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Old 02-11-2014, 10:41   #151
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Re: Class B AIS

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Originally Posted by Ziggy View Post
You will get a much better situational awareness displaying AIS information on a chart plotter. However, one benefit of also displaying the AIS information on the VHF is that you can make a DSC call to a vessel without manually entering its MMSI. Standard Horizon Matrix GX 2000 does not have a built-in AIS receiver but it can display the AIS target if you have a separate receiver or transceiver.
That's exactly what I'm thinking. I believe there is also a version of the iCOM 506 w/o an AIS receiver. I will definitely have my chartplotters displaying AIS data, but having the VHF also display AIS data to avoid keying in an MMSI number would be very nice.
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Old 02-11-2014, 13:12   #152
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Re: Class B AIS

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If one has a transceiver like the XB-8000, does it make sense to purchase a VHF that can display AIS (even though it doesn't have its own AIS receiver)? I believe the some versions of the iCOM 506 can do this. Or, is it cheaper and easier just to use the chartplotter to display AIS?
I think it makes sense to get a VHF that includes AIS display capability. Since I already have a Class-B transponder, I got a Standard Horizon Matrix GX2000 which does not include a receiver. The AIS display and functions are primitive, but when I am at sea this lets me turn off the chartplotter to save power, and still have a usable AIS display / alarm on the VHF (both at the navstation, and at the helm with the remote mike.

I would not want to have top depend on the VHF AIS display in close-quarters situations, but that's when I turn on the chartplotter.

I would check the specs on the VHF you consider. It may be possible to disable an internal AIS receiver and instead use the output of your transponder. I don't know, but that seems like a reasonable feature to design in.
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Old 02-11-2014, 13:46   #153
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Re: Class B AIS

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Originally Posted by Paul Elliott View Post
I would check the specs on the VHF you consider. It may be possible to disable an internal AIS receiver and instead use the output of your transponder. I don't know, but that seems like a reasonable feature to design in.
I don't understand why this would matter? We have a VHF with AIS and a separate AIS transponder. The transponder is connected to the chart plotter, computer and instruments and the VHF is standalone. They both see the same targets and we have the ability on the VHF to make direct DSC calls to ships.

I don't see the advantage of turning off the VHF AIS receive and using an external AIS input.

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Old 02-11-2014, 14:16   #154
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Re: Class B AIS

Here is what I did.

Found a used Garmin AIS600 from a Hong Kong seller. No warranty. Ended up paying something like $225 delivered. Paid about $80 for the cables. Unit tested fine except for silent mode. Opened unit up, reconnected harness for silent / alert mode internally to fix silent mode. Sent unit to Garmin where I paid $50 for them to reprogram MMSI and do a full test. I already had a gps antenna from an old Garmin chartplotter which I recentlu replaced with a Zeus2 in the cockpit with built in antenna.

So, for me, $350 total including the built in active vhf splitter. A gamble, but it worked out. I think there are still more on Ebay, and you can make an offer...
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Old 10-11-2014, 22:53   #155
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Re: Class B AIS

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AIS transmissions are one-way broadcast transmissions. Please explain how there is error correction.
More geekery, and completely unrelated to the OP's question:

We've discussed Forward Error Correction (FEC), and the fact that AIS doesn't use this. Well, it turns out that the underlying HDLC protocol being used over the VHF contains a Cyclic Redundancy Check (CRC), which some researchers have used to implement a low-performance FEC. It wasn't designed to do this, and I don't know if any receivers are trying to use this CRC FEC, but it is at least theoretically possible to wring a few fractions of a dB improvement out of the VHF link this way.

I hadn't known this, but I've been studying AIS demodulation methods lately, and ran into the research paper. Who knew?!?
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Old 10-11-2014, 23:42   #156
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Re: Class B AIS

That CRC might be used as a form of forward error correction is interesting.

I would expect that a receiver that was able to achieve an improvement in its performance would state that improvement in its specifications, that is, they would be able to say the receiver could receive signals of lower level with a certain bit error rate than a competitor's receiver that did not employ their processing methods.

When a receiver manufacturer publishes the sensitivity of their AIS receiver, it is not reasonable to suggest that the performance will improve beyond specification on the (false) assumption that there is error correction. If there is error correction being done, the effect of the error correction will appear in the specified sensitivity.
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