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Old 29-08-2010, 07:14   #1
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AIS - Can I Recieve Permanently but Choose to Switch On / Off Transmitting ?

I cannot seem to get any clear answers to this question, and was wondering if there was an AIS unit that gives one the flexibilty of choosing to switch off transmission of signal whilst still recieving? (eg. in waters where hostile craft are - would like to recieve AIS but not transmit my signal)
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Old 29-08-2010, 07:35   #2
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AIS silence

I too am interested in this. We have added a Raymarine Class B transceiver. I think you can run silent with it, for races, for instances, but I don't know if you then receive or not. I am currently 1000 miles from the boat but heading back soon.
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Old 29-08-2010, 07:58   #3
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I guess for safety reasons the manufacturers of AIS transceivers, even class B ones, make it as difficult as possible to turn off transmissions. Nevertheless I would guess there is an option to do this via software or hardware with most units.

I contacted Comar about this for my CSB200 class B transceiver & persuaded them to supply me with a dealer only firmware version that does allow transmissions to be turned off using their PC software interface. There is also a hardware option to do the same by grounding pin 6 on the RS232 interface. In this "silent" mode it still receives AIS transmissions though.

With its 2w short term transmissions, the extra power use is almost negligible though so rarely use the option.
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Old 29-08-2010, 08:05   #4
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Only vessels under International SOLAS regulation are required to broadcast Class A AIS signals. If you are under SOLAS regulation you may not turn off your signal. AIS is not required in Inland waters except under national regulations.

For Class B AIS you may want to review the following:

Nautical Institute AIS Forum: Technical Feedback

This talks about the ineffectiveness of using Class B Transponders for avoiding collisions. It essentially places these units in doubt based on a collision this past June with a Bulk Carrier and a yacht. Both vessels were not providing a proper lookout and the bulk carrier failed to render assistance. The yacht skipper was dismasted and he was able to recover the mast and cut away the head sail and motor back to port.

Since your yacht is not required to broadcast AIS signals the use of them in not mandatory. There is regulations stating that if you install and AIS transponder you are required to use it. Should you have a collision and are not broadcasting it could be considered a violation.

Having an AIS receiver is of substantial value but the use of transponders to warn other vessels of your location is not so clear cut. Your obligation to maintain watch and augmented radar visibility is not reduced by broadcasting AIS. While it might aid in your visibility is is not a means to place the obligation totally on the other vessel.

If you have security concerns about divulging your position the range of AIS is no greater than VHF signal propagation. If you are in areas where you might require a higher attention to security you would normally be farther off shore than VHF transmission range any way.

Also note Military ships are NOT required to broadcast AIS and I would think in areas where you might want yours off they would certainly have theirs off.

AIS has it's positive uses but it really does not change the game very much for a small boat. You still have to do all the things you would as if you had no AIS. The times when you might be concerned for your own security would also correlate to the areas where having a signal to identify yourself would also avoid a collision.

Having a receiver I feel is a good idea and a valuable tool, but broadcasting when you are not required to do so has a lower value since based on the one incident a large ship didn't see the small yacht, failed to detect the AIS broadcast, collided with the yacht and went on with the trip as if nothing happened. For that reason, you are better served to be more defensive in maintaining watch using a receiver but not as a well served as a broadcaster. The big ships just don't care about you and would choose to ignore you as much as possible. It is illegal for them to do so but you have obligations as well. That fact seems to weigh heavier than the notion of piracy on the high seas. The number of confirmed piracy actions on the high seas with yachts is minuscule. Collisions with large ships is far greater. Playing the low percentage rather than the big percentage that carries a high death rate just isn't good bet.

It is a game of percentages. The only high percentage for you is the most active defense. Hiding would be a lower percentage success rate.
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Old 29-08-2010, 09:16   #5
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The Comar and AMEC AIS-B units do provide a switch for the purpose of ceasing transmission while continuing to receive. Go here for the specifications and description of the feature: Welcome to Milltech Marine - your AIS experts.

Some early Class A AIS equipment lack the ability to receive AIS-B signals. This may be the cause of the incident mentioned above. Some such commercial gear can be updated with software changes to correct it. It is an evolving system. When the Class A equipment was first introduced, Class B was not an established standard. I expect over time commercial vessels will all receive AIS-B signals.

In heavy fog I am in favor of getting any AIS plots from either class.
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Old 29-08-2010, 09:24   #6
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Quote:
Originally Posted by impi View Post
I cannot seem to get any clear answers to this question, and was wondering if there was an AIS unit that gives one the flexibilty of choosing to switch off transmission of signal whilst still recieving?
AIS still seems shrouded in mystery and rumour..

My unit has a switch on the front, which you can define as "transmissions on / off" via the config programme.
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Old 29-08-2010, 09:24   #7
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YES. We have the ACR unit, and the 'transmission' function is easily disabled via either a switch or via a computer keyboard (if a computer is connected to the AIS, usually for a navigation program). In either case, the AIS 'receive' function is still active.
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Old 29-08-2010, 09:38   #8
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The ACR Nauticast-B transponder has a pair of wires in the power/data cable that can be wired to an external "Disable Transmit" switch. As Steve notes, this function can also be software-controlled.

As for older equipment's incompatibility with Class-B, while some very old gear doesn't understand Class-B at all (the early Nasa Marine AIS Radar, for example), most gear can decode at a minimum the Class-B "dynamic" message (position, course, and speed), but may not be able to decode the "static" messages (vessel name, callsign, etc). They will see you -- if they're paying attention -- but may not know your name.
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Old 29-08-2010, 11:51   #9
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My Simrad NAIS300 unit has an optional switch that allows you to not transmit while still receiving. You need to provide the switch and do the wiring yourself, but it is very easy and we have implemented it.
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Old 29-08-2010, 18:58   #10
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The big ships just don't care about you and would choose to ignore you as much as possible.
That statement is so incorrect that it hurts!

I've been a merchant seaman and I can assure one and all that the vast majority of merchantman out there do everything they can to insure that you have a safe passage.

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Old 01-09-2010, 09:05   #11
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I found the information in this thread very useful and would like to thank everyone VERY much for the information. I am now armed to purchase the correct AIS. Thanks again
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Old 01-09-2010, 09:52   #12
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I found the information in this thread very useful and would like to thank everyone VERY much for the information. I am now armed to purchase the correct AIS. Thanks again
There is one more brand, called EasyAIS that has a model labelled TRX. It also has a couple of additional wires that can be connected to an external switch. You can then configure what that switch does. One of the options is to switch the transmission off.

We have now used the device for one season and we are very happy with it. We choosed it for the good price and for that it is water proof and - unlike many other brands - does not need to be installed inside. However, we ended up installing it inside, so I cannot comment how well all the connections would have done on the deck in the real life.
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Old 03-09-2010, 06:56   #13
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Raymarine's class B AIS 500 comes on in the 'silent' transmission mode upon powering on while still being able to receive targets.

One must go into the menu to turn the 'silent' mode off, a good feature.
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Old 13-09-2010, 09:59   #14
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Hi ronbo1,
I have completed my investigation and must admit that with my setup all being raymarine E120 plotter and seatalk integrated, the AIS 500 is EXACTLY what I was looking for. Strangely enough, the Raymarine agent in Cape Town had not thought they had a product that could give this option. Thank you to everyone who contributed to this thread.
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Old 14-09-2010, 07:03   #15
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[Having a receiver I feel is a good idea and a valuable tool, but broadcasting when you are not required to do so has a lower value since based on the one incident a large ship didn't see the small yacht, failed to detect the AIS broadcast, collided with the yacht and went on with the trip as if nothing happened. For that reason, you are better served to be more defensive in maintaining
Pblais much of your post does nor concur with reality and may be based on reading the net too much.

I recently sailed in the med with a raymarine 500 aid class b transponder ( which has a silent mode as does nearly every leisure ais unit )

It was very clear that ships were taking avoiding action to ensure they didn't get anywhere near us . This was all taking place over the visible horizon. It was very humbling to see large container and tanker vessels make obvious course changes to seek to avoid coming close to us

Ais is the closest thinly to PFM I've seen on a small boat. One word of advice buy a transponder forget the receive only at this stage

DAve
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