AIS transponders group test?

[Seven Spades]

Has anyone seen an AIS Transponder group test anywhere. I crossed the North Sea on a rally last year and if you followed the boats on Marine Traffic some were tracked all the way across the North sea others like mine were not picked up once 5 miles off shore. This is not related to mast height as most of the boat were over 45 feet. What I cannot work out is where this difference is arriving is it the power of the AIS transponder, the quality of the splitter or "dodgy" aerials.

If anyone can point to a group test please let me know and I will try to buy a back copy. I have looked through my YM archive but can't see one.

 

[prv]

My guess would be quality of antenna and particularly wiring; also note that not all boats transmit AIS from a masthead antenna. But nevertheless, if there’s a difference in performance from similarly-specced transmitters, it would be interesting to know.

(I say similarly specced as I understand there are now two subtypes of Class B with different power levels.)

Pete

 

[FlyingGoose]

I have also heard through others so cannot confirm , that it can take up to 24 hours for these websites to put your data up on their sites . leave it on for 24 hours and then check

 

[Seven Spades]

(I say similarly specced as I understand there are now two subtypes of Class B with different power levels.)

Pete

Are you able to enlighten me further about the "subtypes" and how this affects the output power.

 

[pvb]

Are you able to enlighten me further about the "subtypes" and how this affects the output power.

Class B transceivers are now available in 2W or 5W power outputs. The higher power ones also use a slightly different method of getting their transmissions in to the system more quickly. In practice, it doesn't make much difference for sailboats. In the same way, a 2W output is perfectly sufficient to give adequate AIS range for sailboats.

 

[Seven Spades]

I have also heard through others so cannot confirm , that it can take up to 24 hours for these websites to put your data up on their sites . leave it on for 24 hours and then check

It is nothing to do with that. We could clearly see some boats at long distance and others only became visible when within 2-5 miles. It was pretty consistent so it must be differences in the transmitting power of some boats. We traveled with 18 boats and as we cruised around it was interesting to as they pop up and disappear on AIS. We were making comparisons with their positions on YB tracker as well so the differences are real.

I know that my AIS broadcast power is poor. It is difficult to track the fault. I had the mast-top aerial replaced it made no difference. It could be the aerial cable, the splitter, the TV multi-plexer or the power of the AIS unit itself. There are so many vaiables.

 

[Sandy]

I often call the local NCI station for a radio and AIS check. If you have one local to you why not arrange for them to check you 5, 10 and 15 miles from their location. This rules out the use of Marine Traffic or other websites.

My AIS set came with the boat and were were a bit surprised that we could only pic up ships at 3 miles, a quick check showed that somebody had not plugged the splitter in and we were picking up the signal just via the coaxial!

 

[[3889]]

Not too sure about this but once offshore aren't Marine Traffic data reliant on relays from other vessels with satellite/ HF links? It's a VHF signal so surely limited in range regardless of transmitter. I stand to be corrected.

 

[Seven Spades]

Well that's the point isn't it. You have 18 boats roughly going in the same direction you can follow how well the shore bases receivers can pick them up. At what point do they go out of range. The range will be related tot he strength of signal, this is affected by the height of the aerial and ... the strength of the signal.

 

[prv]

Not too sure about this but once offshore aren't Marine Traffic data reliant on relays from other vessels with satellite/ HF links?

No - they just stop showing up.

Pete

 

[GHA]

Not too sure about this but once offshore aren't Marine Traffic data reliant on relays from other vessels with satellite/ HF links? It's a VHF signal so surely limited in range regardless of transmitter. I stand to be corrected.

Satellites pick up the ais transmissions, even class B a lot of the time. You need to pay to get the boat name.

 

[MapisM]

I know that my AIS broadcast power is poor.

What do you mean exactly by poor?
From your OP, I gather that your boat is visible within a range of at least 5Nm (probably more, when measured across open sea).
And if so, that's more than enough to cope with the main AIS purpose, i.e. collision avoidance.
Tracking boats on internet websites is just an amusing extension of the AIS concept, not the real thing.
Any boat or ship around you is strictly relying on the AIS signals received directly through their own AIS instruments (if any).
Marinetraffic or any other websites are as useful as a chocolate teapot, for collision avoidance.

 

[Seven Spades]

Ok so when sailingnaccross the NorthmSea we disappeared 5miles off Ramsgate and reappeared near Den Helder, some other boats were monitored all the way across. We see boats appear and disappear even at close range. Our boat was not alone in this. My suspecision is that it is differences between transponders but I can’t prove it which is why I was asking if there is a group test.

 

[MapisM]

We see boats appear and disappear even at close range. Our boat was not alone in this.

Ok, but my point was, is the visibility on marinetraffic website what you are meaning by "see" other boats?
If so, that is NOT the way AIS is supposed to work for collision avoidance: ship-to-ship signal is, and by definition that doesn't rely on any internet connection.
In other words, your 5Nm range transponder will ALWAYS be visible to any ship equipped with their own AIS receiver/plotter, no matter how far you are from any coast. It's sufficient that the other ship is within your 5Nm range, irrespective of whether your signal also reaches any land station connected to marinetraffic or not.

Website visibility is just a gadget, useful for someone else to check where you are from their armchair.
And in this sense, of course when your signal doesn't reach a land station connected to the website network, nobody at home can track your position anymore.
But beware of using it for collision avoidance: that's not meant to serve this purpose, and it could be downright dangerous to rely on it.

Anyway, sorry but I can't answer your question about how to measure the signal strength of your system, and neither I'm aware of any comparative test.
All I'm saying is that if you checked that when you are 5Nm offshore you normally appear on marinetraffic, you can trust to be ALWAYS visible from any other AIS-equipped vessel AT LEAST within that distance, and probably more.
And that remains true also if and when your signal disappears from marinetraffic.

 

[Daydream believer]

2 of our club members both fitted ne VHF sets & AIS transmitters. They both thought them to be Ok when they did VHF radio checks with the CG not realising that they were actually only checking with Bradwell aerial which is powerful and only 3 miles away. Similarly they both checked AIS with each other at close range. Neither realised that they had dodgy transmissions until they got out of range of the rest of us on a particular cruise. They still got the CG. The problem was dodgy aerials, plus old aerial cables. Once they were all replaced they had no problems.

Personally I prefer a separate aerial for my AIS. Although it is considerably lower than some of my fellow cruise partners, I still get as good range. I suspect this is due to having an aerial that suits AIS.
Perhaps others on the forum will say that is tosh & perhaps it is. But even on the mast on the pushpit, I sometimes get ships at 50 miles on a good day. AIS online normally looses me at around 25, sometimes 30, miles offshore.

 

[DJE]

I can't see what a group test would achieve. Transmitting power is limited by the AIS system spec. The variations - as noted above - are due to height and/or quality of antenna.

 

[GTom]

You can choose from the list: antenna mounted on the stern rail instead of the mast, antenna interference if a secondary antenna is mounted on the mast, broken/corroded cable or connections, wrong antenna choice/bad VSWR, high loss splitter with 2W only CSTDMA transmitter, poor/too many connectors, too much cable loss (e.g. 100' rg58)...

To be seen beyond 5 miles, you have to do most if not all of the above right.

 

[yachtbits]

Whilst marinetraffic seems to have the highest number of feeding receivers, the servers are painfully bad at offering duff and regurgitated data. By comparison, vesselfinder updates faster and doesn't store old positions.

Also worth checking when using a single antenna is the tuned frequency. A lot of VHF antennas used to be tuned to 159MHz which allowed AIS to work fine. However, one manufacturer changed theirs to 156.6MHz (ch16) without telling anyone, but this then caused SWR and transmission errors when used with AIS! Guess how I discovered that!

SOTDMA vs CSDTMA comparing the two types of Class B

 

[zoidberg]

I'm not a radio ham, but - for my sins - I've listened to a clutch of them ( we need a 'generic' label for a bunch of them - how about a 'babble of hams'? ) on occasion.

Yachtbits and GTom seem to be on the right track. It's not simply 'an old aerial and dodgy cable' issue. Even with fresh kit, the signal losses from imperfect connections and less-than-ideal cable shielding can be very large. The antenna might be slightly the wrong size ( 'mismatching' ) for the frequency.... and other causes of cumulative loss mentioned. I'm told by experts that the V'Standing Wave Ratio is 'a measure of the reflected power within a transmission line' which is a measure of the Loss. I'm also told that the correct place to measure VSWR is at the connection of the feeder cable to the radiating antenna.

That's up the top of the mast, for most of us!

Someone who clips his SWR Meter to the connection at the back of the transceiver is certainly going to read a misleading indication. Those who have an antenna mounted on a pushpit will be able to take an accurate reading AT THE ANTENNA and remedy any poor showing.

 

[lw395]

You need to have a pretty poor SWR to impact range very much, unless it causes the transmitter to back off the power.
People get excited about losing half the power in the antenna cable, that's 3dB. The ratio of transmit to receive power can be around 100dB, so having a couple of extra dB of loss is not usually a killer. When VHF is noticed not to work, things are normally broken by tens of dB.

One thing to note is that aerial issues work both ways, so a problem with your transmission also affects your reception. And any imperfection with the other guys system is added to the imperfection in yours.
So while you may see distant ships with nice high, well installed aerials on your pushpit aerial, you might not see a compromised installation much closer.

There are a multitude of other ways of reducing the range of a VHF link, such as blocking from strong signals in other channels. When you see what it takes to put a military radio through a comprehensive set of tests, you might suspect that cheap sets are likely to be more prone to problems.