Big ship and no AIS on your screen.

[actionoptics]

What would be the correct response to seeing a large ship in your vicinity
but not seeing an AIS message/icon on your screen?
Should one call the Coast Guard or try to raise the ship on VHF
even if it's name could be read?

 

[jonjo]

What would be the correct response to seeing a large ship in your vicinity but not seeing an AIS message/icon on your screen?

My first thought would be to wonder if the ship is in my AIS blind spot, I have a low aerial.

Then I would wonder about a software bug.

Then I would check the north-up or heading orientation of my AIS display.

Finally I would shrug my shoulders in the knowledge that the CG are progressively tightening up on AIS non conformance. However if there was a small craft alongside the ship and packages were being transferred... hmmm!

 

[Woodlouse]

What sort of AIS reciever do you have? There are two bandwidths that AIS transmits on and some units can only receive one at a time, I think switching at few minute intervals, which means for that period some vessels won't show up. Also the rate that the vessel is transmitting might be low. I think AIS units automatically vary their transmit rates depending on the vessels speed and heading, if it's going fairly slowly in a straight line then it'll only update every few minutes, if it's going fast and turning then it'll be updating every few seconds.

Great though AIS is, it still ain't perfect. And there are a lot of ships trying to transmit, so it's hardly a surprise the odd one doesn't show up.

 

[lw395]

The correct course of action would be to avoid being run over by it.
The navy do not always transmit AIS.
On other ships it may be broken, or your set has missed it somehow. If their gps is not talking to the ais properly, the ship may transmit but your receiver will ignore it as too distant.
I don't regard AIS as totally dependable, but its a very good aid.

 

[srm]

The simplest explanation is that the ship's AIS is not switched on.

AIS is not recognised as an aid for collision avoidance so just keep clear and continue on your way. If you do recieve AIS info, very nice, but don't expect it always to be accurate. I have watched a vessel with AIS transmission saying anchored steaming away at around 20 knots because this is a manual input and the OOW obviously forgot to update it.

 

[jonjo]

AIS is not recognised as an aid for collision avoidance

Strange then that the big ship professional mariners navigating through the English Channel often quote their AIS observations on the VHF when trying to persuade another ship that it is obliged to keep clear.

 

[jonjo]

And there are a lot of ships trying to transmit, so it's hardly a surprise the odd one doesn't show up.

That is one thing you don't have to worry about

 

[srm]

Strange then that the big ship professional mariners navigating through the English Channel often quote their AIS observations on the VHF when trying to persuade another ship that it is obliged to keep clear.

Indeed, it is there to give a whole swathe of information most important probably being identification and position, followed by ground course and speed from a gps input. However, according to the MCA Guidance Notes (MGNs) it does not provide sufficient information by itself for another vessel to act on to avoid collision - for a number of practical and technical reasons.

As a simple example course, speed, and rate of turn are calculated from the movement of a gps antenna - if the ship is maintaining a steady course and speed its usually accurate. However, a large vessel that is starting a turn to starboard may well have its gps antenna near the stern measuring a turn to port as the stern swings around the ship's pivot point. It does not take much imagination to realise that this would cause serious confusion in collision avoidance - "his AIS said he was turning to port but he swung his bow to starboard and hit me"

 

[jonjo]

However, according to the MCA Guidance Notes (MGNs) it does not provide sufficient information by itself for another vessel to act on to avoid collision

However, a large vessel that is starting a turn to starboard may well have its gps antenna near the stern measuring a turn to port as the stern swings around the ship's pivot point.

UK officialdom's reaction to AIS has been ignorant, belated and led by observation. What else could we expect from a branch of Government mugged by ex. charity sector executives and that has an institutional compulsion to discipline a junior staff who used initiative to rescue a teenage girl clinging to a cliff face?

The MCA is a disgrace and hopefully the future Tory government has the MCA's executive jerks clearly targeted for removal.

Re. the turning scenario... that is why big ships transmit their rudder position in the AIS data packet.

 

[BrianH]

Here are two passenger vessels that were not transmitting any AIS signals for the entire time they were in sight during my cruise last year. Both were operating in the central Croatian coast and islands area. They were observed at the same time that I was receiving other ships' AIS signals and plotting their positions. However, they were the only culprits amongst many others that I monitored throughout the Adriatic.

P.S. I'm glad the visibility for the first was good, I had already had a close encounter with a sister ship, a high-speed, catamaran ferry doing 33 knots, earlier in the cruise. Scary.

 

[BrianH]

P.S. I'm glad the visibility for the first was good, I had already had a close encounter with a sister ship, a high-speed, catamaran ferry doing 33 knots, earlier in the cruise. Scary.

​

With reference to this photo from my previous posting - here is a screen snapshot taken at the same time as the photo:

Which shows nothing between me (the large red arrow) and the island of Premuda, but note the target on the other side of the island (small red icon) that was being dynamically updated. This unnamed ship (highlighted in the report list) still has to have its static data sentence received (sent every 6 minutes)

This target was travelling fast, almost 30 knots, but the interesting point was that although only 2.9nm distant it was on the other, seaward side of Premuda with its high spine of hills. This illustrates an advantage of AIS over radar, which would never have shown the target on the screen because the land mass of Premuda would have hidden it by virtue of the characteristics of the higher transmission frequencies involved.

Of course I am not saying AIS is better, only that in some cases, such as this, they can complement each other.

 

[lw395]

As a simple example course, speed, and rate of turn are calculated from the movement of a gps antenna - if the ship is maintaining a steady course and speed its usually accurate. However, a large vessel that is starting a turn to starboard may well have its gps antenna near the stern measuring a turn to port as the stern swings around the ship's pivot point. It does not take much imagination to realise that this would cause serious confusion in collision avoidance - "his AIS said he was turning to port but he swung his bow to starboard and hit me"

I believe the rate of turn info comes from the gyro compass rather than the GPS.

 

[mike_1982]

I believe the rate of turn info comes from the gyro compass rather than the GPS.

Ones I have worked with use GPS as the Gyro isn't connected to the AIS in anyway.

Mike

 

[srm]

I believe the rate of turn info comes from the gyro compass rather than the GPS.

Belief is a dangerous thing. In fact gps or gyro may be used depending on the amount of integration (or otherwise) of the bridge equipment.
However, in the example I gave the transmitted course from the GPS antenna will be very different from the course another vessel would observe either visually or by ARPA.

As to Jonjo's little tantrum (only noticed him in a few threads but this does seem to be a weakness of his) - I have had quite a lot of dealings with members of the organisation over about 30 years, from Chief Examiner downwards, and have always found the surveyors and examiners that I came into contact with totally professional and pleasant to deal with.

 

[Oldhand]

What would be the correct response to seeing a large ship in your vicinity
but not seeing an AIS message/icon on your screen?
Should one call the Coast Guard or try to raise the ship on VHF
even if it's name could be read?

When I called up a vessel on VHF and reported I was not receiving any AIS messages from them, it was quickly turned on and thanks given for pointing out their error. However, it was a vessel I recognised and I was able to call it by name, which made it easier than in your case.

 

[fmoran]

Maybe it was a Navy ship, they rarely transmit AIS.

A related issue which may be of interest:

AIS Enabled Buoys
Good talking to you yesterday at the SBS. Surprised that no one on the Raymarine stand was even aware of this pretty basic problem with the
Raymarine equipment.

Let me recap on the problem I have:

Raymarine AIS250 and E120 installed and working - in 2nd season.
Sailed round Ireland in 2009
Picking up ships ok on the AIS, E120 displaying and providing the details.
E120 not displaying the AIS Enabled buoys - Trinity House / CIS have now a number enabled and working on the East Coast of Ireland.

Questions:

When will the Raymarine "system" start showing these buoys
Is the problem in the AIS250 not being able to pick up, or the E120 not being able to decode the AIS250 info and display.
Is the problem in the hardware or software.
Will a fix be in a software release, or will I need to buy new hardware / equipment.

The AIS250 is by no means a cheap AIS option, in fact the most expensive on the market, so as a minimum I would expect it to let me
"see" AIS enabled buoys.

There is a lot of confusion about on this at present, and even articles in the Yachting press which are unaware of the problem.

 

[jonjo]

As to Jonjo's little tantrum

Tantrum? Hmm let's see...

1 - I raised a specific example about the unhealthy career development merry-go-round that exists between the executives of the UK's burgeoning charity industry and high level posts in Government. If you don't understand my point, read up on the background of the current head of the MCA.

2 - When AIS first arrived, UK officialdom dismissed the technology as an interesting gimmick to assist making initial contact via VHF. The MCA people who should have understood the implication of the technology but could not grasp its significance. Their current stance, as quoted by you, indicates the organization is still conceptually trailing the real AIS use in the field.

3 - I commented on the MCA's obsession with Health & Safety which caused them to discipline a CG staff member who courageously rescued a teenage girl hanging on a cliff and then there was the petulant action of the confiscation of the Hope Cove inshore rescue boat after local CG staff ignored a safety prohibition order on the craft to go to the rescue of swimmers in distress.


You might call it a tantrum but I call it highlighting contemporary affairs with a maritime flavour. There is major regime change coming to this country in the next year and I hope the new brush sweeps away some of the dysfunctional executive detritus at the MCA.

 

[bitman]

How AIS works

just to stop lots of 'I think it works like this' here more info about AIS http://en.wikipedia.org/wiki/Automatic_Identification_System

 

[Scotty_Tradewind]

What would be the correct response to seeing a large ship in your vicinity
but not seeing an AIS message/icon on your screen?
Should one call the Coast Guard or try to raise the ship on VHF
even if it's name could be read?

Are you seeing other ships on same screen. Check your own screen is receiving o.k. perhaps first.
If you cannot identify ship to call directly then CG on a working channel if possible but a CH 16 call without making it an urgent call unless you see otherwise.
They will soon confirm to you AIS signal or otherwise.
Any vessel over 300T should by law be transmitting I believe.... "belief being a dangerous thing"

 

[lw395]

Belief is a dangerous thing. In fact gps or gyro may be used depending on the amount of integration (or otherwise) of the bridge equipment.
However, in the example I gave the transmitted course from the GPS antenna will be very different from the course another vessel would observe either visually or by ARPA.

.

I understood that a rate of turn indicator or gyro was supposed to be used if this info is to be broadcast. The wikipedia article implies the same, doesn't mean that Moroccan Decca installers will make it work like that!
Belief is indeed a dangerous thing, I'm quite happy to have my beliefs and assumptions challenged on here, it's better than having them challenged by a freighter mid channel.
Even if the rate of turn does come from a gyro, I would not depend on it that much, as it may be a short sample rather than a useful average. Ships sometimes turn a little one way before turning much further the other way, for example, moving toward the middle of the channel before turning off. I tend to just take a non-zero rate of turn as a general warning of turning, unless I'm watching a string of consistent figures.