DJE
Well-known member
I suppose the moral is if you are expecting others to avoid you then hold your course and speed.
AIS Class B delays
- Thread starter Roberto
- Start date
Way
Member
Read this with interest as I've just fitted a new plotter with AIS class B transceiver. Does the situation get better say for eg when you're out mid channel and their are less boats say than in the Solent? Starting to feel that perhaps AIS wasn't a great spend. But then others have said they've valued it in situations like my example.
Roberto
Well-known member
FWIW
my personal observations seem to indicate that delays begin to appear with over 15-20 Class A targets, they begin to be significant say when over 25ish. Of course just an impression, though I have spent some time in checking this againg and again.
Invariably, when I cannot see delays in my position there are few Class A, on the other hand when I see the "lost target bar" over Class B (it s a small bar Opencpn draws over AIS targets when their position is older than xx minutes, I have put 1 minute), then if I check the Target List I find one or two pages, meaning 20-30-40+ targets.
There is also another factor: how does one's Class B decide when to send its signal?
I try to explain.
My ais receiver (combined with vhf) has a masthead antenna: it consistently receives signals that are 30-40-50 miles away (even much more but then it s more a sporadic propagation situation), the total number of signals is very high.
My ais transceiver antenna is at about 4m height on the stern gantry, so much lower and should not receive the more distant targets, yet at times it gets swamped by the number of Class A transmissions.
Deduction: if my transceiver antenna was at the masthead, with a much higher number of received Class A targets, it would struggle a lot more in finding a free slot to send its signal. It might actually be better to have a lower transceiver antenna, reducing the range of our transmissions but having a higher chance of sending regular updates.
Does that make sense ?
Roberto
Well-known member
Idea
An idea: right now I am not on the boat and I cannot check by myself, if anyone has a transponder with ProAis software it might be interesting to check if it allows to set a distance filter to the received signals.
For example, one might set a filter at say 3-4-5 miles, the transceiver would consider only the targets inside this perimeter to decide when there is a free slot.
Not sure if possible nor if AIS protocols work like this, but maybe worth checking ?
An idea: right now I am not on the boat and I cannot check by myself, if anyone has a transponder with ProAis software it might be interesting to check if it allows to set a distance filter to the received signals.
For example, one might set a filter at say 3-4-5 miles, the transceiver would consider only the targets inside this perimeter to decide when there is a free slot.
Not sure if possible nor if AIS protocols work like this, but maybe worth checking ?
You seem to be overthinking this instead of looking for simpler explanations for your AIS troubles.
The AIS system has 4000 slots available per second and overload rules to accommodate extra transmitters when under stress. Are we to believe that you regularly sail in overloaded AIS regions or is a simpler explanation most likely?
- Is your boat generating local VHF interference leading your class-B transceiver to think all AIS slots are occupied?
- Could your class-B device have a substandard algorithm for distinguishing background VHF noise from genuine AIS squawks?
- Is your antenna faulty causing your boat to transmit garbled AIS data?
Class-B is great out in the Channel, I reckon +20% of offshore yachts now have a class-B transmitter fitted. I began to feel like an receive-only AIS freeloader sailing around between the Solent, Cornwall and Channel Island over the past 2 weeks. Two weeks ago I think I perturbed another forumite south of Salcombe in the dark as I had multiple broaches in his direction. I knew from his AIS we were 1 to 2 miles apart but from his perspective my nav light signature frequently changed from white to green and then red+green at a range he had to visually estimate.
Roberto
Well-known member
Well, you believe what you want, I am just noticing and reporting a number of cases where class B transmitters, *not only mine* but every class B transmitter I have received, are operating in a perfectly normal way when there is a limited number of class A signals, whereas they begin to show significant delays when a few tens of class A are around. The list of possible causes you indicate should then apply to every class B I have received, as the behaviour is the same.
I am simply reporting, up to each one -should they wish so- to make his/her own tests and personal opinions and believe what they want.
Anyway, it s easy to check for everyone: have a look at the Position Age in seconds/minutes of other class B targets when in crowded waters
lw395
Well-known member
Re: more information
If there was another class B transponder 2 miles to the north west of you, they might be oblivious of each other and sharing some slots.
Likewise all those AIS stations in the harbour won't have line of sight to the yacht.....
AIUI there is some mechanism so that slots are less likely to be overspoken repeatedly in this way.
A class b signal at 3.4 mile range could be transmitting in the same slots as something in another direction. Your set will only receive the biggest signal in a given slot.as fog and windless days continue, I keep on gathering data
this class B position (not myself, another pleasure boat) shows a delay of about 4 minutes, at say 6 knots-3m/s, 4 minutes = 240 seconds --> difference in position 240*3 --> next to half a mile
there were about 35 AIS targets, mostly class A.
I *think* I have discovered something: while watching the "position age" on the various class B targets, myself included, I have seen that the "ages" before any update are always multiple of 30s.
IMHO, to be verified, a class B sends its position, then waits the regular 30s (if speed above 2kt), at the 30th second it checks if there is a carrier (airwave carrier), if there are none it sends its update message, if it finds a carrier then it puts itself on standyby for another 30 seconds. My impression is it does not monitor constantly what happens on the frequency, trying to send its data as soon as a slot is available, but rather checks every 30s if there is a free slot to send its position, if not then waits 30 more seconds before trying again.
The one above was udated at 4m:30s
If there was another class B transponder 2 miles to the north west of you, they might be oblivious of each other and sharing some slots.
Likewise all those AIS stations in the harbour won't have line of sight to the yacht.....
AIUI there is some mechanism so that slots are less likely to be overspoken repeatedly in this way.
FullCircle
Well-known member
From the Vesper marine instruction manual:
"Your position will be transmitted every 3 minutes when your speed is under 2 knots or every 30 seconds otherwise. Your vessel information (e.g. name, callsign, etc) is transmitted every 6 minutes."
"Your position will be transmitted every 3 minutes when your speed is under 2 knots or every 30 seconds otherwise. Your vessel information (e.g. name, callsign, etc) is transmitted every 6 minutes."
Roberto
Well-known member
Re: more information
Indeed, hopefully so.
I think the way to ascertain what this all *may* involve would be to ask Class A users to check the magnitude of "Position Age" of the Class B signals they are receiving when in heavy traffic, after all it's the most important aspect.
regards
Indeed, hopefully so.
I think the way to ascertain what this all *may* involve would be to ask Class A users to check the magnitude of "Position Age" of the Class B signals they are receiving when in heavy traffic, after all it's the most important aspect.
regards
Re: more information
Thinking more about this leads to a conclusion that a class-b vessel should mount its antenna as high as possible. Roberto's theory about AIS saturation with a few dozen class A transmitters in the area remains suspect in my opinion, however the limited view of a low antenna could lead to a class B station being frequently overspoken by a larger class A vessels +20 miles away but when given a better AIS perspective the class B station could find genuinely free AIS slots to squawk in.
Sandy
Well-known member
Re: more information
I find AIS a useful aid to navigation, but have usually spotted the big stuff as they come over the horizon. Seeing the system do the maths takes a bit of stress out of the day, but I am not planning to get anywhere that close to the big stuff that needs pinpoint accuracy.
I find AIS a useful aid to navigation, but have usually spotted the big stuff as they come over the horizon. Seeing the system do the maths takes a bit of stress out of the day, but I am not planning to get anywhere that close to the big stuff that needs pinpoint accuracy.
Re: more information
Strange. I usually find myself looking for the big stuff to show up visually after first being alerted via AIS.
BrianH
Active member
Re: more information
A case in point that demonstrates that was back in 2009 when I had an early AIS receiver feeding a notebook running OziExplorer - I've moved on a bit since but they worked.
Screen shots of crossing of the Gulf of Trieste from Croatia to Italy. The first shows a RoRo ferry (no name, the scourge of single channel receivers) which gave me a CPA (143m) too close for comfort - I rolled the genoa and slowed down.
But it was the first of a long convoy exiting Trieste and the second in line, Star Lady, gave me a CPA of 391m that was sufficient to cross.
Do you never have to cross a shipping lane? Because that is when AIS target reports can be useful.
A case in point that demonstrates that was back in 2009 when I had an early AIS receiver feeding a notebook running OziExplorer - I've moved on a bit since but they worked.
Screen shots of crossing of the Gulf of Trieste from Croatia to Italy. The first shows a RoRo ferry (no name, the scourge of single channel receivers) which gave me a CPA (143m) too close for comfort - I rolled the genoa and slowed down.
But it was the first of a long convoy exiting Trieste and the second in line, Star Lady, gave me a CPA of 391m that was sufficient to cross.
Last edited:
JohnGC
Active member
Re: more information
But who is monitoring class B transmissions?
Vessels requiring class A are able to filter out class B. Some equipment does this at the flick of a switch, see RYA link below.
http://www.rya.org.uk/cruising/Pages/AIStransmissionsfromsmallcraft.aspx
So you can't rely on your class B position being seen on the bridge of a large vessel.
Many smaller vessels do not have AIS receivers, turn the alarms off or are unable to respond if short handed and busy. So you can't rely on them seeing your transmissions either.
Another question is how you or another vessel respond to AIS alarms.
The AIS traffic is likely to be busy in areas where larger vessels are restricted in their ability to manoeuvre (ports, TSS). So even if there was no delay to your class B, it is you that will be the give way vessel. The larger vessel could use your class B information to confirm you are not in the way but it shouldn't effect their course.
For me AIS receive is an invaluable extra pair of eyes and is particularly useful at night or in fog while on passage. It is also helpful around ports and other busy areas but it is primarily the class A vessels I'm concerned with.
Recently I was on a short handed passage from Holyhead to Millford at night with 3 reefs. My AIS (receive only) showed a CPA of <20m with a cargo ship doing 10kn 3 miles away and I suspected my radar profile wasn't very good. The sea was sufficiently lumpy to visually hide the ship from me and wouldn't make their job of spotting me easy. The action I decided on was to call the vessel (by name from the AIS data) and inform them of the CPA, our vessel type and that we where sailing. I did not propose any action. The OOTW immediately responded, said he would alter his course to starboard for which I thanked him. Within a few seconds my AIS showed his course change.
In similar circumstance I would do this pseudo AIS again. I wouldn't dream of doing it routinely or around a port unless I had manoeuvring problems. But it did get me thinking about the whole class B issue. 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? IE no regular transmission cluttering up displays and RF bandwidth. That way class A vessels wouldn't and couldn't filter out class B, but nether would they need to declutter regular class Bs. The onus would be on the class B vessels to monitor and respond to AIS issues. Or maybe the button press at class A priority would be in addition to the automatic class B, low priority transmissions.
But who is monitoring class B transmissions?
Vessels requiring class A are able to filter out class B. Some equipment does this at the flick of a switch, see RYA link below.
http://www.rya.org.uk/cruising/Pages/AIStransmissionsfromsmallcraft.aspx
So you can't rely on your class B position being seen on the bridge of a large vessel.
Many smaller vessels do not have AIS receivers, turn the alarms off or are unable to respond if short handed and busy. So you can't rely on them seeing your transmissions either.
Another question is how you or another vessel respond to AIS alarms.
The AIS traffic is likely to be busy in areas where larger vessels are restricted in their ability to manoeuvre (ports, TSS). So even if there was no delay to your class B, it is you that will be the give way vessel. The larger vessel could use your class B information to confirm you are not in the way but it shouldn't effect their course.
For me AIS receive is an invaluable extra pair of eyes and is particularly useful at night or in fog while on passage. It is also helpful around ports and other busy areas but it is primarily the class A vessels I'm concerned with.
Recently I was on a short handed passage from Holyhead to Millford at night with 3 reefs. My AIS (receive only) showed a CPA of <20m with a cargo ship doing 10kn 3 miles away and I suspected my radar profile wasn't very good. The sea was sufficiently lumpy to visually hide the ship from me and wouldn't make their job of spotting me easy. The action I decided on was to call the vessel (by name from the AIS data) and inform them of the CPA, our vessel type and that we where sailing. I did not propose any action. The OOTW immediately responded, said he would alter his course to starboard for which I thanked him. Within a few seconds my AIS showed his course change.
In similar circumstance I would do this pseudo AIS again. I wouldn't dream of doing it routinely or around a port unless I had manoeuvring problems. But it did get me thinking about the whole class B issue. 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? IE no regular transmission cluttering up displays and RF bandwidth. That way class A vessels wouldn't and couldn't filter out class B, but nether would they need to declutter regular class Bs. The onus would be on the class B vessels to monitor and respond to AIS issues. Or maybe the button press at class A priority would be in addition to the automatic class B, low priority transmissions.
john_morris_uk
Well-known member
Re: more information
Why? AIS is an aid to collision avoidance. Why not use it?
Exactly. But it doesn't stop me looking out all the time either.
Some people seem to think that shipping is just out to get them.
Why? AIS is an aid to collision avoidance. Why not use it?
Exactly. But it doesn't stop me looking out all the time either.
Some people seem to think that shipping is just out to get them.
Colvic Watson
Well-known member
Re: more information
Receivers are really useful, my transponder is much less so but came as a free function with the Matsutec AIS plotter I bought. It's been a useful function twice knowingly - once in a mayday, how many other times I'm not sure.
Receivers are really useful, my transponder is much less so but came as a free function with the Matsutec AIS plotter I bought. It's been a useful function twice knowingly - once in a mayday, how many other times I'm not sure.
Sandy
Well-known member
Re: more information
The AIS system is at my nav table, I like to be up in the sunshine/wind/rain/hail <delete as necessary>.
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.
Re: more information
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?
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?
