"A" versus "B" AIS - worth transmitting?

[CaptTom]

Originally Posted by Kenr74

Thanks for the input. I am new(wish) to sailing, and have yet to venture into very congested areas. In the sailing I have done on the Great Lakes, it seems all but impossible to get anyone to answer the radio on 16. I had hoped DSC would have better success. At least with AIS, you can call by name instead of, "Hello big ship bearing down on me...."

...Which is what I recommend you plan to do.

I bought an AIS receive-only VHF and tied it in to my NMEA network. All it did was alarm because it detected my AIS transponder in the same location. Yes, both units were programmed with the same MMSI. Other VHF radios might be smarter, but it's something to think about.

I also thought I'd be using the PPI display (like a little radar screen) on the VHF, and contacting other vessels via DSC. I never have. The AIS targets appear on my chart plotter, and I can call them by name far more easily than going through the menus on the VHF.

The two times I've tested DSC, just by calling a friend's nearby boat, all it did was surprise the operator and have them searching their boat, trying to figure out where that funny noise was coming from.

[Cpt Pat]

Originally Posted by tanglewood

Just a few comments.



What filtering does make sense is based on speed, distance, and possibly status. That's the solution to clutter from moored boats. Filter them out because they are not moving. If your display is too stupid to do that, get a better display. Don't ask the whole world to turn off their AIS because you have a dumb display. Besides, Class A AIS are supposed to be left on all the time.


Class A has some extra features, but don't underestimate the effort require to correctly install one. Class B will work stand-alone without any input data from other instruments on the boat. Class A will not. Class A requires heading, rate of turn, SOG, COG, and position from you boat's primary instruments. The Class A's built in GPS is a backup only, and the device will alarm continuously if the requisite input data isn't present all the time. Also, Class A requires true heading as an input, and that can be harder to get than you might think. Magnetic compasses report magnetic heading, not true heading. And even if your display can correct to true, it doesn't mean that true heading is present on 0183 or N2K. It just means the display is calculating it for it's own use. The only devices that produce a native true heading are sat compasses and gyros.

I have Vesper Marine display (one of the older LCD screen types) in both my cockpit and cabin that does an excellent job of filtering out non-conflicting targets.

The only reason I have a Class A transponder is that I was able to find one cheap ($700) and I wanted to replace my old ACR Class B that used CSTDMA protocol. The post above is correct: installing a Class A is much more complex that a Class B. As the above post points out, you'll need a second GPS source and, although it's said a non-SOLAS voluntarily-equipped vessels aren't required to send true heading and rate-of-turn; once you've already installed the Class A, it's a shame not to send the info. I installed a navigation reference unit (NRU) (http://www.navteam.com/products/navi...ence-unit.html) that cost another $1,000 (but I have additional uses for that info). It combines GPS and GLONASS for a slightly more accurate positional output, and failover if either system (bot obviously not both) is subject to interference.

I am able to get a "pseudo-true" heading from the NRU that's derived from magnetic heading reference. The NRU has a database to convert magnetic to true based on GPS position that's calculated against the magnetic variation for the GPS position, along with outputs from mufti-axis rate gyros. However, local magnetic anomalies and variation drift over time will result in inevitable errors. Big ships use much more accurate and expensive methods.

There are some Class B stations that also send heading, as it is supported for Class B, if that's a feature you want. They all support COG derived from GPS which generally is be good enough - although sway from heavy seas will skew the accuracy of GPS COG calculations (as anyone who has watched their COG fluctuate wildly on a chartplotter that is not configured to perform averaging, while in heavy seas, will have observed).

But if I hadn't found a cheap Class A that cost the same or similar to a SOTDMA Class B, I would've chosen a Class B SOTDMA transponder, and that's what I'd recommend for anyone who doesn't want to undergo a major project.

[Paul L]

Last night off the eastern Australian coast in a very busy big ship transit area at 2am it was dumping down rain, blowing 20kts with 2-3kts of current. We were tracking a Japanese ship heading up on us on AIS, lights not visible in those conditions, and seeing a really close CPA. At about 3.5 to 4 miles away it changed course to open up the CPA to 1 mile. Whenever I see that 1 mile CPA I almost always know they've seen me. The 3.5 to 4 mile range is probably just about the range of my 1watt AIS Tx in those conditions. A couple of seconds latter we got a call on VHF 16 from the freighter to us by name. The captain wanted to let us know that he changed course and to hope we have a warm watch.
I love AIS and wouldn't be without it on a cruising boat.

[EngNate]

I haven't read the whole thread to see if this has been said already, but there are three main differences between AIS B and A transmissions. One is power and range. A is 12.5 watts and can reach 25 miles or more over open water. B is 2.5 watts and has a max range of up to 8 miles, but usually 5 or 6, often less.

Both A and B transmit at different intervals depending on vessel status, speed and turn. A transmits at more frequent intervals than B.

Most important though is that A transmissions have priority over B. In the scheme of the system each minute is divided into time slots and every unit listens before transmitting and keeps track of the time slots being used by other units in range - that data is part of each transmission. Class A units reserve their time slot(s) in the system, while class B units only transmit in unreserved slots. In very high traffic areas a B unit may not always have a slot available and will wait until there is. It is definitely something to consider around busy ports, but there are time slots for about 750 full messages each minute, more than enough in most places most of the time, for now anyhow.

Port control facilities can silence both A and B units, either specifically by MMSI or over a blanket area. A units do not silence B units, except by the mechanism above.

A, B or C, it's an enhancement to safety, not a guarantee of it. It would be an unwise assumption that upgrading from B to A will insure that you are noticed and avoided.

[CSY Man]

. I just completed the ARC and can tell you that you cannot expect a large tanker or a container ship to alter course for you, even if you have the right of way, even if you tell them on VHF that you have the chute up.

Nobody has the “Right of Way” on the water. Calling a tanker on the VHF to tell him you have the chute up sounds as ludicrous as transmitting RAM on AIS because you are sailing, but entertaining thread.

[Dockhead]

Originally Posted by EngNate

I haven't read the whole thread to see if this has been said already, but there are three main differences between AIS B and A transmissions. One is power and range. A is 12.5 watts and can reach 25 miles or more over open water. B is 2.5 watts and has a max range of up to 8 miles, but usually 5 or 6, often less.. . .

I would make a small quibble with this -- the range is much more limited by antenna installation, paritcularly (a) feedline and connectors; (b) quality of the antenna itself (and whether it is properly tuned for AIS); and (c) antenna height -- than transmission power.


Ships have professionally installed systems with antennas up to 50 meters or more above the water. In a well installed system in a ship, even a Class B set will reach far greater distances than the average sailboat installation.


If you replace a Class "B" set with a Class "A" set and use the same feedline and antenna, do not expect a dramatic increase in range, and certainly do not expect to get the range ships do. I used a continuous length of fat RG214 with high quality and well soldered connectors, and a high quality antenna, for my AIS, and I am typically received at 25 miles or more by ships, even though I have a legacy Type "B" set. I will replace that this winter with either a SOTDMA set or a Class "A" and I will not expect a big difference in range.



That being said -- I don't think long range is as important as totally reliable reception at 10 to 15 miles. To achieve that, though, you really need a good installation, which is far more important than transmission power. Don't use a splitter, by the way, and avoid unnecessary connectors in the feedline. Use an antenna tuned for AIS, not an ordinary VHF antenna -- that also makes a pretty big difference.

[Dockhead]

Originally Posted by CSY Man

Nobody has the “Right of Way” on the water. Calling a tanker on the VHF to tell him you have the chute up sounds as ludicrous as transmitting RAM on AIS because you are sailing, but entertaining thread.

I agree with "no right of way", but I would beg to differ that it is "ludicrous" to discuss your chute with ships. In my experience, 90% of ships will happily comply with a polite request for a small alteration in course due to some circumstance like having a chute up, or being hard on the wind, or whatever, so long as they are not in traffic or in a TSS or otherwise subject to constraints of their own. They ask each other for such favors all the time.



Just do it in good time -- remembering that they are usually totally committed to how they will cross with you from at least 5 miles off. Don't ask them to change course 3 or 4 miles off.

[MartinR]

Originally Posted by Cpt Pat

RAM is a designation that I use only in extremis, as an AIS equivalent of a sécurité call. Licensed professional mariners: people who actually had to pass the USCG Rules of the Road written test, understand its meaning: "there are courses I cannot adopt." It's a prompt to make a radio call to arrange a crossing. It's those large vessels that have the ability to even see my AIS navigation status on their navigation display that concern me. For your vessel, we'd probably just exchange paint in a collision.

A sailing boat is by its nature restricted in ability to change course. That is inherent in the designation. Also, a sailing boat has right of way. So what would be gained by hoisting signals and changing AIS designation to "restricted". Commercial vessels are also sometimes restricted in their ability to manoeuvre, for example in heavy weather. That does not mean we see us as "restricted" under regulations.

Normally commercial vessels do not check the status of other vessels on AIS. They do this by looking out the windows. A dredger is a dredger, a sailing boat is a sailing boat. Very clear on how to proceed. AIS does not come into the decision making here, and actually very seldom in any case. For collision avoidance we use ARPA. Small sailing boats are helped by AIS, they show up in a seaway on AIS, but often not on RADAR.

[MartinR]

Originally Posted by Dockhead

That being said -- I don't think long range is as important as totally reliable reception at 10 to 15 miles. To achieve that, though, you really need a good installation, which is far more important than transmission power. Don't use a splitter, by the way, and avoid unnecessary connectors in the feedline. Use an antenna tuned for AIS, not an ordinary VHF antenna -- that also makes a pretty big difference.

Long range is of no importance. If I see a sailing boat on AIS at 3 miles is absolutely enough. I do not expect the sailing yacht to hole her course, as I would from a commercial vessel, so I alter my course much later. Maybe 1 or 2 miles off.

[MartinR]

Originally Posted by Dockhead

Almost never does a ship answer a DSC call. They respond to voice calls on 16. Once I had a difficult situation with a large passenger ferry, who refused to answer my voice calls. Apparently because I was mispronouncing the Swedish name, the barsteward. He did finally answer a DSC call. That's about the only time I recall ever getting a ship to answer a DSC call.

There is a reason. It is called coast radio stations. Some of these tend to announce their broadcasts on DSC. Fair enough. But on my bridge for example I have 4 DSC radios, all alarming at the same time and in different places. So the easiest way to deal with this is just to press the cancel button.

Unfortunately this happens to individual calls also.

Mayday calls are different, there is a separate alarm system for these, so these are not missed.

[MartinR]

Originally Posted by CarlF

I’ve really lost patience with the folks who are too cheap or too stuck in the past to install an AIS transciever.

Money is not a good excuse now that transceivers are under $500 - easily paid for by a few nights of anchoring out instead of taking a slip or slowing down a bit and burning less fuel.

The “recieve only” folks are the most agravating. They’re happy to use my AIS signal to make THEIR navigation easier but refuse to do the same for me by transmitting AIS. It’s inconsiderate - like not slowing down and giving me a huge wake when passing.

I’ve taken to hailing boats offshore that look like they can easily afford AIS with “Captain, you might want to check your AIS, it doesn’t seem to be transmitting”

I am sorry, I do not need an AIS transceiver. I have my eyes to look out. I want to sail my boat, not play a bloody computer game.

Enough of this at work;-)

[Dockhead]

Originally Posted by MartinR

Long range is of no importance. If I see a sailing boat on AIS at 3 miles is absolutely enough. I do not expect the sailing yacht to hole her course, as I would from a commercial vessel, so I alter my course much later. Maybe 1 or 2 miles off.

3 miles is not enough for a SHIP to see YOU. Maybe in coastal waters, but not in open sea.


Their decision point in open sea is generally about 10 miles. They need to be aware of you well before that in order to set up the crossing with you properly.


It's true that ARPA, and not AIS, is their primary collision avoidance tool, but very often small boats are not picked up properly on radar, so AIS can be really important (another topic is the importance of good radar reflector).

[leftbrainstuff]

There are 3 AIS options now. A which is low power, no priority slot, B which is high power commercial and a new interim A which has power between A and B but still no priority slots.

Anyone can shut off their AIS either by power off or by rigging a silent mode.

As for detection distance. Anything is better than the eyeball at night. Having at least 2 or 3 miles warning is sufficient to route around fishing fleets and nets.

Note that some nations like Thailand and Indonesia are mandating AIS for cruising vessels. I'm not sure why but it certainly isn't for safety as they both have squillions of non compliant vessels.

In busy areas, greater than 2000+ vessels with B class AIS then any A transmitter wont be heard due to the scheduling protocol used. But there are few places in the world where this is an issue.

Personally AIS, A class, is a fantastic safety and situational awareness tool. It gives us good situational awareness of transmitting vessels. This allows us to use the old eyeball to look out for those boats not transmitting. It makes long voyages less fatiguing.

I've even found it encourages people who get distracted and don't have the discipline to be safe watch keepers to pay more attention. That little moving triangle makes them curious.

I suspect that AIS will become mandatory for all vessels over time. This I see as a good thing.

[dmkkeng]

The short answer is yes - here are my thoughts on AIS. I debated the same thing when I first started to look into AIS. Class A (receive only) is easier to install and cost less so my first thought was just install a Class A system. In terms of collision avoidance it only allows you to use the system to make course changes. With a Class B system it provides the potential for everyone with AIS capability to make course changes or at least provide notification in the scenario where they cannot make course changes. To get the full benefit of AIS you need a Class B (transmit & receive) system. Most Class B systems have the option of disabling transmit if you are inclined to do so. I have yet to do so. I do however shut my AIS system off while at dock - at times it can be difficult to differentiate someone who is at dock or is just leaving dock. I think it prudent for users of AIS systems to shut their system off while at dock as the main purpose for AIS is collision avoidance. I have disabled the audible alarm on my AIS for this reason, however my AIS system is integrated with my other navigation systems and a single display. Someone is always at the helm to see what is happening visually. From a sailing perspective I used it to make course changes well in advance of any collision, often the boat that I am on the collision course with will do the same. Usually I can make minor course changes to avoid the collision which often allows me to stay on the same tack and head in the same direction. Prior to having AIS I either made unnecessary course changes or waited to long and had to make drastic changes in course to avoid a collision. AIS definitely improves sailing efficiency.

[Wotname]

Originally Posted by leftbrainstuff

There are 3 AIS options now. A which is low power, no priority slot, B which is high power commercial and a new interim A which has power between A and B but still no priority slots.
..........

Hmm... I think you have your A and B reversed.

A is high power commercial
B is low power recreational