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Developments in AIS


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Developments in AIS

One of KM’s comments, along with a few of the things coming across my desk in the last few weeks, made me think perhaps some crew.org members might find this useful.

 

Please note This is a simplified description! It’s a complex topic that cannot be covered in detail here. There are some links at the bottom for those who want a more in depth description.

 

Most of you will be aware of AIS, even if you don’t have one. AIS, or Automatic Identification System, is a VHF radio device that provides collision avoidance information to other vessels.

 

Most of you will also be aware that there are two main types of AIS, class A and class B:

Class A is mostly for ships and vessels required to be fitted with it.

Class B is mostly for vessels who elect to install it. That’s most of us.

 

So, AIS class A normally transmits at 12.5 watts (your VHF is 25 watts on high). It provides lots of info about the vessel, name, type, dimensions, MMSI,  position, speed, course, heading, cargo, destination, callsign etc etc.

Class B normally transmits at 2w, and provides limited info –Name, callsign, type, dimensions, MMSI, position, course, speed etc.

 

Both share the same radio frequencies, and have to manage those frequencies, which can be congested, without transmitting over the top of each other. To do this, Class A uses (among others) SOTDMA – Self Organized Time Division Multiple Access. Class B uses CSTDMA – Carrier Sense Time Division Multiple Access.

 

SOTDMA is a good system that ensures all transmissions can be received. CSTDMA is a less good system that can result in some transmissions being lost in some circumstances.

 

Also, class B, with its limited output power, has proven to provide less than 100% reliable links with satellites, meaning offshore coverage is limited, or non existent.

 

So, we have recently see the first of the new ClassB SO devices, which are the same as classic class B AIS units (Now called Class B CS ) BUT – transmit at 5w, and use the same SOTDMA (plus RATDMA and ITDMA) that the class A units use.

 

This may well be the unit of choice for sailors, as it will mean rescue services, families etc can track a vessel anywhere at any time.

 

So currently the best of the class B units are class B SO.

 

if you want  more, try http://www.allaboutais.com/jdownloads/Access%20schemes%20technical%20downloads/ais_tdma_access_schemes.pdf

 

 

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So IT, if a yacht had the class B SO AIS unit, lets say a yacht such as the Nina, the classic American wooden yacht that went missing mid Tasman in June 2013, if the Nina had this AIS unit, then the family could have followed the track, and if anything happened, then the rescue services would have had a confirmed last position, speed and heading information?

 

Am I correct in my understanding that this unit can be tracked by satellite, so you can follow it on one of those ship movement websites (like flight tracker but for shipping?)

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OK, stage two;

Other stuff AIS does.

Not just collision avoidance, also navigation.

Another feature of AIS is aids to navigation, or AtoN. AtoN's are virtual markers. They can be temporary or permanent. They give EXACT positional information of whatever they are marking, and can be real (Physical devices on the location they are broadcasting)  or virtual. Virtual devices means there is no gear on the site, and is great for environments that are difficult to build or anchor anything on the site. There are multiple AtoN's around Auckland.

 

If you have a receiver, you will see the AtoN's on your plotters in real time. That will give you accurate distance off etc, and makes for safer navigation. AtoN's can also broadcast additional data - tide rates, wind speed and dir, phone numbers, whatever the owners wish you to know.

 

AtoN's are becoming the marker of choice for many harbors - cheap to install, no real maintenance costs, and smart.

 

Most AIS receivers and Transponders now can "see" AtoNs. Some may require updated software to do so. Many new VHF radios now have AIS receivers.

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Also, class B, with its limited output power, has proven to provide less than 100% reliable links with satellites, meaning offshore coverage is limited, or non existent.

 

How would an AIS unit with only VHF radio connect to a satellite? Or do you mean for position information? I always thought a marine traffic subscription uses other means to get tracking information whilst not near shore station. 

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Willow, some satellites have AIS receivers - they are not that high in the sky that the transmissions wont reach them...

Direct from Wikipedia;

"since 2008, companies such as exactEarthORBCOMMSpacequestSpire and also government programs have deployed AIS receivers on satellites"

Plus if you look on the yacht positions on Marine traffic, the ones offshore usually say "position received via satellite" 

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OK, AIS Section 3 safety

 

AIS is also an aid to safety. Some units have a distress function (my unit has a big red button on the case). These units can act like a SART (Seach and rescue transponder) and will cause other AIS units (including satellites) in range to Alarm, and provide an immediate bearing and range to the source of the signal. When a signal is received, alarms will go, and the distress icon will display. The icon and display method varies between manufacturers, but all the ones I've seen are obvious.

 

ALL newer AIS units will receive SART/emergency information, but be warned, they may need a software update to do so.  Check your unit, the manual, or with the manufacturer. 

 

If you have a box unit (no display, displays on plotter/MFD), then the MFD may also need a software update to display all AIS info, including AtoN and SART info. Please check this as well. 

 

So, read the manual, and find out if your unit can send distress info, and how to do it. Some have to be set up with an (optional) external switch, or distress must be set via software that needs a PC (Thats STUPID imo).

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If you have a receiver, you will see the AtoN's on your plotters in real time. That will give you accurate distance off etc, and makes for safer navigation. AtoN's can also broadcast additional data - tide rates, wind speed and dir, phone numbers, whatever the owners wish you to know.

 

 

Most AIS receivers and Transponders now can "see" AtoNs. Some may require updated software to do so. Many new VHF radios now have AIS receivers.

At this stage it seems few units can see the weather data transmitted by AIS, and that includes  Class A.  Like being able to AtoN this will come to more units over time I suspect.

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Yes Grant, that is a development area. Not many ais units currently transmit weather info either, but it's coming. The data sentences are available, they just need to be implemented. It should be a firmware update for most to be able to use it, once it's there to be used.

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early advice if you only want to receive and not transmit AIS was "If you have a laptop, a GPS and a VHF radio. Than you already have 99.9% of the hardware for AIS reception.......Don't duplicate, just add a small circuit to your vhf." How does this advice stack up these days?
 

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I tried that buying from someone on trademe an adjustment to my vhf radio to be able to receive ais onto a laptop but I was never able to make it work which was probably operator error. But have now bought a matsutec stand alone unit which was not that much more is simple to use and sends and receives. Wouldn’t argue if ais was required for clearance to sail offshore.

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Great stuff IT.. It may be of interest to some people that AIS signals being picked up by satellite is in fact a happy accident.

It was never intended to be the case, but due to the fact that it is "line of sight" and the atmosphere has less interference as you go higher, it has ended up working. As IT has said, the standard class B system is right on the edge of reasonable reception but newer versions should sort that out making a ready made very cheap global tracking system. It has been a huge success with commercial shipping companies because suddenly an existing and legally necessary system (class A) has become a tracking transmitter...

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I don’t know that it’s a recent development, but AIS MOB is a great technology also, we just loaded our life jackets up with a Man Over Board AIS transponsder. We ran it in test mode and the Vesper Marine XB 8000 just worked out of the box. I love the general AIS features particularly on coastal passages, but this feature is an excellent source of peace-of-mind when crew are alone on night watch.

 

I’m hoping Vesper Marine keep updating their firmware as new features become available.

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Yes Robin, the Mob function is a SART, that I was talking about in my safety post above. You are right though, if anyone goes overboard offshore, their best bet is the boat they fell off, if they are equipped with an AIS MOB SART.

A PLB is not so good, as the yacht does not normally carry the equipment to locate that. A combi AIS/PLB would be perfect though, but I don't know if any have made it to market yet. There are several under development.

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