Understanding AIS

[C08]

If you have a Class B transponder how can you be sure that other large vessels using Class A can see you and your signal has not been "prioritised out" for want of a better term in the situation where there is a lot of tother AIS traffic?

 

[AntarcticPilot]

If you have a Class B transponder how can you be sure that other large vessels using Class A can see you and your signal has not been "prioritised out" for want of a better term in the situation where there is a lot of tother AIS traffic?

If there's a potential collision situation, no AIS will be "prioritized out". But class B has a slower update cycle, so it could be quite a long way (several minutes) behind your actual location. Problem with our vessels is that we can change direction very quickly, much more so than bigger, faster commercial vessels. So things like CPA are less reliable and potentially more confusing to interpret. I know that even under autohelm, waves can knock my head off the set course by ten or more degrees, and it can take several minutes to get back on track

 

[LadyInBed]

how can you be sure that other large vessels using Class A can see you

Give them a call and ask!

 

[KompetentKrew]

There are 2250 AIS transmission slots per minute, so I doubt there's any chance of there being no available slots at the speeds boats are typically moving.

I'd have to check how often class-A vessels transmit and do the maths to say for sure, but I doubt this is a consideration, even in high traffic areas.

Buy a class B+ if you want more reassurance.

 

[mm42]

Any time AIS is mentioned on here the same old arguments on the lines of "big ships turn off class B so you'll not be seen" get wheeled out.

As a professional mariner, I've never been on a ship where we've had class B filtered out on our navigation equipment, we find it really useful.

 

[Roberto]

If you have a Class B transponder how can you be sure that other large vessels using Class A can see you and your signal has not been "prioritised out" for want of a better term in the situation where there is a lot of tother AIS traffic?

I have both a transponder (simple class B) and a receiver (integrated into the VHF), with the receiver I can "see" my own transmissions: in places with a lot of traffic I have experienced delays of 4-5+ minutes among two of my transmissions; likewise, I often had virtual collisions with the positions of other friends' boats when they were actually several cables away (so their class B was delayed too). One should set the display parameters as "Mark target as lost/old after x seconds" but you cannot be sure that other people do it.
When I notice such delays I switch the transponder off and keep just the receiver, which unfortunately means also when crossing shipping lanes (though I rely on an RTE), should upgrade to a B+ really.

edit
Opencpn allows several inputs, I brought both the transponder flux and the receiver, switching from one to the other makes for interesting comparisons, like range for example (receiver antenna at masthead, txr on the stern about 4m asl), one can definitely tell when another sailboat has its antenna on the stern, it can disappear on one but still be caught by the upper antenna.

 

[Sandy]

If you have a Class B transponder how can you be sure that other large vessels using Class A can see you and your signal has not been "prioritised out" for want of a better term in the situation where there is a lot of tother AIS traffic?

In simple terms you can't.

Why you can't make a 20° change in course away from the danger is another question.

I work on the principal I can change course faster than she can, but the big stuff is in my experience, very good and I was surprised to see One Manhattan make a 2° change to her course as she overtook me in the Thames Estuary last August.

How seafarers ever survived before AIS is another taxing question. I am a huge fan of the mark one eyeball.

 

[KompetentKrew]

How seafarers ever survived before AIS is another taxing question. I am a huge fan of the mark one eyeball.

I am looking forward to the release of the Mark II eyeball, as correct focussing of my Mark I's requires the emplacement of additional corrective lenses on the outside.

 

[Uricanejack]

I the short answer is you can’t.

I wouldn’t worry about it. I think it’s probably an urban myth ships turn class B off.
Some might. I can’t think why, severe attacks of Luddism perhaps
As someone who has spent some time using it. I would never consider turning Class B off.

I expect most well reputed operations probably have ISM policies specifying the company required settings on Navigating Equipment which would require it to be on.

When in confined harbour limits. Or Pilotage Waters Stationary AIS targets can be a PIA when passing close by docks, anchorages ect.
Setting off unwanted CPA alarms ect. Most of which are probably Class A.
This can be managed by setting the on board radars not to display those alarms for stationary targets.
Small vessels underway setting of TCPA or CPA alarms not a problem as a diligent watch keeper you would want to ignore.

 

[Bilgediver]

Give them a call and ask!

Preferably using DSC and not channel 16 if you want a response .

 

[AlexLbk]

I dock in a very busy commercial waterway. Tankers, barges, tugs go by 24x7. I encountered a few fully loaded container ships in that channel as well. My vessel is slow and not very agile with a single screw. On every occasion where there was it even a remote risk of collision the ships or tugs reached out using boat name (the name is only shown on the steps behind a folding swimming platform, so you can’t see the name when the platform is folded). The only way they could have obtained that is from AIS transponder, so Class B AIS is definitely seen by large ships.

 

[LadyInBed]

Preferably using DSC and not channel 16 if you want a response .

I don't have DSC.
A quick call on 16 saying that I believe we are on a collision course, what is your intention? does the job.
I use 16 as it's monitored by CG, so if the brown stuff hits the fan, the ship will have difficulty arguing his way out of any subsequent proceedings.

 

[DAVAIS]

There is no priority for Class A or Class B, the difference is the transmission power and the interval time. Class B updated the information every 3 mins or 30 seconds(when it moving) while Class A updated every 6 seconds. Was told by a cargo ship they normally see the small boat/vessel on their AIS system firstly and then by the eye.

 

[KompetentKrew]

… for Class A or Class B, the difference is …

Another difference is that class A reserves slots in the schedule, whereas B transmits in slots which it believes to be empty.

I would assume it's theoretically possible for class A ships to reserve all the slots in the schedule (there are 2250 AIS transmission slots in a minute) and for there to be no spaces in which class B's can transmit, but I can't imagine how congested the area would have to be for this to happen.

It seems more possible that congestion would leave relatively few slots available for class B transmissions, increasing the likelihood that two class B vessels would chose the same slot to transmit in, so that one vessel transmits "over" the other. It seems likely to me that this occasionally happens, but probably not enough to worry about.

 

[emandvee44]

There you go

How often do AIS transponders broadcast position and static information reports?
Transponders send out position updates based on the speed over ground (SOG) of the vessel and updates occur at different intervals depending on whether a Class A or Class B transponder is used. Depending on whether you are using a single channel or dual channel AIS device, reporting intervals will vary based on the tables below:
Receiving Class A AIS Transponder reports

Ships Dynamic Conditions	Dual Channel Receiver	Single Channel Receiver
Ship at anchor or moored	3 min	6 min
SOG 0-14 knots	10 sec	20 sec
SOG 0-14 knots and changing course	3.3 sec	6.6 sec
SOG 14-23 knots	6 sec	12 sec
SOG 14-23 knots and changing course	2 sec	4 sec
SOG >23 knots	2 sec	4 sec
Ship Static Information	6 min	12 min

Receiving Class B AIS Transponder reports

Ships Dynamic Conditions	Dual Channel Receiver	Single Channel Receiver
SOG <2 knots	3 min	6 min
SOG >2 knots	30 sec	1 min
Ship Static Information	6 min	12 min

Until recently I used to spend a lot of time on ships, mostly large tankers. I asked the officers on several occasions if they ever turned off class B reception and the answer was always "we do not have the facility to do that"

M.