DJE
Well-known member
I suppose the moral is if you are expecting others to avoid you then hold your course and speed.
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.
You seem to be overthinking this instead of looking for simpler explanations for your AIS troubles.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 ?
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.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.
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?
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
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.
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.
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.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.
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.
Strange. I usually find myself looking for the big stuff to show up visually after first being alerted via AIS.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.
Do you never have to cross a shipping lane? Because that is when AIS target reports can be useful.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.
Scary!
It seems to me that at the very time you require accuracy of yoursystem i.e. in busy shipping areas,it actually increases the danger you are in!
This has convinced me NOT to fit AIS other than a receiver.
Yet another reason not to waste money on a Class B transceiver! Putting the money towards an active radar transponder buys more certainty and, arguably, potential safety.
Strange. I usually find myself looking for the big stuff to show up visually after first being alerted via AIS.
The AIS system is at my nav table, I like to be up in the sunshine/wind/rain/hail <delete as necessary>.Strange. I usually find myself looking for the big stuff to show up visually after first being alerted via AIS.
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.The AIS system is at my nav table, I like to be up in the sunshine/wind/rain/hail <delete as necessary>.
Professional mariners.But who is monitoring class B transmissions?