AIS Alarms on Ownship

[Neptune's Gear]

OK guys, I'd like to get my head around how opencpn actually is supposed to recognize your own ship MMSI. Most other Nav programs allow you to enter this data ...

I've checked this problem on OCPN 3.2 and the current beta, 3.3.1824.

In this instance, with a Matsutec AIS, latest firmware, the ownship MMSI causes collision warnings (with itself!) AND it is listed in the AIS targets list. The docs say this issue is fixed, but it's still current. However, it is not immediate, nor is it repeated in any sort of time cycle. Seems random, and sometimes takes 1/2 an hour of operation to see it. Below is the NMEA data (from VDR) that will cause the issue I'm talking about.

$IIRMC,032837,A,3637.996,S,17447.867,E,2.8,260.0,, ,E,A*33
$IIHSC,260.0,T,241.0,M*42
$IIRMA,A,3637.996,S,17447.867,E,,,2.8,260.0,,E,A*3 C
$IIMTW,34.4,C*10
$GPRMC,032839,A,3637.9970,S,17447.8690,E,2.8,259,2 90614,19,E*5C
$IIMWV,137.9,T,4.7,N,A*34
$IIMWV,110.1,R,3.3,N,A*3C
!AIVDO,1,1,,,B7`B5t@06s82TLJhW<:QcwkVSP06,0*5A
$IIRSA,-3.0,A,,*01
!AIVDM,1,1,,A,B7`B5t@06s82TLJhW<:QcwkVSP06,0*19
$IIDBT,53.1,f,16.2,M,8.8,F*13
$GPGLL,3637.9971,S,17447.8680,E,032840,A
$IIVLW,3990.254,N,1800.106,N*43
$GPRMC,032840.00,A,3637.99680,S,17447.86390,E,2.88 6,261.00,290614,,,D*78
$GPVTG,261.00,T,,M,2.886,N,5.346,K,D*3D
$GPGGA,032840.00,3637.99680,S,17447.86390,E,2,12,0 .69,4.3,M,28.7,M,,0000*43
$GPGSA,A,3,21,18,22,27,24,31,19,11,42,12,25,50,1.2 3,0.69,1.01*09
$GPGSV,4,1,15,03,03,273,18,11,12,224,14,12,09,091, 42,14,72,236,43*71
$GPGSV,4,2,15,18,42,080,46,19,14,264,28,21,18,017, 45,22,75,144,46*75
$GPGSV,4,3,15,24,24,134,48,25,08,057,41,27,09,296, 35,31,19,345,45*76
$GPGSV,4,4,15,32,04,244,15,42,34,310,38,50,37,316, 39*49
$GPGLL,3637.99680,S,17447.86390,E,032840.00,A,D*76
$GPGGA,032840,3637.9971,S,17447.8680,E,1,9,1.4,13, M,32,M,,
$GPVTG,,T,240,M,2.8,N,5.2,K
$GPZDA,032840,29,06,2014,-12,0
$IIVHW,,T,241.3,M,2.6,N,4.9,K*76
$IIHDG,241.3,0,E,19.0,E*6B
$IIDPT,16.2,-0.3,100.0*58

Any assitance in isolating/fixing this error would be appreciated!

[DotDun]

VDM is AIS data from other vessels
VDO is AIS data from your own vessel

Both the VDM & VDO sentences in that capture are from mmsi=512001521, I have to assume that is you.

I'm not sure of the protocol details, conceivably, VDO could cancel VDM, but I doubt that as it seems like a waste of bandwidth. I would have to capture a working network to see if both VDO and VDM are sent for 'own' vessel, that'll take a few days.

Does anyone else know the protocol details?

[Neptune's Gear]

Hey DotDun, thanks for the response.
Yep, that is the MMSI of the vessel.

Reading the stuff I can find on here, flyspray and on GIT etc, it seems to me that OPENCPN gets your vessel MMSI from AIVDO, and will/should then ignore the AIVDM with your MMSI.

It seems to do this almost all the time, just sometimes at random intervals, it causes this error.... It started doing this after an AIS firmware update, so it could be something to do with the AIVDM or /O sentence sentence structure. However, using these sentences as examples, both of them decode fine on a couple of online AIS sentence decoders I could find....

Matt

[Neptune's Gear]

Anyone??

[bdbcat]

Neptune...

I'll take a look at this real soon.

Dave

[Neptune's Gear]

Cheers Dave, I know you guys are Busy!

Sent from my D5503 using Cruisers Sailing Forum mobile app

[PjotrC]

Matt...

Your recorded NMEA looks strange to me. There are three types of sentences stating the position of the same vessel (own ship): RMC, VDM, VDO. This is not normal.

Normally there is either a VDO or a VDM in the stream. VDM is sent/received on the air. VDO is generated on the local interface from the transponder, Usually the transponder has a config option, whether to send a RMC or a VDO for the own vessel.

The usual "ownship alarm" problem with AIS occurs when people have an independent, second receiver, but your case of NMEA stream is something else...

It looks wrong to me. Probably the GPS-generated RMC is multiplexed with AIS transceiver output, differing by a small amount (it can be seen on the status line). But again, VDM and VDO should not occur together, I think.

P.S. I edited out the VDM from your sample, and all seems to work well - no AIS target for ownship, in spite of RMC+VDO together...

[PjotrC]

Originally Posted by PjotrC

VDM and VDO should not occur together, I think.

I checked the sources available (NMEA 0183 and ITU-R M.1371-4) and the sample recordings I have, and indeed no VDM for own ship there...

I do not have the newest edition of NMEA 0183, but I doubt this has changed since. The language is not 100% specific, but I see no reason to output a VDM for ownship on the local interface. The distinction seems clear: VDM for received data, VDO for broadcast data.

If the problem is random and occurs maybe every 30 minutes, then probably the whole stream should be examined...

[Neptune's Gear]

Hi Pjotrc,

There is already a thread about this, and a feature request;

FS#802 - AIS Track own MOB AIS SART & Enter MMSI and chose an Action

This feature would sort it out, but is not yet implemented.

However, I'm just trying to understand exactly what's happening here.

If you don't see both AIVDO and AIVDM in your data stream, then your AIS must filter it - because it has a transmitter and a receiver - it will "hear" it's own transmission.

I asked the manufacturer of this unit I'm testing (matsutec), and they say they don't filter anything, so that is probably the cause.

It can be corrected with the use of NMEA router (separate program), but I'm interested in why OCPN deal with this is a manner different to most other Nav programs, who simply enter their MMSI numbers so the program can differentiate, and change the VDM sentence with your MMSI to VDO...

Cheers
Matt

[bdbcat]

Matt...

Well, of course we can implement your suggestion, when time and resources allow. Low priority so far, but will probably find its way into the next major OCPN Release.


But I'm with PjotrC on this.

What is the logic of having an AIS transponder generate an AIVDM message for your ownship? This is not a rhetorical question, I really do not see any use case where it makes sense. I can see no conceivable reason to do this. In fact, it places the burden on the data stream receiver to (e.g. OCPN) to sort out the conflict.

Trying not to be too fussy about this, I attach a clipping from the NMEA-2.0 spec.

Am I missing something here?

If you are Beta testing this transponder code, I would report a bug in the transponder logic.

But, anyway, you have the hardware that you have, and presumably some other OCPN users do, too. So, the Feature Request will get a bump, because we aim to please.

Thanks
Dave

[Neptune's Gear]

I agree Dave, and it's understandably low priority. I do appreciate all the work you guys do. What I really wanted to confirm that it's unusual to have the VDM sentence for ownship in your Ais stream. So no one else has this by the look of it. I'll report that to the manufacturer...
Thanks again for the help and info!

Sent from my D5503 using Cruisers Sailing Forum mobile app

[MarkJ]

Originally Posted by bdbcat

Am I missing something here?



Dave

Yes Dave.

You are missing the point that its nice to know if our AIS is transmitting. And precisely when.
As it only transmits every 30 seconds when getting close to a ship it really does matter. Also if using an antenna splitter and talking to the ship on VHF the AIS package may not be sent as the splitter takes priority.

One minute is a hell of a long time when a ship is trying to make fish burley with your boat...


Mark

[PjotrC]

Originally Posted by Neptune's Gear

FS#802 - AIS Track own MOB AIS SART & Enter MMSI and chose an Action

Yes, I have seen the FS#802. There are parts I do not understand there, and parts I do not completely agree...

It mainly concerns "MMSI for all SARTs on board".

The MMSI numbers are specific, as per Recommendation ITU-R M.585-6 :

for AIS SART: 970mmyyyy
for AIS MOB : 972mmyyyy
for AIS EPIRB:974mmyyyy

where mm is manufacturer code, and yyyy is serial number per manufacturer, restarting at 0, when exhausted.

Moreover, every active AIS SART/MOB/EPIRB is (also legally now) a distress signal. There should be no option to ignore it, own boat or other boat...

@MarkJ

re: - know when transmitting

Some AIS transponders (in fact, most I know) have a LED to show TX status.

AIS transponder units usually come with a management software tool, that can set static data, etc. and also show what and when was sent and received.

Such a tool might be integrated into chartplotter software, and in fact, some parts are sometimes included in some programs. But usually this means a myriad of dialogs, panels, buttons, etc. and I find it unusable in action, especially on 1 minute notice, while other things do happen. I am also not sure how far this is product-specific.

Including some visible TX indicator in software might be possible, but NMEA really does not suggest issuing a VDM for own transmission (presumably a VDO is issued at the same time), and I think it is not possible for the transponder to receive its own signal from the antenna (especially if Class B)...

P.S. Anyhow, I would suggest not to follow the logic "I have certainly transmitted now, so she will alter her course promptly"...

[bcn]

Originally Posted by PjotrC

Some AIS transponders (in fact, most I know) have a LED to show TX status.

AIS transponder units usually come with a management software tool, that can set static data, etc. and also show what and when was sent and received.

Such a tool might be integrated into chartplotter software, and in fact, some parts are sometimes included in some programs. But usually this means a myriad of dialogs, panels, buttons, etc. and I find it unusable in action, especially on 1 minute notice, while other things do happen. I am also not sure how far this is product-specific.

Including some visible TX indicator in software might be possible, but NMEA really does not suggest issuing a VDM for own transmission (presumably a VDO is issued at the same time), and I think it is not possible for the transponder to receive its own signal from the antenna...

P.S. Anyhow, I would suggest not to follow the logic "I have certainly transmitted now, so she will alter her course promptly"...

And as the software tools for the AIS units might use the same serial/COM port you can only run the tool or OpenCPN. So only the Tx led will tell you.

The other question is what happens in the case the splitter is suppressing the emission of the position report. Does the AIS notice this? If not it will send the VDO sentence but in reality nothing has been sent.

VDM with own ship data just looks wrong. As earlier said: VDO is for own ship data.
Matt has mentioned in other threads already: the Matsutec AIS is not homologated neither for the US nor for EU waters.

[Neptune's Gear]

That is true, (this particular unit is not FCC or EU approved). Many or most of the Asian ones are not. This does not mean you won't see them. They are sold in many parts of the world, and a ship must comply with the regulations of it's registered country...
This particular one I'm just playing with, to see what can and cannot be done.
If I remove the antenna, it no longer sends VDM with its own MMSI, so it must be hearing it's own transmission as far as I can tell..
The more I learn about AIS the better, so playing is good!
I wonder how many AIS units out there will do this....
Matt