Digital Selective Calling Radios: Four Articles

[continuouswave]

I have now authored four articles about digital selective calling (DSC) radios that might be typically used in a recreational boat installation. I'd like to call readers' attention to these articles, explain what each article examined, and give a short summary of the findings.

The first article examined the behavior of an older but popular DSC radio, qualified to an initial specification for DSC. The older radio was used to send a DSC distress alert message. The most surprising result was that the message sent included only a coarse position for the vessel. This article appears at

DSC Distress Message Test
continuousWave: Whaler: Reference: DSC Distress Alert Message Test

The second article examined the behavior of a very recent model DSC radio, qualified to the most recent specification for DSC Class-D radios. This newer radio sent a DSC distress alert message with a position for the vessel to a resolution of 0.001-minute of latitude and longitude. This article appears at

DSC Distress Alert Message Test 2
continuousWave: Whaler: Reference: DSC Distress Alert Message Test 2

The third article reports on my investigation into the format of the datagrams being sent from a DSC radio when it receives a call. These datagrams are not particularly well documented publicly, and this investigation attempts to discover more about them. Some insight into their structure is presented. This article appears at

Data Interface in Digital Selective Calling Class-D Radios
continuousWave: Whaler: Reference: Data Interface in Digital Selective Calling Class-D Radios

The fourth article extends the communication system being tested to include a chart plotter receiving information from a DSC radio. The test found rather surprising inconsistencies in the interpretation of information across this link. The article appears at

Chart Plotter Interface in Digital Selective Calling
continuousWave: Whaler: Reference: Chart Plotter Display of DSC Data

In the past there have been many articles about digital selective calling radios, but most of them focus on making interconnections between devices and determining what color wire connects to what other color wire. These four article are not about the details of connecting components of a digital selective calling system. Instead, they look at the actual procedures and outcomes from sending and receiving digital selective calling calls of various types. The last article explores some entirely new ground--how a typical chart plotter responds to data from a DSC radio.

[highseas]

Thanks Wave,some valuable information here.

[SaltyMonkey]

Thanks

I'm a bit confused as to the workflow of how to use it in an emergency or non-emergency, so perhaps this will address it. Once a message is transmitted in some way, for example, how does one go about transmitting more specific information to second party or the correct protocol or procedure, next steps. etc

[continuouswave]

In Class-D radios there are only a very limited number of possible DSC calls that can be originated. These are described in

continuousWave: Whaler: Reference: Data Interface in Digital Selective Calling Class-D Radios

See the subheading "Limitation of Class-D Radios" for a list.

Also see the subheading "The Interface Between Human and Machine." This will give you some pointers to reference material that applies to each part of the communication system.

The instruction manual for a Class-D DSC radio should explain the procedure for initiating a digital selective calling call with that particular radio. The behavior of the radio must conform to the ITU-Rec. M.493-13 specification.

For distress signals, a DSC radio will automatically repeat the distress call while automatically switching to Channel 16 to monitor for replies by voice.

[Auspicious]

Good series of articles.

I like your NMEA 0183 adapter. Can you post a circuit diagram?

To my knowledge the only VHF radios with NMEA 2000 interfaces come from Navico (Simrad and Lowrance). All the others are NMEA 0183 only, which is often problematic for installation, especially DIY installation. There are workarounds (NMEA 0183 ports on chartplotters, autopilots, and converters) but it isn't as plug and play as NMEA 2000 has become. Throw in some SeaTalk of various generations and things get complicated. *grin*

As people do refits in whole or piecewise and transition to NMEA 2000 or SeaTalk-ng the interface to VHF radios for DSC (and sometimes AIS) becomes more complex.

Oh - the title at http://continuouswave.com/whaler/ref...lNMEA0183.html should be 0183 v. 1083. *grin*

[steamgoat]

Thanks c-wave,I just purchased a GX1700 so I am glad that it seems I made a good choice. Bruce.

[Prairie Chicken]

Salty, the short non-technical answer is that once the DSC Distress or Urgency message has been sent, you switch to Chan 16 & broadcast your Mayday or Pan Pan just as you normally would. Vessels having received the DSC should have automatically or manually switched to 16 in anticipation of your broadcast.

[continuouswave]

Originally Posted by Auspicious

Good series of articles.

I like your NMEA 0183 adapter. Can you post a circuit diagram?

For more about NMEA-0183 interface, see the general guidelines at

continuousWave: Whaler: Reference: NMEA-0183 Interconnection Guide

and my wiring implementation at

continuousWave: Whaler: Reference: Universal NMEA-0183 Interface

To toot my own horn, my Universal NMEA-0183 wiring implementation works beautifully. You just have to figure out each device and wire it to the standard connector. Then all devices plug together without any fuss or thinking.

Originally Posted by Auspicious

To my knowledge the only VHF radios with NMEA 2000 interfaces come from Navico (Simrad and Lowrance).

Garmin and ICOM have radios with NMEA-2000.

Originally Posted by Auspicious

Oh - the title...should be 0183...*grin*

Yes--thanks for pointing out the typo--I think I have typed 0183 too many times.

[Auspicious]

Originally Posted by continuouswave

Garmin and ICOM have radios with NMEA-2000.

It looks like all Simrad VHF radios, most Lowrance radios, one Garmin, and one Icom support NMEA 2000. That's much more than I thought.

All the Raymarine and Standard-Horizon radios are NMEA 0183 only.

Of course older radios are NMEA 0183 only.

I have to admit to a fondness for the simplicity of NMEA 0183 and the low infrastructure cost.

NMEA 2000 cables and connectors are expensive and there are few alternatives. I appreciate the CANBUS heritage but I wish Ethernet had been the basis for high speed interconnects. The efforts of Raymarine (SeaTalk-hs), Furuno (NSE), and Simrad (Simnet) in that direction had me pretty interested.

[continuouswave]

Thanks for your comments and opinions about the relative merits of using NMEA-0183 and NMEA-2000 to interconnect marine electronic devices. That is an interesting topic, but it is not really of much concern in my investigation of the behavior of digital selective calling radios. I am more concerned with testing and describing what is actually happening in digital selective calling radios and with their connections to other devices.

I have only investigated one radio and chart plotter combination that employed NMEA-2000. I am not so much interested in the relative merits of NMEA-2000 and NMEA-0183, but in how the change in connection might affect the outcome of a digital selective call and its interface to a chart plotter.

As I described in the fourth article, I was very surprised to see that the response of the chart plotter to data from the radio varied with the method of connection. That there should be some difference in how the chart plotter responds to two identical DSC calls based on how it is connected to the radio seems very unanticipated and very unusual.

I have reported this behavior to the manufacturer of the radio and chart plotter (both made by the same people) to alert them to this unusual behavior. So far, I have exchanged about five emails with them, but they seem to be extremely slow in investigating the problem.

As I mentioned, I do not know of any particular specification or international standard that describes how a chart plotter should respond to data it receives by a wired connection from a DSC radio. This seems like a wide open field of study. I can only test the devices I have on hand or on loan. I would encourage others who have different DSC radios and chart plotters interconnected to describe the behaviors they see.