the SC-101 radio only sent the position to the nearest whole minute.

Photo: Screen capture of chart plotter waypoint detail
On the HDS-8, I checked the details of the waypoint
associated with the distress message position sent.
No additional position information was found.

This is really a distressful situation. Here we have a distress signal that originates from a vessel that could be ten-feet away from my DSC receiver, and the position data sent in the message indicates the vessel is 0.65-miles away.

Three Problems

The test revealed three problem in the transmission of a DSC distress message using this older marine radio conforming to RTCM SC-101 recommendations:

  1. interfacing the radio to the GPS receiver was unnecessarily difficult;
  2. initiating the distress message required very careful following of a procedure;
  3. transmitting the actual position used a truncated position of limited resolution.

The problem of interfacing the radio to the GPS receiver can be attributed to the very poor documentation from the manufacturer provided with the radio. But even when the manufacturer followed-on with information in more detail on their website, they appear to have neglected to provide truly provide accurate information. Getting the radio to accept data from its connected GPS receiver was much harder than it should have been, and the process was very poorly documented.

The manufacturer did provide information in their documentation to explain how to initiate a distress message. Since failure to hold the distress button for at least five seconds will terminate any attempt to send a distress message, this requirement should be presented with absolute clarity. It should be presented on the radio itself, on or near the actual button in a clear label. Upon browsing the instruction manual a second time, I did find a highlighted notice in the first few pages explaining the five-second button hold procedure. Otherwise this information was buried the manual contents. The manual also referenced an important warning sticker that must be applied near the radio. Unfortunately, I no longer have that sticker, and its message is not shown in the manual. Perhaps it explained the five-second hold, but I can't say for certain.

The design of the digital selective calling system to transmit only a truncated portion of the position information available is the aspect of this system that is the hardest to understand and rationalize. With the availability of position information with an accurate resolution of 30-feet or less from a GPS receiver, it seems extremely odd to discard some of that data and transmit position data with a resolution of about 6,000-feet.

Who is at Fault


With regard to the problems encountered in interfacing the radio to a GPS receiver, most of the fault lies with the radio manufacturer for failing to provide clear instructions regarding precisely which NMEA sentence has to be provided. I called the technical support line for the manufacturer of the radio to ask about the peculiarity of this radio only accepting one specific sentence from a GPS receiver. The technical support person acknowledged that they had seen a preference for that sentence, but they seemed unaware that only that sentence would work. The technician asked me to verify that the sentences were being sent with the $GP prefix; that prefix indicates the sender is a GPS receiver. I have seen some NMEA data sent for position have the prefix $CP (for chart plotter), but upon checking my test chart plotter's NMEA output, it was clearly sending all of the suggested sentences with the $GP prefix. I will give the manufacturer's technical support a pass on this problem, as the radio is over ten years old. In modern electronics, ten-year-old products are very often not a priority for technical support resources. A certain allowance for unfamiliarity is necessary when asking about an old product like this one. I am sure that ten years ago technical support was probably more aware of this problem than they are now.

As for the general problem of interfacing two NMEA devices, there is certainly room for improvement in both the standard of NMEA-0183 and its implementation by manufacturers following the standard. The NMEA-0183 standard failed to provide for an easily accomplished interface of the two devices. Getting data from a GPS receiver to this radio required practically expert-level knowledge, along with tedious experimentation, to accomplish. This process was unreasonably hard and prone to error. Radio manufacturers and NMEA should review this problem and find improved methods. The existence of this problem has been recognized by no less an authority than the United States Coast Guard, which formally asked NMEA to do something about it. In a letter to NMEA, the USCG requested that "the NMEA 0183 standards committee address this interconnect problem as a matter of urgency and consider revising their standard as appropriate."

Distress Message Attempt Procedure

As for the steps necessary to attempt a DSC distress message, the fault lies mostly with me, the operator. I should have read the manual and studied the distress message attempt procedure before the test was begun. But I think my untrained interaction with the radio may be typical. I pushed the button and the device responded with a beep. The beep seemed like a warning or error message, so the natural response is to stop pushing the button. To hold the button while the radio emits a series of very loud warning beeps is not a natural reaction. Even after reading the manual and learning that the button must be held, to hold the button for five seconds while the radio beeps loudly at you is still an uncomfortable response. I suppose the design of the human interface is intended to make sure that you really want to initiate a distress message, and it is doing its best to make you release the button if you pushed it by accident.

I have looked for a source of the five-second button hold procedure as a specific requirement for compliance. I do not have an actual copy of the RTCM SC-101 recommendations. Even though at one time there was a federal regulation that incorporated the recommendations, the regulations themselves were never actually provided by the federal government for its citizens to read on-line in electronic form; instead, citizens were directed to visit an archive center or to purchase the regulations from the ITU. Since I did not feel like traveling to Washington, DC, to read these regulations nor find them so interesting I wanted to purchase a copy for myself, I cannot say for certain what they have to say about the duration of the distress button hold.

In the annex of the current version of ITU Recommendation M.493-13 I did find some specifics about the distress call button presented as a "design example." In Annex Section 3, I found:

3.1 Dedicated distress button to initiate the sending of the distress alert attempt. This button should have at least two independent actions. Lifting of the protective lid is considered the first action. Pressing the distress button is considered as the second independent action. This button should be red in colour and marked "DISTRESS". Where a non-transparent protective lid or cover is used, it should also be marked "DISTRESS". The cover should be protected against inadvertent operation with a spring loaded lid or cover permanently attached to the equipment by e.g. hinges. It should not be necessary for the user to remove seals or to break the lid or cover in order to operate the distress button. This button should be used only for this purpose and it should be able to perform this function at all times. Use of the button without any previous operator actions to compose the alert should initiate the default distress alert attempt. The "default distress alert attempt" consists of "undesignated" for the nature of distress, radiotelephony for the communication mode, and on HF the transmission of the attempt uses the multifrequency method including all six bands. The distress button should have priority over all DSC procedures.

In the actual regulations themselves, the distress button is not quite as closely defined. It is described in Section 11 only in this way:

11.2 A distress alert should be activated only by means of a dedicated distress button which should be clearly identified and be protected against inadvertent operation with a spring loaded lid or cover. The initiation of a distress alert should at least require two independent actions.

No matter if the five-second hold were the manufacturer's own idea or some federal regulation required it, the button should be clearly labeled with that information. In a real distress situation, the radio operator should not be expected to recall a single sentence in the instruction manual he might have read years earlier.

Loss of Position Resolution

The general design of the digital selective calling radio system to only send position data with a resolution of whole minutes of latitude and longitude, or a resolution of about 6,000-feet, is hard to understand in the context of today's ubiquitous use of global positioning satellite navigation systems with accuracies of 30-feet or better. Limiting position data to one nautical mile of resolution might have seemed sensible in 1960 for a ship at sea. In 2001, when DSC radios were coming into common use in recreational boating, position data from a GPS receiver was available with much more accurate resolution than one nautical mile. The governing body, the International Telecommunication Union (ITU) finally recognized this, and eventually they provided an enhanced position data extension to their initial method. The enhanced position data extension permits sending of position data to one ten-thousandths of a minute of resolution, or about six-inches. Perhaps the global scope of marine radio use necessitated the slowness of the changes, but it is certainly a surprise to find, in 2014, that a DSC radio might only send a distress message with a position resolution of one nautical mile.

The DSC standard has been revised in ITU Recommendation M.821 to provide optional expansion and enhanced position data. Latitude and longitude can now be sent to a resolution of a ten-thousandth of a minute. (See section of the recommendation.) The newer Class-D DSC radios are mandated to use the enhanced position data.

Unanticipated Result

My test of this RTCM SC-101 DSC radio sending a distress message produced very unanticipated results. I was extremely surprised by the difficulty of interfacing the radio and a GPS receiver. Even though I consider myself to be a near expert on interfacing devices using NMEA-0183, I was undone by lack of clear information provided. This task was much too hard.

My first attempt to send a distress message failed because of lack of familiarity with the procedure. Although the procedure was accurately described in the owner's manual, I do not think my initial attempt was much different from a typical user's efforts. It should not be necessary to have the owner's manual on hand to send a distress message. Instructions to push and HOLD for the button for more than five seconds should be on the radio front panel.

I must also comment on the general usability of the DSC radio qualified to RTCM SC-101: it is awful. There is practically no operation of the radio for a DSC function that can be performed without carefully reading and following the directions in the owner's manual. The difference in user interface between my Class-D radio and this older RTCM SC-101 radio is very significant, with the Class-D radio being very much easier to use.

When the technical and operational hurdles to send a distress message were finally overcome, I was disappointed in the outcome. My distress message contained only a rough estimate of the actual position of the source of my radio transmission, with the resolution 300-times less precise than the actual data available to be sent. This is a significant shortcoming of an older DSC radio.


Boaters who are still using older, previously installed DSC radios that only meet RTCM SC-101 recommendations should consider abandoning those devices and upgrading to a modern Class-D DSC radio. While the problems of interfacing the radio to a GPS receiver may remain (due to lack of changes in the NMEA-0183 standards or compliance with those standards), the radio's user interface will likely be much improved. Also, the Class-D radio will take advantage of newer provisions in the DSC standard to permit sending position data with much more accuracy. Since a new marine radio with DSC features complying with Class-D standards can be purchased for less than $200, there is little reason to continue to use an older radio. A more detailed comparison of Class-D to SC-101 radios can be found in another article.

Do not try to test your own DSC radio by making a test Distress Alert, unless you have disconnected the antenna from the radio and substituted a proper dummy load. Even with a dummy load there can be some radiation, so a test should never be done in any close proximity to a Coast Guard radio watch location. Connect the dummy load with very high quality RG-213/U or RG-214/U coaxial cable to suppress emissions from the transmission line itself.

As an alternative to actual testing of the distress message attempt, read the owner's manual for your radio carefully and rehearse the procedure for sending a distress message until you are familiar with it. Verify that your DSC radio is getting proper position data from the GPS receiver attached to it. This can usually be done through the radio itself, and the position data can be shown on the radio's display screen.

DSC capabilities can also be tested by sending a DSC position call to another radio. An exchange of position with another radio should verify the GPS connection and position data being sent by your radio.


I plan to do more testing of actual DSC radios and their interoperation, and I will describe those tests in a future article. I hope to be able to test interconnection of a DSC radio to a chart plotter and GPS receiver using the NMEA-2000 protocol.

Follow-Up 2018: RTCM SC-101 Radio Receiving Capability

I resumed testing of a DSC radio qualified to RTCM SC-101 by investigating its behavior when receiving DSC transmissions.

RTCM SC-101 Radio After Broadcast of Distress Alert

After the RTCM SC-101 radio was operated to make a Distress Alert broadcast, the voice communication receiver remained tuned to Channel 70. This behavior was very strange. There are few marine radios that remain in use that can make a voice transmission on Channel 70; this is only possible with radios made prior to the introduction of DSC, thus radios made prior to c.1990 or more than 25-years-old. Further, it is very unusual to anticipate anyone would reply to a DSC Distress Alert by making a voice transmission on Channel 70, even if their radio permitted it. The implication of this behavior is the radio, now tuned to Channel 70, would not hear any calls on Channel 16--the voice distress channel--from other vessels attempting to contact the vessel in distress.

It was possible by operator intervention to manually re-tune the radio voice receiver to Channel 16, but performing that operation then interrupted the countdown timer that was running which would re-transmit the DSC Distress Alert in about four minutes. If the opertor switches to Channel 16, the automatic countdown to re-transmit is ended.

RTCM SC-101 Radio Receiving Distress Alert

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:

RTCM SC-101 Radio Receiving Position Report

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 transmitting vessel on a non-emergency position report, when it had failed to display the position of the transmitting vessel when sent in a Distress Alert.

RTCM SC-101 Radio NMEA-0183

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.

RTCM SC-101 Radio Receiving Summary

The three 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 odd behavior of the receiver in staying tuned to Channel 70 after the transmission of Distress Alert is also puzzling.

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 revealed 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.

Comments and Questions

I will reply via email to any to inquires or comments emailed to me.


The DSC System, Class-D Devices, and Position Resolution

There may be some confusion among some boaters about present-day regulations governing digital selective calling radios. Perhaps further explanation and details of the regulations will be helpful.

DSC Defined in ITU M.493

The digital selective calling system (DSC) for use in the maritime mobile service is described and defined by the International Telecommunications Union (ITU) in their recommendation M.493. From that document's introduction:

This Recommendation describes the digital selective-calling (DSC) system for use in the maritime mobile service covering general purpose and simplified versions of DSC equipment. A description of a generalized user interface as well as an automated procedure for the operation of shipborne equipment are also included.

The recommendation defines several classes of compliance. Presently there are classes designated A, B, D, and E. Previously defined classes designated C, F, and G have been withdrawn. Of most interest to recreational boaters is the Class-D designated equipment. Class-D is intended: provide minimum facilities for VHF DSC distress, urgency and safety as well as routing calling and reception, not necessarily in full accordance with IMO GMDSS carriage requirements for VHF installations.

(The acronym IMO refers to the International Maritime Organization, a United Nations agency, and GMDSS to Global Maritime Distress and Safety at Sea, an internationally agreed-upon set of communication protocols.)

The ITU-R M.493 has been around for over twenty years, and it has been revised thirteen times. While there may be some historian who has access to every previous version, I only have access to one or two most recent versions. Thus what I will describe as M.493 is based on the version in effect now, ITU-R M.493-13, which has a date of October, 2009. Let us see what it has to say about the DSC system, Class-D DSC radio equipment, and, in particular, about the messages that are to be transmitted. The most interesting message to be transmitted is the Distress Alert message. This message is described in Section 11. There we will also find a mention of two or three other ITU or IEC recommendations.

The Distress Alert may contain more than one consecutive Distress Alert on the same frequency, according to ITU-R M.541. That recommendation says: "A distress alert attempt should be transmitted as [five] consecutive calls on one frequency."

Then ITU-R M.493 says: "...these consecutive alerts should be transmitted with no gap between the end of one call and the start of the dot pattern of the following call to enable bit synchronization to be maintained..."

The position data to be transmitted is called the "distress coordinates". They are defined in ITU-R M.493, and are to consist of the latitude and longitude to a resolution in whole minutes, along with a pointer to the quadrant of the compass from that position in which the vessel is actually located. (See Section 8.1.2)

Finally, ITU-R M.493 says: "Immediately following a distress alert a DSC expansion message giving enhanced position resolution according to Recommendation ITU-R M.821 should be transmitted...."

The expansion message for position enhancement is defined in ITU-R M.821. That recommendation says (in Section 2.1): "The expansion message fields allow more explicit data pertaining to the transmitting station to be passed to receiving stations than is defined in Recommendation ITU-R M.493. The data passed allows for increased resolution in positioning...."

In ITU-R M.821 we find in Section the position can be sent with a resolution of a ten-thousandths of a minute .

If these three recommendations are followed, a modern-day Class-D DSC radio should transmit its Distress Alert message five times in a row, followed immediately by a DSC expansion message giving the enhanced position data. The net effect of this is the distress position should be sent via a DSC Distress Alert to a resolution of 0.0001-minute of latitude and longitude.

IEC 62238

Digital selective calling radios of Class-D are also subject to some addition requirements. They must comply with an international standard called IEC 62238, "Maritime navigation and radiocommunication equipment and systems—VHF radiotelephone equipment incorporating Class "D" Digital Selective Calling (DSC)—Methods of testing and required test results," published by the the International Electrotechnical Commission (IEC).

The IEC is a worldwide organization for standardization that promotes "international co-operation on all questions concerning standardization in the electrical and electronic fields." Unfortunately, I do not have a complete copy of the IEC 62238 standard, but I do have a preview of the publication. The introduction section of the standard does a good job of explaining the purpose of both the standard an the Class-D DSC radio:

Equipment designed to this International Standard is intended to provide compatibility with the Global Maritime Distress and Safety System (GMDSS) for fitting to vessels to which the International Convention on the Safety of Life at Sea (SOLAS) 1974 does not apply. Such vessels are typically small commercial vessels, pleasure vessels, fishing vessels, etc.

The equipment does not meet all the requirements of the International Maritime Organization (IMO) for SOLAS vessels. However, it does meet the IMO guidelines for non-SOLAS vessels in that it is capable of maintaining a listening watch on VHF channel 16 simultaneously with a watch on DSC channel 70.

The equipment is further capable of both transmitting and receiving Distress Alerts by DSC on channel 70, thus providing for the safety of own ship together with the ability to assist other ships in distress.

The emphasis in this standard is on simplicity of operation. The VHF equipment uses DSC controllers based on Class D, which will probably be integrated in the VHF radio equipment but in any event will have the capability to select the radio channels automatically. An input for position information in IEC 61162-1 format is a requirement and the use of automatic position updating is to be encouraged.

Distress calls may only be made by means of a protected dedicated button. Furthermore, channel 16 is required to be selected automatically after transmission of a distress or urgency call.

Position is included in the distress call either through being entered manually or with an internal GPS or an external GPS. Moreover, a DSC expansion sentence is sent after a Distress Alert to enhance the position resolution to better than 1 nautical mile.

Routine calls require only the input of the called MMSI and a channel number in the case of a ship to ship call. For incoming calls, the radio should be easily configured by the operator to either select automatically or manually the channel number given in the message. Means are provided for the user to enter a temporary group MMSI to permit calling amongst a group of related vessels.

The intended aim of designers of equipment to this international standard is that it should take no longer than 10 min for an operator to learn to use the equipment. This is achieved by the provision of clear simple menus with the most frequently used functions at the top of the menu tree.

As an aid to safety, it is a requirement, in order to prevent the transmission of DSC calls from an unidentifiable ship, that DSC operation is inhibited on a new equipment until the vessel's own MMSI has been entered.

Equipment designed to this standard is fitted with a 50-ohm external antenna socket or connector for use on board vessels and operates in the bands between 156 MHz and 174 MHz allocated to the maritime mobile service.

Please note, in particular, the remarks about the Distress Alert message and position: "Position is included in the distress call either through being entered manually or with an internal GPS or an external GPS. Moreover, a DSC expansion sentence is sent after a distress alert to enhance the position resolution to better than 1 nautical mile. [Emphasis added]"

It seems clear that modern DSC radios that comply with ITU-Rec. M-493-13 and IEC 62238 are going to send a distress message that is immediately followed by an expansion message that contains enhanced position resolution.

Although I have not seen the full text of IEC 62238, the preview also makes clear that the standard "specifies technical characteristics, methods of measurement and required test results." I presume that the required test results include transmitting a Distress Alert message immediately followed by an enhanced position expansion message.

Radios for Use in the USA

VHF Marine Band radios sold in the USA are required to be accepted by the FCC for use in the Maritime Radio Service. In Part 80 the FCC rules for STATIONS IN THE MARITIME SERVICES are given. In subpart E the GENERAL TECHNICAL STANDARDS for the radios are given. The federal regulations governing digital selective calling equipment are given in 47 CFR 80.225. This section contains some historical regulations, but the applicable regulations for radios which will be manufactured and sold today are:

"The requirements for DSC equipment voluntarily installed in coast or ships stations are as follows....Beginning March 25, 2009, the Commission will not accept new applications (but will continue to process then-pending applications) for certification of non-portable DSC equipment that does not meet the requirements of ITU-R M.493-13 and, in the case of Class D DSC equipment only, IEC 62238 (both incorporated by reference, see § 80.7)."

It seems overwhelmingly clear that a present-day Class-D DSC radio sold in the USA will send a distress message that is immediately followed by an expansion message that contains enhanced position resolution.

DISCLAIMER: This information is believed to be accurate but there is no guarantee. We do our best!

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Author: James W. Hebert
This article first appeared March 28, 2014. A revision was made April 22, 2014.

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