AIS Transponders for Yachties

[macd]

Plus remember its a low power VHF signal on line of sight so subject to the obvious limitations that entails.

I know 'line of sight' is so in theory, since AIS uses VHF frequencies. But it manifestly is not line of sight in practice: my AIS receiver regularly 'sees' even Class B targets on the other side of islands (such as Faial in the Azores and Majorca, neither of which lack height). Anyone care to explain why?

 

[maby]

I know 'line of sight' is so in theory, since AIS uses VHF frequencies. But it manifestly is not line of sight in practice: my AIS receiver regularly 'sees' even Class B targets on the other side of islands (such as Faial in the Azores and Majorca, neither of which lack height). Anyone care to explain why?

'cos it is not really limited to line-of-sight. Radio signals at those frequencies are deflected in the relatively low atmosphere (as opposed to short wave signals that are deflected in the high atmosphere) by sharp discontinuities in temperature and humidity. The most common effect is a temperature inversion - normally the air temperature decreases steadily with increased altitude but if you get a situation where there is a layer of warm (and possibly moist) air some thousands of feet up with colder air both below and above, then it can act like a duct with the VHF radio signal bouncing along it - possibly many hundreds of miles. You get such conditions more often in the summer than the winter, and it's not uncommon for radio amateurs on the 2m band (almost the same frequency as AIS) to talk to other stations well into central Europe.

 

[laika]

Question for Solent regulars who want to be able to filter out "B" from "A": What do you use the information from "A" for on those occasions when "B" is of no use to you?

After the novelty wore off, I found myself switching off AIS entirely in the solent. Locals will know rather more about where the Wootten ferry or that car carrier coming down the Thorn Channel will be in 10 mins time than their AIS can tell them. Visitors overloaded with information from their charts and pilot books might be forgiven for not knowing which ferry is which and appreciate the turning info perhaps. I can imagine someone planning a tight race strategy might like to know when a large vessel they can't see leaves their berth, but wouldn't ch. 14 tell you this without having to look at the plotter? For non-racey me, keeping an eye on where stuff is likely to appear from (especially if pre-warned by a VTS broadcast) gives plenty of time to get out of the way of anything big. YMMV.

In limited vis when I do have the AIS on, there's not normally many yachts out there, and I frankly don't mind knowing where those with transmitters are.

This isn't a question about the utility of AIS in general, just about use of a receiver in crowded waters like the solent where the commercial traffic generally follows a predictable but non-straight-line route. How do others find AIS A info useful in such a place when B isn't?

(I do recognize that part of this thread raises a concern about "B" clutter unrelated to cluttering up yachties' displays: this question is unrelated to that)

 

[fireball]

Question for Solent regulars who want to be able to filter out "B" from "A": What do you use the information from "A" for on those occasions when "B" is of no use to you?

Its nice to know where that big ship coming in to the solent is going - ie is it going to turn off into Pompey or carry on to Southampton .. AIS gives you (an idea of) that.

I could turn off AIS receiver all together - but it's a PITA as it's not on a separate switch ...
TBH I don't want AIS B at all (most of the time) because not only does it display on the CP - it gets included in the alarms (used x channel) ...

 

[maby]

Question for Solent regulars who want to be able to filter out "B" from "A": What do you use the information from "A" for on those occasions when "B" is of no use to you?

...

I suppose the point is that we small boat owners are more interested in being warned about big boats that have the power to sink us while, in return wanting them to know about us and recognising that they can easily not notice us. As an owner of a small boat, I'm a lot less concerned about the possibility of a close encounter with another yacht and a lot more optimistic that the skipper of the other boat will see me before he hits me. OK - I know that being t-boned by a Beneteau under full sail can do a lot of damage, but we run that risk every week when we race.

 

[laika]

Its nice to know where that big ship coming in to the solent is going - ie is it going to turn off into Pompey or carry on to Southampton .. AIS gives you (an idea of) that.

I don't usually find the Pompey/Southampton question such a problem. By the time it's visible vessel type is *usually* (exceptions obviously) a pretty good clue and unless I'm way east, by the time I see it the aspect should give it away. OTOH medium sized coasters etc. which may or may not be going up the North channel have often left me thinking "hmmmm..." and certainly yes I wouldn't be ignoring the AIS if it was on.

I could turn off AIS receiver all together - but it's a PITA as it's not on a separate switch ...

Blame the cowboy installer ;-)

 

[fireball]

Blame the cowboy installer ;-)

 

[BrianH]

TBH I don't want AIS B at all (most of the time) because not only does it display on the CP - it gets included in the alarms (used x channel) ...

I've just returned from an extended Adriatic cruise and the unnecessary alarms generated by Class B targets were getting to be tedious. I have to go below to the netbook on the navigation table to cancel and would dearly love to filter them out entirely.

On passage I need the ability to detect the ultra-fast ferries, which seem to be driven by cowboys who enjoy frightening leisure sailors, but the spurious Class B alarms are making me turn off AIS altogether.

Perhaps OpenCPN developers could consider a modification ....?

 

[macd]

Maby: thanks for the explanation.

 

[Jumpy07]

Especially on a motorboat on Loch Lomond I spotted on MarineTraffic last week.

I guess it was a visitor hauled there by RangeRover (it was a 30' motor boat) but why even have it switched on!

Some people are to quick to judge..

Or boat was on the Clyde for many years and we moved it to Loch Lomond a couple of years back due to change in personal circumstances.... I run www.aisclyde.com and I use the transponder on the boat as a remote AIS receiving station (I have three round the Clyde), which picks up Loch Long and Loch Fynne from the north end of Loch Lomond. I leave the transponder on tx.. but in reality the data is sent over a 3G network using UDP connection to my server at home.

I could stop my own vessel being plotted.. but when it drops off I know the power card has run out.. so a form of monitoring..

PS I dont own a rangerover.. and the vessel is too big to tow with one in any case... and I also know how to fit the switch to stop the tx !!

Awaiting apology..

Craig