How much use is AIS

[pmagowan]

I've never eaten a whole one in one sitting.
Right on the second count.
You are the master of your vessel, so you have to do what you feel is best for you. I thought it would be helpful to have the perspective from the other side. Unfortunately there are many pro's who have no experience in small vessels, so they are not aware of the unease felt by the little fish. In a busy seaway in particular, an OOW may close the distance (to the inner side of stage 2) before making a manoeuvre. This is to reduce the interactions with third party vessels, and is common practice, but can be downright scary to the small-boat skipper. My personal take on it, is that once I'm within about 5 miles (give or take depending on the situation) of the other vessel, I consider that I'm obligated by the rules - if I'm 'stand on', then I stand on. There are always measures that can be taken, if it's starting to look like the other guy isn't doing what he should be. I don't shy away from using VHF to ask the intentions of the other vessel. Back to the OP, I see AIS as being useful for putting a name on an approaching contact - though I am shocked by how many vessels don't seem to transmit that info - just the MMSI. And if I got to stage 3, I would likely fire up the engine and have it standing by - ultimately I'll be more manoeuvrable than a large vessel, so can afford to get closer, but don't want to risk getting caught in irons. If I haven't gotten a clear indication of his intentions by the time we're 2 miles apart, then I consider it to be stage 3. My 2p.

Sounds reasonable. Essentially you assess when a 'colregs situation' applies and then stick to the rules. You stand on, as required, and then take action at such time as you consider the other vessel has failed to do so or has not taken sufficient action for your comfort. I have never used the VHF to call another vessel for collision avoidance but I would not hessitate to do so if I felt it was the correct course of action, taking into account the warning that it may distract me from other action.

 

[ditchcrawler]

I respect of MMSI number and name of the ship that information is transmitted at different periods.You may only see the MMSI number to start with and the name of the ship will come up later as they are different packages of information.The gap can be several minutes and you may no longer be monitoring that vessel so not notice it.If this has already been mentioned I apologise.

 

[Robin]

I've never eaten a whole one in one sitting.
Right on the second count.
You are the master of your vessel, so you have to do what you feel is best for you. I thought it would be helpful to have the perspective from the other side. Unfortunately there are many pro's who have no experience in small vessels, so they are not aware of the unease felt by the little fish. In a busy seaway in particular, an OOW may close the distance (to the inner side of stage 2) before making a manoeuvre. This is to reduce the interactions with third party vessels, and is common practice, but can be downright scary to the small-boat skipper. My personal take on it, is that once I'm within about 5 miles (give or take depending on the situation) of the other vessel, I consider that I'm obligated by the rules - if I'm 'stand on', then I stand on. There are always measures that can be taken, if it's starting to look like the other guy isn't doing what he should be. I don't shy away from using VHF to ask the intentions of the other vessel. Back to the OP, I see AIS as being useful for putting a name on an approaching contact - though I am shocked by how many vessels don't seem to transmit that info - just the MMSI. And if I got to stage 3, I would likely fire up the engine and have it standing by - ultimately I'll be more manoeuvrable than a large vessel, so can afford to get closer, but don't want to risk getting caught in irons. If I haven't gotten a clear indication of his intentions by the time we're 2 miles apart, then I consider it to be stage 3. My 2p.

So in small boat mode you behave like most of us that obey the regulations and apply commonsense practicality within the rules. I too am surprised this side of the pond just how few transmitting AIS boats display the vessel name and only show the MMSI. I wonder if that is in order not to give away too much information to potential nasty people like possible drug running hijackers or pirates in some areas, or even simply wanting to stay anonymous and not attract unwanted/inconvenient attention from various civil authorities. In our case we do transmit via AIS our vessel name and details as well as our MMSI and international callsign. We do however have an 'AIS Silent Mode' switch installed whereby if we want not to transmit and only want to receive, then we can, but so far have not flicked the switch although if/when we ever get to cruise in some of the dodgier places in the Caribbean we may choose to do so.

I can perfectly understand that the guys on most commercial boats do not appreciate the view seen from lower down where the seas can be lumpier, the spray is flying and the perceived threats from immense lumps of fast moving steel can bemuch more menacing than they are seen from the sheltered stable environment of a big ship bridge bristling with every aid including a coffeemaker, winsdscreen wipers and a comfy chair, where after all in any collision with a small boat they only have a potential enquiry to worry about

Many moons ago we had the Master of a VLCC as a member in our then (very) small boat yacht club and as soon as he had some leave available he took off in his own little 24 footer, usually to France and the Channel Islands. When asked 'why, was it not rather tame?' he said 'because sailing a small boat especially in that area is so much more challenging and the thrill of reaching a foreign destination never fades when done by DR not sophisticated instruments (this was in the days before Decca and GPS on small boats). He was also somewhat reassuring about how they 'saw' us on radar and confirmed that the type of reflector we carried was very good. another comment was that in thick fog they would always have the best guys on the bridge and that actually we in small boats were more vulnerable on a clear summer day when concentration might wander, I don't know if that is a wider held view but I still remember it and certainly the closest encounters of a large and rusty kind have mostly been in very clear vis, not in fog

 

[Cruiser2B]

I respect of MMSI number and name of the ship that information is transmitted at different periods.You may only see the MMSI number to start with and the name of the ship will come up later as they are different packages of information.The gap can be several minutes and you may no longer be monitoring that vessel so not notice it.If this has already been mentioned I apologise.

I was not aware of that - I knew the way timeslots were assigned varied between Class A and B, but didn't realize the info transmitted was also "packeted". Seems like a serious limitation. AIS was only coming into use when I stopped driving big lumps of steel, and I only just put one on my boat (Vesper Watchmate), so still working out how to make the most of it. Perhaps as Robin stated, there are some deliberately suppressing the information. Or perhaps there is some numptyism - I've seen quite a few "moored" vessels going over 10 kts; a couple of them were US Coast Guard.

 

[prv]

I was not aware of that - I knew the way timeslots were assigned varied between Class A and B, but didn't realize the info transmitted was also "packeted". Seems like a serious limitation.

The way it works is that the data is divided into "static" and "dynamic", and they have their own separate refresh rates. I can't remember off the top of my head, but it's something like every two seconds for dynamic data from a ship that's turning, versus every 6 minutes for the static. Dynamic data as the name implies is the stuff like position, speed, rate-of turn, etc. Static data is the vessel name, callsign, destination port, third mate's inside leg measurement... . Both include the MMSI which is how the receiver ties them together.

Almost invariably the first messages you get will be dynamic ones, so to begin with all the display can do is say "there's a vessel here, moving like so, and its MMSI number is x". Then after a few minutes the static message comes in, and the display says "that ship I told you about? I know all about it now..." and the MMSI is replaced with the name and so on.

If the ship is a long way off and you're only picking up occasional messages, you can have an MMSI-only target for much more than six minutes as your odds of catching one of the many dynamic message are pretty good but you can easily miss the single static one each time it comes through.

The reason for the slower static messages is that they can be much bigger, so if you only had one kind of message you'd either have to broadcast fewer of them (meaning slower updates of the position, speed, and turn data) or fit fewer ships into the airspace. By batching up the big stuff and sending small fast updates they make a better tradeoff.

Pete

 

[capnsensible]

Thanks for that excellent explanation, resolves a lot for me! As a user of AIS nearly every day its opened up my understanding far more and will enable me to get more out of an already very useful instrument.

:encouragement:

 

[Robin]

The way it works is that the data is divided into "static" and "dynamic", and they have their own separate refresh rates. I can't remember off the top of my head, but it's something like every two seconds for dynamic data from a ship that's turning, versus every 6 minutes for the static. Dynamic data as the name implies is the stuff like position, speed, rate-of turn, etc. Static data is the vessel name, callsign, destination port, third mate's inside leg measurement... . Both include the MMSI which is how the receiver ties them together.

Almost invariably the first messages you get will be dynamic ones, so to begin with all the display can do is say "there's a vessel here, moving like so, and its MMSI number is x". Then after a few minutes the static message comes in, and the display says "that ship I told you about? I know all about it now..." and the MMSI is replaced with the name and so on.

If the ship is a long way off and you're only picking up occasional messages, you can have an MMSI-only target for much more than six minutes as your odds of catching one of the many dynamic message are pretty good but you can easily miss the single static one each time it comes through.

The reason for the slower static messages is that they can be much bigger, so if you only had one kind of message you'd either have to broadcast fewer of them (meaning slower updates of the position, speed, and turn data) or fit fewer ships into the airspace. By batching up the big stuff and sending small fast updates they make a better tradeoff.

Pete

The only way for me to revise or add to the initially programmed data re the 'static' information on my Garmin however is via a program installed on my laptop from the Garmin supplied CD and which then requires me to use a USB to micro cable to connect to the black box AIS600 which since it is installed out of sight behind a screwed in place panel is never going to happen.

When initially installed the installing dealer programmed ours with Boat name, MMSI, international callsign , that dealer did not add anything else even though I did provide the USCG documented Reg Number, as well as the LOA and draught Info Here in disneyland the data has to be dealer not user loaded and he has to enter his permit reference number too I believe.

 

[Cruiser2B]

Thanks for that excellent explanation, resolves a lot for me! As a user of AIS nearly every day its opened up my understanding far more and will enable me to get more out of an already very useful instrument.

:encouragement:

+1

 

[BrianH]

If the ship is a long way off and you're only picking up occasional messages, you can have an MMSI-only target for much more than six minutes as your odds of catching one of the many dynamic message are pretty good but you can easily miss the single static one each time it comes through.

A great and clear explanation.

The static data sentences are sent every six minutes regardless of whether Class A or B. But what really affects many receivers in the time taken to include in the display is the receiver hardware design. All receivers are two channel ones but not all listen on both channels simultaneously, especially the older and cheaper ones, they have a single processing path and the input stage switches between the two frequencies alternately. This means they can get out of synchronisation with transmissions resulting in long periods before the ship's name, for example, is added to the target list and displayed.

Before buying any AIS system, check that the receiver is a genuine dual-channel one, some manufacturers and vendors are very coy about declaring this. All quote dual-channel but some without clarifying whether it is simultaneous, parallel reception.

 

[syc1]

Thanks for that excellent explanation, resolves a lot for me! As a user of AIS nearly every day its opened up my understanding far more and will enable me to get more out of an already very useful instrument.

:encouragement:

+2

 

[prv]

All receivers are two channel ones but not all listen on both channels simultaneously, especially the older and cheaper ones, they have a single processing path and the input stage switches between the two frequencies alternately. This means they can get out of synchronisation with transmissions resulting in long periods before the ship's name, for example, is added to the target list and displayed.

Correct. I meant to include that in my post, but in trying to make clear why you may miss several static messages even with a parallel receiver, I forgot to mention the further limitations of alternating receivers.

I believe the majority of receivers now sold are parallel though.

Pete

 

[BrianH]

I believe the majority of receivers now sold are parallel though.

Er, yes, but there are still some that are not. For example, the most popular NASA engine never has been, nor is its latest incarnation the 'Ais Engine 3'.

From their current product web page:

"The unit can receive ships on either the A or B AIS channels. In default setting it alternates between the two channels."​

In fact, that is quite an admission, before it was impossible to find out from their specification or data sheet if the unit was a true parallel receiver. As always, you usually get what you pay for.

 

[Robin]

Correct. I meant to include that in my post, but in trying to make clear why you may miss several static messages even with a parallel receiver, I forgot to mention the further limitations of alternating receivers.

I believe the majority of receivers now sold are parallel though.

Pete

Got me checking on mine, a Garmin AIS600 transceiver black box, which is a simultaneous A+B parallel receiver according to Garmin leastways. it contains a built in antenna splitter which apparently does not interfere with AIS transmission or receipt data even if the separate VHF set is transmitting, I suppose I just believe it does what it says 'cos I is a simple numpty

 

[prv]

Er, yes, but there are still some that are not. For example, the most popular NASA engine never has been, nor is its latest incarnation the 'Ais Engine 3'.

What else would you expect from NASA?

Pete

 

[BrianH]

What else would you expect from NASA?

Pete

Agreed, but for a long time they were about the best choice for a cheap solution. Hard to understand how they can still peddle their product now that there are parallel dual-channel receivers out there priced for so little more.

Looking at my post above I can see that it still may not be clear why simultaneous channel reception is important, especially in the light of fast-travelling Class B targets. So here is an addendum ... look away Pete, not for you:

Both Class A and B transponders automatically alternate transmissions between the two AIS frequencies (VHF 161.975 Mhz and 162.025 Mhz), which doubles the system bandwidth. An alternating frequency receiver may not be listening on the frequency carrying a transmission and will consequently miss it. For the Class A dynamic data sentence this does not matter as the frequency (time) of transmission is only a matter of seconds for a sender that is moving and the next transmission will be received when the sender changes to the alternate channel frequency - provided the receiver has not as well changed the listening frequency, in which case he must wait for the next ... or the next.

But Class B dynamic sentences are only sent every 30 seconds, so missing one (or even two with worst case synchronisation) can cause Class B targets to jump across the screen. Even worse is the case of the static data sentence sent every six minutes for both Class A and B. It can often take up to 12 minutes or even longer (by the 6 minute multiple) to be included in the target data. Not good for wanting to call an impending threat by name on VHF to ask intentions ... but then, one shouldn't do that - as I have been reprimanded for so doing on this forum.

Edit:
For Class B dynamic data transmissions, the 30 second frequency applies to vessels with a speed of 2-14 knots. However, for vessels moving at 14-23 knots the frequency is halved to 15 seconds and at greater than 23 knots reduced to every 5 seconds. Even so, a fast planing motor yacht target will, with an unsynchronised receiver, jump spasmodically along its screen track.

 

[franksingleton]

I have often said that you never stop learning about meteorology and sailing. This thread is a good and fascinating demonstration.