AIS failure

[johnalison]

A funny thing happened to us yesterday as we passed the Hook of Holland. Although the AIS was showing vessels all around us and in the harbour, the range seemed limited to a few miles. It was a while before I realised that the sheer number of vessels was swamping my system. A large cargo vessel bearing down on us didn't show until it was a couple of miles away. The same thing has occurred now that we are in the Scheldt.

I think that I shall have to be less cavalier about leaving mine transmitting when in crowded areas. It could have been quite awkward in fog.

 

[pvb]

Don't venture in to the Solent...

 

[GHA]

Sounds odd, what receiver?

Both the system and receiver should be capable of identifying many thousands of signals.

http://www.navcen.uscg.gov/?pageName=AISworks

The required ship reporting capacity according to the IMO performance standard amounts to a minimum of 2000 time slots per minute, though the system provides 4500 time slots per minute. The SOTDMA broadcast mode allows the system to be overloaded by 400 to 500% through sharing of slots, and still provide nearly 100% throughput for ships closer than 8 to 10 NM to each other in a ship to ship mode. In the event of system overload, only targets further away will be subject to drop-out, in order to give preference to nearer targets that are a primary concern to ship operators. In practice, the capacity of the system is nearly unlimited, allowing for a great number of ships to be accommodated at the same time.

 

[johnalison]

It's a Raymarine receiver through an e7 plotter. I'm sure the system is capable of dealing with the numbers but it looks as if my set isn't. I cleared the memory by switching it off briefly and as the signals reappeared a few distant ones popped up and then vanished. Normally I am picking up vessels from up to 40 miles away or so.

 

[lampshuk]

I have experienced the same thing in the Solent with a Vesper XB8000 and the iPad app. With the same receiver connected to my Android phone or to Seaclear on laptop the app presents the data very fast. So I suspect the issue is with the hardware/app used to present the data, not the receiver.

 

[david_bagshaw]

I have had issues where using a lap top powered from an inverter has scrambled signals from the ais.

fitted a smoothing circuit between the inverter and laptop charger, out of a large old tv that fixed the issue. I could imagine a noisy alternator could have a similar effect.

 

[pvb]

It's a Raymarine receiver through an e7 plotter. I'm sure the system is capable of dealing with the numbers but it looks as if my set isn't. I cleared the memory by switching it off briefly and as the signals reappeared a few distant ones popped up and then vanished. Normally I am picking up vessels from up to 40 miles away or so.

Plotters have a maximum number of AIS targets; your e7 will only show 100, and it'll go for the nearest 100.

 

[LadyInBed]

Try reducing the active reporting range on the chart plotter.
What have you got it set to, 50 mls? Try reducing it to 30.

 

[geoid96]

Earlier this season I upgraded from a Comar AIS-Multi receiver to a Digital Yacht AIT3000 transponder. There was an immediate increase in number of both class A and class B vessels tracked and an improvment in number and speed of static messages received. In both set-ups NMEA 0183 was output to PC running OpenCPN. Both systems incorporate a splitter from the masthead VHF antenna. In my case, the difference in performance was therefore most likely down to the hardware not the application.

The AIT3000 also outputs via NMEA2000 to a Furuno plotter on deck. I haven't noticed any more vessel positions displayed on this compared to OpenCPN at the chart table. This implies that it is not an NMEA 0183 throughput limitation.

 

[johnalison]

Try reducing the active reporting range on the chart plotter.
What have you got it set to, 50 mls? Try reducing it to 30.

That's an idea, but the mass of vessels was within a five mile zone.

 

[Twister_Ken]

Same thing in Algeciras/Gibraltar Roads. So many Class A moving or at anchor that the proximity alarm was howling continuously and target positions were overlapping. Turned the whole shooting match off and relied on eyeball.

 

[GrahamM376]

I think that I shall have to be less cavalier about leaving mine transmitting when in crowded areas. It could have been quite awkward in fog.

According to a retired cross Channel ferry master we had on board a couple of days ago, they turn class B off in crowded areas if too many leisure boats clogging up the screen.

 

[RichardS]

Same thing in Algeciras/Gibraltar Roads. So many Class A moving or at anchor that the proximity alarm was howling continuously and target positions were overlapping. Turned the whole shooting match off and relied on eyeball.

Can't you change the parameters do ignore boats moving slower that 1 knot or with a TCPA more that 30 minutes etc etc. This is what I do.

Richard

 

[Twister_Ken]

Can't you change the parameters do ignore boats moving slower that 1 knot or with a TCPA more that 30 minutes etc etc. This is what I do.

Richard

I suppose sensibly, if the AIS/Plotter sees me shaping to sail under the stern of an anchored merchantman, it assumes a collision is possible, even with the parameters set to minimum values.

 

[dunedin]

Same thing in Algeciras/Gibraltar Roads. So many Class A moving or at anchor that the proximity alarm was howling continuously and target positions were overlapping. Turned the whole shooting match off and relied on eyeball.

Surely you can turn the AIS alarm off but leave the rest of the AIS on - we only switch the alarm on in open waters

 

[GHA]

Surely you can turn the AIS alarm off but leave the rest of the AIS on - we only switch the alarm on in open waters

Same here, the alarm is almost always off.

Still not completely convinced about receivers getting swamped with targets, but maybe so . Google doesn't come up with much. Class A transponders look like they have to cope with masses to meet the design standards but not much info about ordinary receivers.