AIS Class B delays

[JohnGC]

Re: more information

Professional mariners.

Those of us with the equipment have seen big ships altering course to miss us prior to AIS class B. Why would you suspect that large ships with crew who are diligent enough to plot the erratic radar pings from a small GRP yacht and take avoiding action, would then deliberately deny themselves the hi fidelity class B AIS info from the same target?

No they do not - or at least a significant number do not.

Why do I suspect this?
1. The RYA link I posted. Well worth reading.
2. The commercial vessel I have some knowledge of, filters class B out all of the time.

Why do they do that?
1. This thread shows that class B isn't all that hi-fi.
2. Clutter.

Unless you asked, you have no idea how you where detected by the ship that avoided you. Other options are visual and radar.

 

[DJE]

Re: more information

Would it be better if class B was a system which transmitted with the same priority as class A, but only at the press of a button?

If you switch off transmissions in congested areas that is almost what we have now.

 

[JohnGC]

Re: more information

If you switch off transmissions in congested areas that is almost what we have now.

Not really. You don't have the option to send a single position report with class A priority.

I'm not saying it's a great idea; just interested in where the discussion leads.

 

[john_morris_uk]

Re: more information

No they do not - or at least a significant number do not.

Why do I suspect this?
1. The RYA link I posted. Well worth reading.
2. The commercial vessel I have some knowledge of, filters class B out all of the time.

Why do they do that?
1. This thread shows that class B isn't all that hi-fi.
2. Clutter.

Unless you asked, you have no idea how you where detected by the ship that avoided you. Other options are visual and radar.

I can understand the justification in confined waters (too many alarms are confusing) but if a commercial ship turned off Class B in open waters and collided with a small craft transmitting Class B the Bridge watch keeper would be hung out to dry. They'd be in deep guano even if the small craft wasn't transmitting AIS.

It's a piss poor excuse not to have Class B if you can afford it.

As in all things at sea and IRPCS, assume nothing and navigate with caution. Refusing to have AIS for the reasons given is foolishness. IMHO.

 

[BrianH]

Re: more information

I can understand the justification in confined waters (too many alarms are confusing) but if a commercial ship turned off Class B in open waters and collided with a small craft transmitting Class B the Bridge watch keeper would be hung out to dry. They'd be in deep guano even if the small craft wasn't transmitting AIS.

It's a piss poor excuse not to have Class B if you can afford it.

As in all things at sea and IRPCS, assume nothing and navigate with caution. Refusing to have AIS for the reasons given is foolishness. IMHO.

I agree completely. I have noted enough input from professional seamen to know that they welcome Class B reports in open waters where their IRPCS responsibilities are assisted by them. Small vessel positions are as important to be aware of as larger Class A report vessels and AIS assists them in being so.

 

[JohnGC]

Re: more information

I can understand the justification in confined waters (too many alarms are confusing) but if a commercial ship turned off Class B in open waters and collided with a small craft transmitting Class B the Bridge watch keeper would be hung out to dry. They'd be in deep guano even if the small craft wasn't transmitting AIS.

It's a piss poor excuse not to have Class B if you can afford it.

As in all things at sea and IRPCS, assume nothing and navigate with caution. Refusing to have AIS for the reasons given is foolishness. IMHO.

What's "a piss poor excuse"? Evaluating a system and discovering that it won't work in my situation?

I was surprised to learn that commercial vessels can and do filter out class B AIS. I haven't refused to have it but it is clear (to me at least) that having a class B transmitter it is nowhere near as useful as I had supposed or hoped.

I have also been led to believe that commercial vessels use radar much more. If that is the case, then I would rather invest in something that gives me a reliable radar profile than a class B transmitter.

It would be interesting to read what those with experience on commercial vessels have to say.

 

[john_morris_uk]

Re: more information

What's "a piss poor excuse"? Evaluating a system and discovering that it won't work in my situation?

I was surprised to learn that commercial vessels can and do filter out class B AIS. I haven't refused to have it but it is clear (to me at least) that having a class B transmitter it is nowhere near as useful as I had supposed or hoped.

I have also been led to believe that commercial vessels use radar much more. If that is the case, then I would rather invest in something that gives me a reliable radar profile than a class B transmitter.

It would be interesting to read what those with experience on commercial vessels have to say.

I have only kept watch on a warship but I've been on the bridge of various commercial vessels at sea round the world. My experience (albeit limited to perhaps 20 hours of commercial vessels bridge time) is the extreme professionalism of those responsible for the safe conduct of the vessel.

Its one of the reasons I have a certain amount of righteous indignation when I read of the almost paranoid attitude of some yachtsmen when faced with a large vessel in in open waters.

 

[RichardS]

Re: more information

I crossed the TSS shipping lanes off Grado and off Venice a couple of weeks ago. This is the first time I've sailed across a TSS and I found the AIS so useful that I showed the screen to the family. There was only one vessel in view but 3 or 4 on the AIS screen and I was able to say to the crew that if we held our current speed and course (which was 90 degrees to the lane, of course) ship A would pass 0.5 miles ahead in 30 minutes, ship B 1 mile ahead in 60 minutes, ship C 0.5 miles aft in 45 minutes and ship D 1 mile aft in 50 minutes.

We held our course and speed and everything happened exactly as predicted. I'm absolutely sure that the OOW on each of the bridges had made the same observation to himself based on our Class B transmissions and this certainly helps to engender the feeling that everything is under control.

This was in broad daylight but could have been at night or in fog, if there ever is fog in the north Adriatic.

I've scrapped the idea of buying radar now as I'm satisfied that I've balanced my personal risk vs cost equation.

Richard

 

[GHA]

Re: more information

It would be interesting to read what those with experience on commercial vessels have to say.

Very much so.

 

[JohnGC]

Re: more information

I have only kept watch on a warship but I've been on the bridge of various commercial vessels at sea round the world. My experience (albeit limited to perhaps 20 hours of commercial vessels bridge time) is the extreme professionalism of those responsible for the safe conduct of the vessel.

Its one of the reasons I have a certain amount of righteous indignation when I read of the almost paranoid attitude of some yachtsmen when faced with a large vessel in in open waters.

The few interactions I have had with commercial vessels support your claim of professionalism. In contrast I have witnessed some truly stupid behaviour on the part of small boats.

What really interests me is the extent to which commercial vessels monitor class B AIS; this is the primary factor for me in considering a class B TX purchase. The argument that a commercial vessel manoeuvred to avoid a small vessel and therefor must have been monitoring class B doesn't hold water. Such manoeuvres occurred before the widespread use of AIS and also occur when the small boat doesn't transmit AIS.

The delays to class B which can occur in busy areas (so well illustrated by the OP) would modify my reliance on class B in those areas but wouldn't be the deciding factor for a purchase.

 

[jonjo]

Re: more information

The argument that a commercial vessel manoeuvred to avoid a small vessel and therefor must have been monitoring class B doesn't hold water.

You have imagined that argument. The argument is that it demonstrates pro mariners have some motivation not to run down small leisure craft and use what observations that are available to them to achieve that outcome.

Such manoeuvres occurred before the widespread use of AIS and also occur when the small boat doesn't transmit AIS.

Which is what I told you earlier in this thread from which we can conclude pro mariners are diligent. The widespread scare mongering on the internet by people like you about reckless pro's disabling disabling class-B are in fact an infrequent occurrence and minor concern.

so well illustrated by the OP[/B]) would modify my reliance on class B in those areas but wouldn't be the deciding factor for a purchase.

It is just one guy on the internet theorizing about something he does not understand as illustrated by his recommendation to lower the antenna height of a class-B vessel to improve matters. I suspect his troubles would go away if (1) he first confirmed signal quality of his transmissions (2) Raised the antenna to mast height in order to give his class-B AIS a full picture of the used and free AIS slots and so allow his set to pick genuinely free data slots.

 

[Sandy]

Re: more information

So do I. In a shipping lane I do not find AIS assessment at the chart table very 5 or 10 minutes to be a burden. The biggest hassle at the moment is waking MS Windows up from sleep mode and waiting a minute for target updates. This hurdle will be removed when I fit one of those Lynx 10 tablets with its low current draw.

I have a https://www.vespermarine.com/ unit on board, no need for anything to wake up.

 

[jonjo]

Re: more information

I have a https://www.vespermarine.com/ unit on board, no need for anything to wake up.

I need to improve my AIS setup for a singlehanded sailing venture I am considering next, I will checkout your suggestion if they exhibit at SBS. My current plan is for a LYNX-10 windows tablet running Open/CPN at the chart table as recommended by the proprietor of visit my harbour at his retail outlet in West Cowes. This will be augmented according to SBS deals by a 7" Raymarine display at the helm coupled with a Raymarine 650 class-B. The IPS boating titles seem to have an editorial downer on Raymarine at the moment and score their products harshly without explanation. The only downside I see with Raymarine is the laggard CPUs they fit in their MFDs, this has been a Raymarine failing for years.

My fear of big shipping in the Channel has passed since fitting an AIS receiver, the small stuff is my greater concern. I had more close encounters with yachts during my last Channel crossing as we were all transiting the route between Alderney and the Needles. I had to keep reminding myself less than 50% of yachts have class-B AIS, I can feel my first investment in radar coming on for the solo venture.

 

[JohnGC]

Re: more information

You have imagined that argument. The argument is that it demonstrates pro mariners have some motivation not to run down small leisure craft and use what observations that are available to them to achieve that outcome.

RichardS in post #48 makes exactly that argument. He may be "absolutely sure" that the OOW saw him on AIS. But there is no evidence in his anecdote that he was seen at all let alone on AIS. Of course I have no evidence that he wasn't seen on AIS either.

Which is what I told you earlier in this thread from which we can conclude pro mariners are diligent. The widespread scare mongering on the internet by people like you about reckless pro's disabling disabling class-B are in fact an infrequent occurrence and minor concern.

Oh come on! I have never suggested that pro mariners are not diligent. You have taken my reporting that some of them filter out AIS class B to be scare mongering; that is your interpretation not mine. Do you also think the RYA are scare mongering? To be clear; what I am suggesting is that some (possibly a significant number) of commercial vessels do not monitor class B AIS because they find radar and visual observation sufficient. I'd like to know how widespread that is. The RYA article I linked to suggests it could be a common practice.

It is just one guy on the internet theorizing about something he does not understand as illustrated by his recommendation to lower the antenna height of a class-B vessel to improve matters. I suspect his troubles would go away if (1) he first confirmed signal quality of his transmissions (2) Raised the antenna to mast height in order to give his class-B AIS a full picture of the used and free AIS slots and so allow his set to pick genuinely free data slots.

I think this touches on a good point. Standard VHF antennas are not optimal for AIS, so ensuring the installation and antenna type are as good as possible must be worthwhile.

In post #25 you stated that there are 4000 slots per second for AIS transmission; I believe this is actually 4500 per minute or 75 per second. Taking the higher 4s transmission rate for messages gives bandwidth for up to 300 vessels. Allowing for the additional slow rate transmissions and slot finding time it's probably closer to 250. That should be plenty for even the most crowded locations.

But, there may be other bandwidth restrictions within the AIS RX chain. NMEA 0183 at 38400 baud has bandwidth for 83 messages per second and should be able to keep up, but can the plotter it is connected to? There is also the question of how many AIS targets a particular piece of RX equipment can handle at any one time. My own AIS receiver which is integrated with DCS VHF doesn't specify this. Does anyone have a figure for their own equipment? Does anyone know how many AIS targets OpenCPN can handle at once?

 

[jonjo]

Re: more information

Yes my mistake over the AIS 4000 slot interval, I am used to calibrating the performance of larger computer servers that deal with with 1000's of requests per second.

75 messages per second for Open/CPN running on a PC should not be a struggle, the software is crafted using an older programming language known for its efficiency. Who knows with lower-end custom devices but generally CPUs can chew their way through a mind boggling number of logical operations per second when not reaching out across a network.

From casual observation I reckon 30% of ships are doing under 14knts SOG, 60% 14 to 23 knots and the rest are faster. I make that about 1100 slots per 100 vessels plus say a margin of 10% for turning vessels upping their transmission rate. My masthead antenna picks up some AIS targets at around 40 miles and even in the central English Channel I doubt Open/CPN has shown more than 50 targets. Yes the AIS station slot tables must be getting busy but we are some way from class-B stations getting squeezed out and self muted. Given the limited 2W transmit power of class-B it would take an incredible number of class-B stations to get into an unproductive dispute over which vessels get the remaining free slots at random.

As to monitoring, how many large vessels do not have integrated displays these days? Yachtsmen can purchase integrated AIS and Radar displays for < £1000.

Could some of the confusion over class B monitoring on large ships be related to alarms rather than active monitoring? I can see that in Southampton water a ship might disable AIS alarms from class B stations due to all of the spoof alarms raised by leisure boats exiting the Hamble.

 

[JohnGC]

Re: more information

Yes my mistake over the AIS 4000 slot interval, I am used to calibrating the performance of larger computer servers that deal with with 1000's of requests per second.

75 messages per second for Open/CPN running on a PC should not be a struggle, the software is crafted using an older programming language known for its efficiency. Who knows with lower-end custom devices but generally CPUs can chew their way through a mind boggling number of logical operations per second when not reaching out across a network.

From casual observation I reckon 30% of ships are doing under 14knts SOG, 60% 14 to 23 knots and the rest are faster. I make that about 1100 slots per 100 vessels plus say a margin of 10% for turning vessels upping their transmission rate. My masthead antenna picks up some AIS targets at around 40 miles and even in the central English Channel I doubt Open/CPN has shown more than 50 targets. Yes the AIS station slot tables must be getting busy but we are some way from class-B stations getting squeezed out and self muted. Given the limited 2W transmit power of class-B it would take an incredible number of class-B stations to get into an unproductive dispute over which vessels get the remaining free slots at random.

As to monitoring, how many large vessels do not have integrated displays these days? Yachtsmen can purchase integrated AIS and Radar displays for < £1000.

Could some of the confusion over class B monitoring on large ships be related to alarms rather than active monitoring? I can see that in Southampton water a ship might disable AIS alarms from class B stations due to all of the spoof alarms raised by leisure boats exiting the Hamble.

I think we agree that there are enough AIS slots for 100s of vessels. Although if you take a look at Shanghai via MarineTraffic it may not always be the case.

What OpenCPN could do verses what it actually does is down to the programmers. I agree that on a PC OpenCPN could handle all of the available slots but does it?

No I'm not referring to AIS alarms but to the removal of class B AIS targets from displays.

Snips from the RYA link (the bold highlights are mine),

But and it’s another big but, the revised performance standard requiring the integration and display of AIS data on radar displays includes a requirement to be able to reduce clutter on displays so that detection performance is not adversely affected.
The same standard goes on to state that ‘Reported targets provided by the AIS may be filtered according to user-defined parameters’; to you and me that’s the man on the bridge and he is not going to be drilling down through layers of menu levels to find the filter!

..............
Certainly, a number of manufacturers have incorporated Class B filtering on their products and I currently write this from a vessel that has this feature available at the flick of a switch!

..............
Which leaves the obvious question: "Is it worth investing in AIS Class B if ships are going to switch it off?"

Or as I put it in my first post? Who is monitoring class B AIS? Would I be better off spending my dosh on something which ensures I have a reliable radar return?

There is no doubt in my mind that AIS RX linked to my plotter was a very good investment in my safety and those who sail with me.

 

[BrianH]

Re: more information

No I'm not referring to AIS alarms but to the removal of class B AIS targets from displays.

Snips from the RYA link (the bold highlights are mine),

Or as I put it in my first post? Who is monitoring class B AIS? Would I be better off spending my dosh on something which ensures I have a reliable radar return?

There is no doubt in my mind that AIS RX linked to my plotter was a very good investment in my safety and those who sail with me.

The article was written by Stuart Carruthers, RYA Cruising Manager, who is more than qualified to comment on the possibility of AIS Class B filtering, being a surveyor and engineer himself. However, the points made are, in my opinion, speculative with no evidence that professional deck officers make a habit of sailing with AIS Class B targets removed from their target displays and alarms on a regular basis. I am more than a little surprised that the RYA lends itself to publishing what could be interpreted as a non-proven, scaremongering article.

I am not implying that the functionality of removing Class B targets is not possible, we all know from earlier threads and Panbo articles that it is. What I would like to refute is that a typical commercial ship in open waters has AIS Class B targets removed from its displays as a matter of course.


.

 

[JohnGC]

Re: more information

<snip>. What I would like to refute is that a typical commercial ship in open waters has AIS Class B targets removed from its displays as a matter of course.


.

I would like to see it refuted as well but do you have evidence to do so? Without evidence your statement is also "non-proven".

The RYA article draws attention to a rumour, examines some evidence but doesn't reach a conclusion regarding how widespread the practice is. In my book that is not scare mongering but the sensible raising of an issue many small boat owners may not be aware of. Of course if you have AIS class B TX and assumed all commercial vessels monitor it I can see you might get a fright.

It cannot be argued that not using AIS class B equals poor watch keeping. If it did there would have been no correct watch keeping before AIS.

It can be argued that reliance on class B TX to inform commercial vessels of the presence of a small boat is questionable. I have no argument with it as a supplementary aid.

 

[RichardS]

Re: more information

I would like to see it refuted as well but do you have evidence to do so? Without evidence your statement is also "non-proven".

The RYA article draws attention to a rumour, examines some evidence but doesn't reach a conclusion regarding how widespread the practice is. In my book that is not scare mongering but the sensible raising of an issue many small boat owners may not be aware of. Of course if you have AIS class B TX and assumed all commercial vessels monitor it I can see you might get a fright.

It cannot be argued that not using AIS class B equals poor watch keeping. If it did there would have been no correct watch keeping before AIS.

It can be argued that reliance on class B TX to inform commercial vessels of the presence of a small boat is questionable. I have no argument with it as a supplementary aid.

But to refer back to my post, for commercial vessels transiting the TSSs in the North Adriatic the most likely collision risks will almost certainly be the many leisure craft crossing the lanes on passage between Italy and Croatia/Slovenia. As you say, I have no proof that those commercial vessels are monitoring Class B but they would have to be mad not to be.

Richard

 

[Roberto]

It is just one guy on the internet theorizing about something he does not understand

Let me just laugh

Have fun