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ContinuousWave: Small Boat Electrical
DSC Distress Alert Test
|Author||Topic: DSC Distress Alert Test|
posted 03-28-2014 12:36 PM ET (US)
Recently I set up a test configuration for sending an off-air DSC distress alert message using an older VHF Marine Band radio that conforms to the c.2001 RTCM SC-101 recommendations. I describe the test and the results in detail in a new Reference Section article. See
I will be glad to answer any questions related to the article in this message thread. I hope you read the article and find it informative. I was very surprised by the outcome of my testing, and I suspect most readers will be similarly surprised, too.
posted 03-29-2014 09:55 AM ET (US)
Jim--I read your report. It was very interesting and informative.
I believe your test set-up may have some commercial viability. I have no idea of the potential demand but it may be worth a survey on ContinuousWave.
posted 03-31-2014 11:30 AM ET (US)
Jim--I, too, read your report and was curious about my setup. Until recently I had my Standard Horizon radio connected to my Garmin GPS 2006C chartplotter. I removed it and installed a Simrad NSS8. I found in that manual the option of RS-232 or RS-422 communications. I tried to connect with RS-232 first and no joy. I changed the settings and realized that plus to plus and minus to minus is the protocol for RS-422 (I realize that is not correct terminology)and that is what is shown on the Standard Horizon pages. So stripped the wires and re-terminated for RS-422 and they started to talk. I agree that NEMA-0183 may be a reliable comms protocol, but the implementation of the protocol is about the worst I have seen. I am about to dip into the NEMA-2000 interface and see how good that is.
posted 03-31-2014 12:54 PM ET (US)
FNO--I am fairly certain that the NMEA0-0183 standard calls for, or at least prefers, use of differential electrical signals for sending the data. The RS-422 protocol is a differential electrical signal, while RS-232 is not.
It seems to me that a good part of the confusion that exists about interface of NMEA-0183 signals stems from the mixture of single-ended and differential ports among various manufacturers and models.
Lowrance and Simrad are somewhat unique in my limited experience in offering the option to use either single-ended or differential ports. Standard Horizon has been using only differential ports for many years. If all NMEA-0183 devices provided differential ports the interconnection would be simpler.
It is not difficult to interconnect RS-232 to RS-422, but it can become confusing.
posted 03-31-2014 01:47 PM ET (US)
The Coast Guard of the USA has some history of digital selective calling on their NavCen website. In particular, they say:
The salient data in that long quoted material is the notion that the FCC required type-accepted VHF Marine Band radios sold after June 17, 1999 to conform to at least RTCM SC-101 recommendations. This means that from 1999 until 2011, a period of over ten years, VHF Marine Band radios sold in the USA were very likely to be DSC radios with only RTCM SC-101 features.
posted 04-02-2014 09:49 AM ET (US)
Let me add a bit more information about the DSC distress message:
When the DSC message was received by my Class-D radio, the radio sent a NMEA-0183 sentence to the chart plotter. This sentence informed the chart plotter about the DSC distress alert. The chart plotter responded with a visual alarm notification on screen, and with a new waypoint, created at the position of the vessel sending the distress alert.
The chart plotter allows for the user to position the cursor over the waypoint and get more information. I show this in the article as a screen capture. The chart plotter names this screen "Edit Waypoint." In that screen there is a field called "Description." For the DSC distress alert message, the description read "Undesignated distress."
In the DSC protocol, a vessel sending a DSC distress alert message can specify the nature of the distress. There are many categories for distress, such as , grounding, sinking, on-fire, disabled, and so on. Since the SC-101 radio has no means for the operator to enter a category of distress, the distress condition "Undesignated" is sent. (Or so I presume.)
Having previously experimented with this same chart plotter in exchanging of non-distress DSC messages, for example, exchanging position data, I noticed that the chart plotter created a similar waypoint for the DSC vessel that sent me their non-distress message including a position. And the chart plotter also gave that non-distress waypoint the description of "Undesignated distress." I don't believe the non-distress waypoint should be described that way. A non-distress message should not be marked with a notation of distress.
The icon used by the chart plotter for the distress waypoint was different from the icon used for non-distress waypoints created by DSC messages. In this way there was a the least some visual distinguishing appearance for the distress alert vessel location.
posted 04-02-2014 12:18 PM ET (US)
Jim,--Do you know if sending the distress out through your newer Class D VHF would provide the full GPS coordinates? Or do the Class D radios also send truncated information?
posted 04-02-2014 12:55 PM ET (US)
A Class-D radio is mandated to use the enhanced position extension. It sends the position to a resolution of six-inches.
posted 04-02-2014 01:36 PM ET (US)
I mentioned the enhanced position extension use with Class-D radios in the main article:
Note that an SC-101 radio typically will not be aware of the enhanced position extension of the DSC protocol, and it will probably ignore that data in any DSC message it receives. The result is that when an SC-101 radio receives a distress alert, it will only receive the position to a resolution of a nautical mile, even if the distress alert were transmitted by a Class-D (or higher) DSC radio that has sent the position data with higher resolution.
By sending the position in the DSC message with the coarse position, then sending a second message immediately following with the enhanced position extension, the older and newer radios can interoperate. If there were an entirely new distress alert message format in use, then all the older radios would not be aware of it. They could not decode that new message format. By sending the distress alert in the same (older) format, then sending a second message with the position enhanced data, both old and new radios can interoperate.
posted 04-03-2014 10:20 AM ET (US)
Regarding the resolution of the position into whole degrees and minutes of latitude and longitude, I don't know for certain how the radio came up with the position it sent, that is, if it just truncated the data from the GPS or if it applied a rounding of the data to the nearest whole minute.
In the test case, the GPS was in a position where the decimal part in both the latitude and longitude was between n.000 and n.500 minutes. Because of this, I don't know if the radio just truncated to n.0 minutes or if it properly rounded down to n.0 minutes. In order to test that, I would have to move my whole test bench to a new position in which the decimal part of both latitude and longitude was between n.5001 and n.9999. There I could observe if the position were rounded (up) or truncation to whole minutes.
posted 04-05-2014 11:09 AM ET (US)
I have added an addendum to the article that cites the appropriate standards for digital selective calling radios, gives hyperlinks to those standards, and excerpts portions of those standards to further explain in detail how a modern DSC radio should send its position information in a distress alert message.
posted 04-17-2014 12:52 PM ET (US)
I have a new radio on the bench for testing with DSC distress alert messages. This radio is a recent model that is sold in the USA under the FCC requirement to comply with ITU-Rec. M.493-13 for digital selective calling. (The radio is a Standard-Horizon GX1700.)
The M.493-13 radio transmits a distress alert message with the position information expanded so that the position of the vessel in distress is sent with a resolution of 0.001-minutes of latitude and longitude (about six feet). This is exactly as I anticipated would occur, based on the recommendations of the ITU-Rec. M.493-13. The position is sent with one-thousand-times better resolution than in the older radio tested previously, which sent only a position to a resolution of a whole minute (about 6,000-feet).
The ITU-Rec. M.493-13 radio also incorporates a number of other differences in the digital selective calling features compared to the older radio tested. I will elaborate more about this in a future follow-up article with illustrations. However, I want to mention that two important added feature are:
--the ability to send a distress alert CANCEL message, and
--the ability to designate the nature of the distress in the distress alert message.
Standby for more about the advantages of the newer radio.
posted 04-28-2014 08:29 AM ET (US)
The results of testing a newer VHF Marine Band radio which is compliant with the current FCC requirements, that is, it complies with ITU Recommendation M.493-13, are presented in a second test report at
The radio under test was a Standard-Horizon GX1700.
posted 11-22-2014 11:09 AM ET (US)
I have revived this thread because I wanted to mention that this initial article has since grown into a series of four articles in which I perform tests of digital selective calling (DSC) distress alert transmissions with various VHF Marine Band radios. The four articles in the series are mentioned in the preface to each of the articles, so navigation among them is simplified.
This series of four articles gives a great deal of information about digital selective calling distress alert messages, the standards that define these messages, the way the standards are implemented on various radios, and the observed behavior of radios and connected chart plotters when receiving these messages.
It's now well past boating season in the northern USA, so perhaps more time is available for reading about boating and boating electronics than there is for actual boating and use of boating electronics. I suggest these four articles as a good and quite thorough look at digital selective calling and distress alert messages. The four articles are:
Part 1: DSC Distress Alert Message Test: the initial article
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