AIS and Open CPN Own Boat showing trailing........

TSB240

TSB240

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I have been more than happy using a Matstutec HP33A in conjunction with Open CPN for over two years.

However I have recently found that in more crowded (as in AIS transmitter crowded) areas that I get the Open CPN little green ais target with my own mmsi and boat details following me in some cases quite a distance behind me..
I have also had open CPN collision alarms with my own boat! Only comes up on Open CPN it has never displayed on the HP33A.

I suspect this may be a bug in the Matstutec software and an upgrade to the latest issue is probably overdue. I have used a short term work around which is to program Open CPN to ignore my own boat MMSI.

This does reassure me that I am actually transmitting an ais message.

It however raises a big secondary concern that maybe in more crowded areas or where there are more class A transmissions than class B position update timescales can be lengthy. I never use just AIS in poor visibility as I also have radar.

This is the only explanation I can give for my own boat position to be lagging my actual position by up to 1/4 mile.

Is there a potential and dangerous flaw in AIS only use and in particular for class B users that this bug has exposed. AIS transmissions in my area are relatively low density.

Any comments.
I suspect you have no way of checking your own boat transmitted position unless you have a separate secondary receiver for AIS!

Steve
 
Yes, you're right. In crowded signal conditions the Class B transmissions can suffer quite lengthy delays before being able to find a transmit opportunity. It has been noted before and all you can do about it is be very cautious and be extra vigilant if there's any chance of another boat coming close.
 
You can double click on your boat ais icon and it will tell you the age of the data. You can also program Opencpn to display targets with a line through them if the data is older than a set time, default is 2 minutes for a line and 10 minutes target disappears.
 
GHA said:
You can double click on your boat ais icon and it will tell you the age of the data. You can also program Opencpn to display targets with a line through them if the data is older than a set time, default is 2 minutes for a line and 10 minutes target disappears.
Click to expand...

I will try this and see if I can get any solid data.
But lets say this is a genuine problem especially with a Class A user possibly travelling at 15 -30 knots versus a class B user doing 4 knots. (Perhaps 30 knots is not so likely in poor viz?)

This could mean a CPA of 1/2 or 1/4 of a mile could actually be zero?

I recently had a long trip back from Southern Ireland to Anglesey in zero viz and was very thankful that all Class A ships showed at least 1.5 NM and more CPA's. I don't think we ever had more than 10 Live AIS targets on screen at any one time and doubt there was any significant transmitted position lag due to a high density of AIS use!
 
TSB240 said:
I will try this and see if I can get any solid data.
But lets say this is a genuine problem especially with a Class A user possibly travelling at 15 -30 knots versus a class B user doing 4 knots. (Perhaps 30 knots is not so likely in poor viz?)

This could mean a CPA of 1/2 or 1/4 of a mile could actually be zero?
Click to expand...

I've seen class B transmissions missing a few messages in a row even with hardly any other targets around so treat class B with caution, though from the track you can usually see if something is amiss. Not sure AIS is really the sort of tech to be relying on for anything less than a mile away..
 
TSB240 said:
However I have recently found that in more crowded (as in AIS transmitter crowded) areas that I get the Open CPN little green ais target with my own mmsi and boat details following me in some cases quite a distance behind me..
I have also had open CPN collision alarms with my own boat! Only comes up on Open CPN it has never displayed on the HP33A.

I suspect this may be a bug in the Matstutec software and an upgrade to the latest issue is probably overdue. I have used a short term work around which is to program Open CPN to ignore my own boat MMSI.
Click to expand...

Your transceiver should be sending a VDO message for your own boat, which display software should ignore. It could be an Open CPN issue, or your transceiver may be sending your own boat's data as a VDM message instead.
 
pvb said:
Your transceiver should be sending a VDO message for your own boat, which display software should ignore. It could be an Open CPN issue, or your transceiver may be sending your own boat's data as a VDM message instead.
Click to expand...
Definitely a bug in HP33A It is sending it in VDM and VDO.
 
pvb said:
It has to send it in VDM, otherwise other vessels wouldn't see your position.
Click to expand...

VDM is an NMEA0183 sentence. It travels in wires around your boat, not to others.

On an AIS unit's NMEA interface, messages about other vessels should be in VDM sentences, messages about your own vessel in VDO.

On the radio interface, the distinction doesn't make any sense since the protocol is not NMEA.

Pete
 
prv said:
On the radio interface, the distinction doesn't make any sense since the protocol is not NMEA.
Click to expand...

Exactly. The AIS messages are defined by an ITU spec but I've never got my hands on it to check the details. I hate ITU specs as they're pretty expensive and are hard to get even when they could be justified for the day job, which to be honest AIS can't.

The NMEA0183 VDM sentence simply encapsulates the undecoded AIS message recieved over the air. If you capture them you can see the 6-bit encoded message but I've never tried decoding one. Pretty difficult without the spec anyway.
 
lpdsn said:
If you capture them you can see the 6-bit encoded message but I've never tried decoding one. Pretty difficult without the spec anyway.
Click to expand...
AIS decoder will do it for you, I have 13Mb of raw messages & 17Mb decoded csv file recorded at anchor over a few weeks if anyone is interested in checking how often class B gets received in the real world.
 
lpdsn said:
Exactly. The AIS messages are defined by an ITU spec but I've never got my hands on it to check the details. I hate ITU specs as they're pretty expensive and are hard to get even when they could be justified for the day job, which to be honest AIS can't.
Click to expand...

There's a US government site which gives a lot of detail, all the different message types and fields etc. Not sure if it gets down to the encoding format but it may well do.

Pete
 
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