Icom DSC Radio and OpenCpn NMEA output.

[GrahamC]

There an interesting feature of OpenCpn (5.8.4 in my case) when configured to output all NMEA sentences on a COM port that is connected to both the input and output of an ICOM DSC radio that is setup to receive and transmit DSC NMEA sentences.

My ICOM -M423G was connected is this manner and received an all ships urgency DSC call from Solent Coastguard.

OpenCpn then simply turned round this sentence and spat it out back to my radio which then proceeded to transmit another All ships Urgency DSC call with its MMSI number - my yacht!

This resulted is some communication with the coastguard to ensure I was not in trouble. - They were in fact handling a PAN PAN at that time.

If it had been a Mayday then the situation would have been worse.

I just wanted to highlight that this can happen to anyone who has a system configured in a similar manner without any filters applied.

It may be better to build such a filter into OpenCpn so it cannot inadvertently happen?

BTW:
I am in the process (with some success) of using the javascript plugin and suitable script to enable the initiation of a DSC call to a vessel transmitting AIS using the mouse and also from a configurable buddy list.

Although this has been poo-pood by some as unnecessary This was requested by a friend who has crossed the Atlantic single handed in a 30ft yacht a couple of years ago and is going again this year.

His take on the matter is that some ships don't respond on ch16 (asleep?)
and being in close quarters in heavy seas is not the time to be getting the manual out and keying in the MMSI of the ship.
DSC would at least sound some sort of alarm on the ship hopefully.

Anyway I just thought it worth mentioning.

[Hakan]

GrahamC..
Good to hear the live DSC messages was recognized by OCPN as it should.
One Q though: Was was your intention to output all to the radio. Are there any messages coming from other sources you wanted to send to the radio? If so I'd suggest a filter to send out only these.
If you set an output connection to send all everything received to OCPN will be sent out. So in this case what's received from the radio will immediately be echoed back. Also the DSC/DSE message(s). OCPN does nothing but echo out on such a connection. So I agree one have to take care.

The radio's function to act on a received "own" message as you've described may be strange? So thanks for the post.

[GrahamC]

Hi .
I now filter DSC messages on that portas transmitting only JSDSC (javascript) so messages received from the radio as CDDSC are ignored.
There needs to be a warning somewhere in case anyone else accidentally echos a distress message. As the radio substitutes the received MMSI with its own MMSI......

[Hakan]

I see. The goal is to send JSDSC to the radio. Then it's wise to restrict output messages to only that. And maybe JSDSE if applicable?


I have not before heard of what's your experience with received NMEA DSC for a VHF radio. And that's really valuable knowledge you've announced here.
Thanks.

[stevead]

Fortunately Hakan was able to complete the DSC feature in recent versions of OpenCPN using "artificially generated" DSC sentences, so good to know that DSC distress alerts are displayed correctly in a real situation.

Interesting that the radio transmits a DSC All Ships message using the repeated sentence, especially given that the second field of the DSC message you received would have contained the MMSI number of the vessel broadcasting the message and not your vessel's MMSI number. The MMSI number substitution is strange behaviour indeed. Perhaps a bug in Icom radios!

Quite some time ago I raised a feature request for OpenCPN to initiate DSC calls. At the time my testing correctly initiated DSC All Ships and Individual routine, safety and urgency calls, however I never tested DSC Distress & Distress Alert relays for obvious reasons.

I guess at some stage we should revisit DSC Distress alerts for OpenCPN's NMEA 2000 implementation. I'm reasonably sure that PGN 129808 is not supported.

And I would really love to see OpenCPN implement an alert log facility where various alerts (lost GPS fix, MOB, AIS Safety messages, AIS SART,/EPIRV/MOB messages, DSC alerts etc.) are centrally stored.

[rszemeti]

Originally Posted by stevead

The MMSI number substitution is strange behaviour indeed. Perhaps a bug in Icom radios!

Not a bug at all, but entirely correct. No DSC radio should ever transmit with an ID other than its own.

[stevead]

Not a bug at all, but entirely correct. No DSC radio should ever transmit with an ID other than its own

Agreed. However if I send an initiate DSC sentence to a DSC radio, with anything other than my assigned MMSI number, I would have expected the correct behaviour for the radio would be to not initiate the call, rather than to "transparently substitute" the MMSI number and make the call.

What would be "fun" to test is to send the initiate DSC call with invalid values for the format specifier, category, first and second telecommands and observe what happens.

[Antipole]

Originally Posted by stevead

I guess at some stage we should revisit DSC Distress alerts for OpenCPN's NMEA 2000 implementation. I'm reasonably sure that PGN 129808 is not supported.

For information, the forthcoming JavaScript plugin v3 has the ability to receive and send NMEA2000 messages and can, independently of OCPN, decode and encode any that has a Canboat PGN descriptor. This includes PGN 129808.

Anyone wanting to play with this in advance of release (likely March) contact me by PM.

[GrahamC]

Originally Posted by Hakan

I see. The goal is to send JSDSC to the radio. Then it's wise to restrict output messages to only that. And maybe JSDSE if applicable?


I have not before heard of what's your experience with received NMEA DSC for a VHF radio. And that's really valuable knowledge you've announced here.
Thanks.

HI Hakan
I am monitoring local VHF for DSC calls and did get a real distress call from Solent coastguard which showed up on the OpenCpn with a red jagged circle around the waypoint.
This was as well as the now infamous All Ships Urgency echoed call.
This had no positional information as it was a PAN PAN Telephony relay by Solent coastguard.
$CDDSC,16,0023200110,10,00,26,900016900016,,,,S, -- All Ships Urgency
At That point I hadn't coded decoding the DSE sentence.
It is a very quiet time for coastguard DSC traffic as not many folk are sailing at the moment.
I have 2 radios in my office one with a dummy aerial and the other still picks up DSC calls with the aerial 2 metres away.
I need to move the radio with a dummy aerial to determine the maximum distance calls are received by the radio with an aerial or put it in a faraday cage before trying out the distress calls !!

[rszemeti]

Originally Posted by stevead

Agreed. However if I send an initiate DSC sentence to a DSC radio, with anything other than my assigned MMSI number, I would have expected the correct behaviour for the radio would be to not initiate the call, rather than to "transparently substitute" the MMSI number and make the call.

What would be "fun" to test is to send the initiate DSC call with invalid values for the format specifier, category, first and second telecommands and observe what happens.

Absolutely! I have often wondered how Icom radios accepted and calling instructions over the networ, the manual is opaque on that, but as they do work with other Icom AIS products to allow a "click to talk" feature that sets up a DSC call, I knew it was possible, it seems reasonable that DSC alerts etc would follow the same path.



Thinking about it, I suspect the radio is not even looking at the vessel ID in the incoming data, as it knows it will be generating that itself, MMSI/vessel ID is probably not even a input parameter ...



What would have made more sense would have been of the dsc data was more explicitly encapsulated in "send message" or "setup call" command, I've not examined the actual data, but it sounds like as simple DSC outgoing packet was treated automatically as a command in itself ...

[stevead]

If you're interested in decoding NMEA 0183 DSC & DSE sentences and NMEA 2000 PGN 129808 messages, the TwoCan plugin source might be useful.

In addition, NMEA 0183 DSC & DSE decoding is also present in core OpenCPN. (IIRC for DSE, at least enhanced position and ship's name)

And I know somewhere Hakan has a set of assorted NMEA 0183 DSC sentences that were used to test DSC Distress, Distress Alert Relays, and DSC Position Updates.

Connecting two radios together for testing requires an attenuator circuit that supports 50 ohm loads and reduces the 25W output (which is about 45 dBm). Here's an interesting video if you wish to learn how to build your own.

I tested initiating DSC calls with Icom radios quite some time ago, it was pretty simple, although for obvious reasons I never tested DSC Distress and Distress Alert relays. Unfortunately integrating this kind of feature with a plugin for OpenCPN is a bit pointless because there is no way to add a menu item "Initiate DSC Call" to either the AIS context menu or the MMSI Number (Buddy) context menu.

[GrahamC]

Actually Steve the javascript plugin allows you to add menu items to the main context menu . That is how I get it all to work quite successfully.
In fact either clicking on the actual target or near a target on the displayed chart and selecting "make dsc call" from the main context menu (added by the javascript) will produce a popup dialogue with the chosen targets MMSI number and channel to 'chat on'. Clicking on OK/YEs will initiate a DSC call.
Thank you Antipole for making all this possible.

[bdbcat]

Stevead...
Today is a reasonable time to add utility functions to the plugin API. O5.10 will not ship for a couple of months yet.
PRs welcome.



Dave

[stevead]

Tony (Antipole) has already added the following PR (#3586) for enabling a plugin to add context menu items to the AIS, Route/Track/Waypoint Manager and MMSI Buddy Lists.

Re: GranhamC's last comment. Cool! I didn't think that right clicking near a MMSI target and invoking the Main Context Menu would provide a plugin with the details of the nearby target (MMSI number etc.)

The intent of the PR is to allow a plugin to add the "Make DSC Call" menu item to the AIS Context Menu, instead of the Main Context Menu.

[GrahamC]

In response to the comment below:

I actually find the nearest AIS target from the cursor by scanning the AIS list.
So if there are few or just one target you don't even have to click on the target anywhere on the chart will do! The problem comes when your buddy has switched his AIS off at anchor so hence the need for a buddy list and another menu item perhaps to Make a buddy dsc call. Not got that far yet!


"Re: GrahamC's last comment. Cool! I didn't think that right clicking near a MMSI target and invoking the Main Context Menu would provide a plugin with the details of the nearby target (MMSI number etc.)"