Big ships v small boats at night

[2nd_apprentice]

[ QUOTE ]
[ QUOTE ]
Are you confusing non-parallel multiplex with single frequency receivers?

[/ QUOTE ]
No, are you? Are there any "single frequency receivers"?

Common terminology seems to be gravitating to alternating dual channel receivers being called "single-channel scanning", see here.

Even the Nobeltec 100, which everyone refers to as "single-channel" has both AIS frequencies and alternates between them each 36 seconds - not the 6 minutes that you earlier implied.

[/ QUOTE ]

Can't see your point there. It does say that they're multiplex AIS receivers so of course they get both frequencies. They only have one channel but they're not single frequency receivers.

 

[graham]

I dont think anyone would "rubbish" AIS. The point that I think some are making is the need to be aware that it isnt a reliable method of identifying all ships all of the time.

Its a great tool but reliant on factors completely out of your control.

 

[jonjo]

[ QUOTE ]
You are appearing to suggest that you as a small vessel would keep out of the way of a commercial vessel if the CPA was very small

[/ QUOTE ]
No, I am suggesting that it is preferable for a leisure craft to make a small but early course change if it does not cause confusion. If all yachtsman were to ping merchant vessels every time their AIS unit shows a cpa between 0.5 & 1nm there are going to be many sharp exchanges on the airwaves.

[ QUOTE ]
then imply that a commercial vessel was solely to blame for a collision/near collision when a similar small vessel didn't.

[/ QUOTE ]
No I am saying that established P&O practice of making slow course alterations, so as not to perturb passengers, is the only explanation I can think of that explains why the Ouzo ended up under the bow of the PoB.

 

[BrianH]

[ QUOTE ]
I dont think anyone would "rubbish" AIS.

[/ QUOTE ]

Then I am guilty of hyperbole and being too sensitive when I read:
"Having 'played' with AIS for 4 years now, I am firmly convinced that it is a not a reliable technology."

And:
"The cheap sets only receive on one frequency so you won't see 50% of the transmissions. How bloody useful!
So called "multiplex" receivers alternate between 161.975 MHz and 162.025MHz which sounds much better. They switch over every 6 minutes."

The first author later admitted, with endearing grace and humour, that his conclusion was not objective and was based solely on his own less than perfectly performing system.

The second author is thoroughly confusing me with irrelevancies about single-frequency systems, which are not identified when requested and wouldn't alter the principle of how long critical data can be delayed, so I shall just bow out of a senseless discourse.

 

[jonjo]

In an earlier post you implied channel limitations on cheaper sets seriously undermines the usefulness of AIS because...

[ QUOTE ]
They switch over every 6 minutes. In this time PoB will have sailed 2nm.


[/ QUOTE ]
I think other research in this thread has debunked the blind for 6 minutes theory.

I have a £200 AIS set (EasyAIS) and significant targets rarely go more than 10 secs between position squawk updates. The rotation speed of a typical yacht's radar scanner equates to 2 second updates so the performance of these two technologies is not vastly different in this respect.

 

[Danny Jo]

[ QUOTE ]
Can you not recall the CPA and tCPA figures that prompted the call? If I made such a call the numbers would be imprinted on my brain cells.

[/ QUOTE ] I have a £220 Nasa unit. It does not appear to be capable of delivering the CPA and tCPA. This was a bit of a disappointment when I first used it, but then a more expensive receiver would not have suited me, because I don't have a chartplotter. (My 20 year old B&G instruments are all wired into a Philips AP Navigator, and while they continue to work well I am loth to attempt to add or change components of the system.) I take your point about "bothering" commercial seamen, and in my defence I should make it clear, as I did in the exchange I described, that my stated aim is to avoid impeding the other vessel. (The unstated aim, of course, is to establish that the other vessel knows of my existence.)