My experiments have been with sentences based on the research
of the format by James Hebert. He has written a few articles, this is one of them continuousWave: Whaler: Reference: Data Interface in Digital Selective Calling Class-D Radios
. In this he describes a sentence he trapped from a radio sending a cancel message. "$CDDSC,12,3381581370,12,06,00,1423108312,0236,338 1581370,,S,*20". His tests were done near Detroit as the lat/lon in field 6 indicates. His understanding of this sentence is that when a radio sends a distress message (the '12' in the first field) with it's MMSI (field 2) then repeats it's own MMSI in field 8, it is to be interpreted by the receiving radio and attached NMEA listeners as a cancellation.
I created a test DSC Distress message with expansion DSE for my own location in Sydney Australia
Also a Cancel message with DSE:
OCPN (V4) drops a Distress indicator in the correct position but when I send the cancel it seems to just register a new Distress. I have made that diagnosis by seeing that the pop-up message for the second (cancel) sentence starts a new time count.
So far I haven't actually used my radio to generate these messages. Partly because of access to my boat and partly because some of the tests would entail sending a DSC Distress message which would not be welcome by the SAR monitors or other boaters! That's one of the problems of testing this kind of thing. Jim Hebert was able to send and trap the distress messages because he used transmissions into a dummy load and in a relatively shielded area so as not to trip real-world alarms.
I hope to go to my boat in the next few days and try some of the tests with the radio connected, but not the distress message. On the other hand I will also have the opportunity to try my emulator messages into a Raymarine
E120W to see how it responds to the Distress and Cancel messages.
I will trap/log as much as I can and report my findings so we decide what info might help you.
Oh, perhaps you can answer a query. During my experiments I occasionally saw a " -
" black bar dropped over a previous DSC sent vessel location "green diamond" icon. I wondered whether it meant cancel but I haven't seen it across a red distress marker. Is there any listing or description of the symbols/icons used by OCPN for the DSC messages?
Keep up the good work on OCPN.