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Old 26-01-2015, 10:31   #16
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Re: OCPN support VHF DSC Position NMEA sentences?

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Originally Posted by Gypsy23 View Post
OCPN deletes either Distress or Position msgs after 10 mins with a warning black bar across the marker 2 mins before the delete if the msgs have not been refreshed/resent in that period
This is new indeed... in my original implementation in 2.5 there was no timeout for Distress, and I think there should be none, certainly not the regular timeout setting as for the regular targets.

One should not assume that the operator will attend to OCPN in any fixed time at all, so the target should not disappear before a mark is made somehow...

I have recently passed the renewal exams for my GOC license, so this may also reinforce this feeling, but the chartplotter is not a GMDSS device, and it should only offer absolutely basic support, preferably not requiring the user to perform any immediate actions... the temptation to implement all possible sequences of sentences and automate the response to Distress alerts would not be a good thing... the focus should still remain on the DSC receiver.

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Originally Posted by Gypsy23 View Post
Cancel Distress - Both E120W and OCPN threat this as a new Distress msg.
There is no such thing as Cancel Distress sentence in the DSC protocol, you probably mean Distress Acknowledge, which does not cancel the alert. Whether to skip it or not? Accepting as a new distress is safer, considering the chance of losing the original alert (not a real alternative, I think), but maybe it is better to skip it, for reasons explained in the preceding paragraph: the main thing is the radio set, chartplotter should not try to pose as a more comfortable alternative...

Better to keep things simple and obvious...
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Old 26-01-2015, 14:46   #17
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Re: OCPN support VHF DSC Position NMEA sentences?

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Originally Posted by Gypsy23 View Post
Hello Doc
I confirm I was able to receive and display DSC sentences on OCPN but not yet seen a successful DCE sentence. Also, the DSC message did not come through on the first transmission of a single sentence, it required 2 transmits in close succession.

Either or both of these problems could be due to my test sentences or test rig so it is too early to be sure yet. I hope to experiment with real-life radios in the next few days and will report back then.
R
I posted earlier that I found my test DSC/DSE messages sometimes needed 2 transmits to be picked up by OCPN.

I have since made direct serial-to-serial port tests from the msg generator to OCPN and I no longer see any missed messages. OCPN reacts reliably as required to each message. The problems must have come through the internal TCP server and network connection I was using.

Sorry for any concerns I may have raised.
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Old 26-01-2015, 15:25   #18
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Re: OCPN support VHF DSC Position NMEA sentences?

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Originally Posted by PjotrC View Post
This is new indeed... in my original implementation in 2.5 there was no timeout for Distress, and I think there should be none, certainly not the regular timeout setting as for the regular targets.
I just did another test of this - If there has been no operation of OCPN since the first Pop-up and marker, after 10mins the marker is deleted but the Pop-up remains on screen without any Position info or timer and --- in place of Bearing and Range.

Quote:
One should not assume that the operator will attend to OCPN in any fixed time at all, so the target should not disappear before a mark is made somehow...

I have recently passed the renewal exams for my GOC license, so this may also reinforce this feeling, but the chartplotter is not a GMDSS device, and it should only offer absolutely basic support, preferably not requiring the user to perform any immediate actions... the temptation to implement all possible sequences of sentences and automate the response to Distress alerts would not be a good thing... the focus should still remain on the DSC receiver.

There is no such thing as Cancel Distress sentence in the DSC protocol, you probably mean Distress Acknowledge, which does not cancel the alert. Whether to skip it or not? Accepting as a new distress is safer, considering the chance of losing the original alert (not a real alternative, I think), but maybe it is better to skip it, for reasons explained in the preceding paragraph: the main thing is the radio set, chartplotter should not try to pose as a more comfortable alternative...
VHF-DSC radios have logs for DSC msgs so the Op can look back to recent activity even if the screen no longer shows the msg, but also, it is my understanding that DSC Distress calls are repeated by the radio until an Ack is sent, usually by a base station, so the OCPN alert will remain current until 10mins after the last repeat by the Distressed vessel.

Actually, I am not campaigning for any more/less features on this for OCPN. I have done the testing so I understand the system and what to expect and I am documenting it because there is very little detail about how it works in the Help file. Added to that, we clearly have some special 'features/implementations' by some radio manufacturers which cause different results on any device monitoring the DSC data.

It is great to have such good feedback about what is intended and the history behind the software development. Tks for your input and of others.
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Old 28-01-2015, 02:34   #19
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Re: OCPN support VHF DSC Position NMEA sentences?

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Originally Posted by PjotrC View Post
There is no such thing as Cancel Distress sentence in the DSC protocol, you probably mean Distress Acknowledge, which does not cancel the alert.
Gypsy23...

I stand partly corrected here... according to ITU-R M.493 Table 4 a Distress Acknowledge with the same MMSI used for the sender and for vessel in distress should be interpreted as self-cancellation. Written in a very fine print indeed.

Interesting, but never in my trainings and study, as well as in none of the equipment I used nor official guidelines issued I have seen this feature mentioned.

Your example indeed shows the same MMSI for sender and casualty, BUT it uses the format specifier 12, which is a Distress. As far as I can see, the Distress Acknowledgement should use format specifier 16. Format specifier is in the first numerical field of the sentence. So your example would literally be a (slightly incorrect) new Distress.

I have no real case of this kind of traffic. OCPN will likely skip the Acknowledge, looking just for Format 12. It would be interesting to see, what E120 will do with Format 16 self-cancellation... can you check this?

DSC is Pandora's Box, my original thinking was this: upon reception of anything like a Distress, just silently plot the position, and let the user follow the radio procedures as per regulations.

That's why I do not like the alarm on a DSC Distress in the chartplotter. One event - one alarm. It looks good on simulation, but in reality the alarm goes off in the radio first, so the second one is not needed at all.

As to the enhanced indication of position plotted - it is easy to find just by zooming out a bit - always on top.

So if I were programming this today, I would likely -

0. skip Acknowledgements (?)
1. turn on tracking for DSC targets always
2. do not timeout Distress targets ever
3. do not show AIS alarms on DSC targets
4. do ensure the AIS Target List is visible

Thanks for bringing the DSC features to attention.

A Pandora's Box, I am afraid

Cheers,

Piotr
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Old 28-01-2015, 05:25   #20
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Re: OCPN support VHF DSC Position NMEA sentences?

Just tested a Format 16 self-cancellation with OCPN 4.0:

A Distress target changes to a regular Position Report (green). I think it is perhaps OK...

But an acknowledge by another station is shown as a separate Position Report with acknowledge sender mmsi and vessel in distress position... perhaps not so OK...
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Old 28-01-2015, 14:40   #21
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Re: OCPN support VHF DSC Position NMEA sentences?

OK. Interesting! I'll try a Format 16 message on the E120W in the next day or two - I have to get out to my boat.
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Old 29-01-2015, 15:55   #22
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Re: OCPN support VHF DSC Position NMEA sentences?

A couple of questions:

  • What Format number is a routine DSC call?
  • And if I take an existing sentence and modify it by changing GPS coordinates, Format #, etc., how do I calculate the new checksum? (The online calculators don't seem to replicate the values posted in your sentences.)
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Old 29-01-2015, 21:58   #23
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Re: OCPN support VHF DSC Position NMEA sentences?

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Originally Posted by RhythmDoctor View Post
A couple of questions:

  • What Format number is a routine DSC call?
A message to an individual MMSI is a Format 20. Take a look at the article I linked in my msg #3. It is not the actual NMEA spec. but it does help understand some of the options.


Quote:
  • And if I take an existing sentence and modify it by changing GPS coordinates, Format #, etc., how do I calculate the new checksum? (The online calculators don't seem to replicate the values posted in your sentences.)
I used a program "NMEATALKER" to generate the parity/checksum. It worked well but you only get 100 checksums before you have to pay for the software. Remember, you don't include the *xx in the string you are trying to generate a checksum for and any extra space will cause a different checksum, possibly cause the message to fail. However, after generating new checksums for my messages I realised that OCPN has an option in the comms setup area to ignore parity on any particular connection so I think you could put any number after the * and it will work.

A point here though, from what I can see and comments back from the development guys, this DSC feature in OCPN isn't designed to act on the full range of signals which may be sent out from a DSC radio. PjotrC can correct me if I am wrong, but it seems the software looks specifically for Format 12 messages and shows them as Distress but treats all other messages as Postion msgs.
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Old 30-01-2015, 02:00   #24
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Re: OCPN support VHF DSC Position NMEA sentences?

On Windows (I hope new editions still support Command Line) one can use a simple program I wrote many years ago. Executable and sample screen attached. Note that input to this program consists of a complete line without checksum, but including the initial $ and final asterisk...

Quote:
Originally Posted by Gypsy23 View Post
A point here though, from what I can see and comments back from the development guys, this DSC feature in OCPN isn't designed to act on the full range of signals which may be sent out from a DSC radio.
That was my original intention and I would still vote for it. But it is Open Source...
Attached Thumbnails
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Old 30-01-2015, 02:30   #25
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Re: OCPN support VHF DSC Position NMEA sentences?

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Originally Posted by PjotrC View Post
On Windows (I hope new editions still support Command Line) one can use a simple program I wrote many years ago. Executable and sample screen attached. Note that input to this program consists of a complete line without checksum, but including the initial $ and final asterisk...

That was my original intention and I would still vote for it. But it is Open Source...
I tried to open your pdf NMEA checksum batch file attachment but it failed. Can you check if it is ok pls.

Tks
Ray
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Old 30-01-2015, 02:47   #26
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Re: OCPN support VHF DSC Position NMEA sentences?

Download the attachement.
Strip the ".pdf" extension (it is there only to circumvent file type restrictions).
Unzip the "nmea_checksum.7z" archive into any directory.
There will be 2 files inside: "nmea_checksum.exe" and "nmea_checksum.bat"
Run the .bat script.
It is possible to copy/paste to a Command window by rightclick on the top and choosing "Edit..."
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Old 30-01-2015, 02:54   #27
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Re: OCPN support VHF DSC Position NMEA sentences?

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Originally Posted by PjotrC View Post
Download the attachement.
Strip the ".pdf" extension (it is there only to circumvent file type restrictions).
Unzip the "nmea_checksum.7z" archive into any directory.
There will be 2 files inside: "nmea_checksum.exe" and "nmea_checksum.bat"
Run the .bat script.

Hah, hah, we lay people don't recognise these tricks. I actually stripped the 7z out!!
Tks
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Old 30-01-2015, 04:09   #28
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Re: OCPN support VHF DSC Position NMEA sentences?

Shouldn't some of this find its way into the wiki if it is not covered in some areas? Just to make wiki clearer?

Sent from my SCH-I545 using Cruisers Sailing Forum mobile app
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Old 30-01-2015, 04:24   #29
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Re: OCPN support VHF DSC Position NMEA sentences?

RD
I use NMEA MTK checksum calculator which works for me. Make sure you trim the sentence as indicated.
Tony
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Old 30-01-2015, 04:49   #30
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Re: OCPN support VHF DSC Position NMEA sentences?

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Originally Posted by Gypsy23 View Post
Hah, hah, we lay people don't recognise these tricks. I actually stripped the 7z out!!
Tks
Got it, it works. Thanks, I'll put it into a safe place for more use later.
Ray
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