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Old 01-11-2016, 04:25   #1
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AIS shows many long distance targets

I just started OpenCPN and noticed there is an abnormally large number of long distance AIS targets being displayed. Is there an unusual atmospheric phenomenon causing this, or are there repeater stations (do they even retransmit AIS?).

I am currently anchored a few miles south of Mackay, on the Queensland coast and I can see targets ranging between 600 miles to the north and 300 miles to the south, with one being 260 miles directly out to sea from me. Most are Class A, with some Class B. The furthest Class B is at Lady Musgrave Island, about 220 miles from me, and 30 miles offshore.

I remember the last time I was there having some difficulty reaching the mainland via VHF.

As I wrote this post, I noticed a few dropped off, but there are still nearly 50 targets over 100 miles away.

For the record, I have a cheap dedicated AIS antenna mounted at the spreaders, feeding into a Vesper XB8000.
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Old 01-11-2016, 04:38   #2
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Re: AIS shows many long distance targets

I just looked at the BoM site and there appears to be a big high pressure ridge running up the Coral Sea just now so it could be down to atmospheric ducting...
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Old 01-11-2016, 05:55   #3
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Re: AIS shows many long distance targets

I seem to recall reading that in some areas there are AIS repeaters, which might account for this. Of course, the AIS frequencies are very close to the amateur radio 2-meter band, and I know that there are a lot of 2-meter repeaters out there, so maybe some of those are forwarding the signal along. The only other explanation I can think of would be, as El Pinguino mentioned, unusual atmospheric conditions.
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Old 01-11-2016, 06:49   #4
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Re: AIS shows many long distance targets

Call up local traffic control and ask. I bet they have repeaters along the coast.

What you are experiencing is normal but not common unless your maritime administration are serious guys who DO CARE about the coral reef.

Let us know when you find out the actual explanation. I bet my money on repeaters.

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Old 01-11-2016, 08:04   #5
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Re: AIS shows many long distance targets

I would guess atmospheric ducting, rather than repeaters. I occasionally pick up targets more than 600 miles away, off the coast of Baja. AFAIK there are no repeaters along that coast.
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Old 01-11-2016, 09:06   #6
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Re: AIS shows many long distance targets

I don't see how repeaters working on the official AIS frequencies could work..... mayhem is what I can see ensuing if anybody tried it....
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Old 01-11-2016, 10:29   #7
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Re: AIS shows many long distance targets

I've experienced conditions in which VHF communications of several hundreds of miles is possible; I expect that this is what's going on.
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Old 01-11-2016, 10:38   #8
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Re: AIS shows many long distance targets

Quote:
Originally Posted by El Pinguino View Post
I don't see how repeaters working on the official AIS frequencies could work..... mayhem is what I can see ensuing if anybody tried it....
1) You can ignore an AIS target 600 NM away with confidence: it's basically irrelevant.

2) The AIS frequencies are at the high end of the marine VHF range. I've heard some (very occasional) "bounced" VHF from a distance, just as one can occasionally hear AM signals across a continent if the ionosphere favours it. I haven't seen it personally with AIS, but I can understand that it's not that unusual.
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Old 01-11-2016, 10:57   #9
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Re: AIS shows many long distance targets

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Originally Posted by El Pinguino View Post
I don't see how repeaters working on the official AIS frequencies could work..... mayhem is what I can see ensuing if anybody tried it....
They work. We saw vessels hundred of miles away consistently while sailing around the Galapagos. Here's a write-up on their AIS repeater installation
Ais - Galapagos
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Old 01-11-2016, 10:59   #10
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Re: AIS shows many long distance targets

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Originally Posted by S/V Alchemy View Post
......
2) The AIS frequencies are at the high end of the marine VHF range. I've heard some (very occasional) "bounced" VHF from a distance, just as one can occasionally hear AM signals across a continent if the ionosphere favours it. I haven't seen it personally with AIS, but I can understand that it's not that unusual.
Ducting common in hot dry air...
I've heard a ship on VHF Vox at about 400 miles once.... and worked stations at 180 miles.
Greatest distance worked that I have heard off was in the 60's... a ship off Bombay working a ship near Aden that was working a ship near Suez.
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Old 01-11-2016, 11:04   #11
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Re: AIS shows many long distance targets

Definitely VHF atmospheric ducting. This is a very common and well-known phenomenon. No repeaters involved.

The VHF signals, usually in paths over water, travel thru atmospheric layers ("ducts") over long distances, often 300-400 miles. The condition comes and goes; doesn't last long.

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Old 01-11-2016, 12:26   #12
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Re: AIS shows many long distance targets

Quote:
originally posted by Paul L:
They work. We saw vessels hundred of miles away consistently while sailing around the Galapagos. Here's a write-up on their AIS repeater installation
Ais - Galapagos
Repeaters do not work by transmitting the same information out on the same frequencies. They would re transmit the information out on a different frequency that an AIS Transceiver would not receive.
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Old 01-11-2016, 12:49   #13
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Re: AIS shows many long distance targets

VHF has an amazingly strong sky wave that travels straight up for thousands of miles and is received by satellites. Sky waves are no use in ship to ship VHF radio calls because radios are picking up the ground waves that are only good for line of sight.
I'm no radio expert and do not have my GMDSS text books handy for reference, so I would like to hear from someone who really knows about this stuff. Can the sky waves sometimes bounce off the ionosphere and be received by ordinary VHF receivers?
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Old 01-11-2016, 13:10   #14
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Re: AIS shows many long distance targets

Targets. to me is the wrong term. Your system is receiving information transmitted by those vessels, not receiving information based on a a return from a signal you sent in their direction (as would occur with radar). Just my mindset.
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Old 01-11-2016, 14:33   #15
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Re: AIS shows many long distance targets

Once we heard Coast Guard Tin Can Bay , on Queensland's East coast, on Ch. 16 VHF, in Noumea, New Caledonia. That had to be some kind of skip! Also, about 600 mi., once while we were in Baja. Radio reception weirdnesses happen.

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