AIS surprise

[Reefmagnet]

Originally Posted by GILow

This is interesting, I’d not heard of AIS repeaters. Do you know of a map of the repeaters anywhere? I couldn’t find one with a bit of googling.

They don't seem to be publicized. There's some along the shipping lanes of the GBR which act essentially the same as a VHF repeater with about the same extension of range.

[Dsanduril]

AMSA operates the AMSA AIS Base Station Network, which is a part of the Australian Maritime Identification System (AMIS). AMIS is intended to monitor and identify all vessels within 1000 miles of Australia. There isn’t much published about either system without digging deep in government files. In their 2019-2020 transparency report AMSA did say:

AMSA operates a national AIS network for the purpose of tracking vessels to assist in their safe navigation. The network also provides data to the Under Keel Clearance Management (UKCM) system in the Torres Strait and the Great Barrier Reef and Torres Strait Vessel Traffic Service (REEFVTS). Both enhance the safety of shipping in these environmentally sensitive marine areas.

The network is continually reviewed to ensure coverage remains effective and complements AMSA’s traditional aids to navigation network.

.
There are a couple of amateur radio sites that seem to have dug up some information (vklogger for one) but you need to be a member to see the info.

I started really looking at this along the GBR, in the NMEA sentences you can see the forwarding field, 00 indicates reception direct from ship, 01 is one repeat, 10 is two, 11 is three. Along the GBR we saw lots of 01 and 10, by Darwin we were only seeing 01 single repeats. Have never sailed down on your side, but would be surprised if there wasn’t a repeater out by the lighthouse at Cape Borda or up on the Yorke Peninsula.

[GILow]

Originally Posted by Dsanduril

AMSA operates the AMSA AIS Base Station Network, which is a part of the Australian Maritime Identification System (AMIS). AMIS is intended to monitor and identify all vessels within 1000 miles of Australia. There isn’t much published about either system without digging deep in government files. In their 2019-2020 transparency report AMSA did say:



.

There are a couple of amateur radio sites that seem to have dug up some information (vklogger for one) but you need to be a member to see the info.



I started really looking at this along the GBR, in the NMEA sentences you can see the forwarding field, 00 indicates reception direct from ship, 01 is one repeat, 10 is two, 11 is three. Along the GBR we saw lots of 01 and 10, by Darwin we were only seeing 01 single repeats. Have never sailed down on your side, but would be surprised if there wasn’t a repeater out by the lighthouse at Cape Borda or up on the Yorke Peninsula.

Fascinating stuff.

I’ll see what I have on board that can view those sentences. I have a backup AIS unit set on receive-only that I could feed to my laptop. Probably no time before I leave South Australia, but plenty of opportunities later.

[StuM]

Originally Posted by GILow

Fascinating stuff.

I’ll see what I have on board that can view those sentences. I have a backup AIS unit set on receive-only that I could feed to my laptop.

OpenCPN ?

[GILow]

Originally Posted by StuM

OpenCPN ?

Funnily enough I just happen to have that installed on the laptop.

Of course, the darn laptop doesn't have a serial port so the idea will have to wait until I get to my next destination for me to go shopping.

[StuM]

Originally Posted by GILow

Funnily enough I just happen to have that installed on the laptop.

Of course, the darn laptop doesn't have a serial port

What doe USB stand for?


(I know, I know - you are talking about an RS-232 port )

[clownfishsydney]

Originally Posted by Dsanduril

Australia has a pretty extensive network of shore repeater stations. These are intended to help “see around corners, over hills” and are used by governments for tracking. An AIS position message has a specific two-bit field that is incremented each time the message is forwarded by a repeater, thus a message can be forwarded three times, any repeater receiving a message with both bits sets knows it has already been forwarded three times and drops the message - it doesn’t get forwarded any further. If you can look at NMEA from the AIS unit or dig around on the target info you can see if the message was received directly (no forwarding bits set) or how many hops it has been forwarded. This greatly extends range and explains many long-distance contacts in places with AIS shore stations.

I have seen this said before (not just about Australia, but also that ships can retransmit) but have never seen any government or any other factual evidence/statement that this is true.

I volunteer as a NSW Marine Rescue radio Watch Officer, and I can say that this has never been said to us and that our AIS fixed base does not pick up any ships/boats further than I would expect it to.

My yacht would regularly pick up ships 80 nm away, yachts 40 nm and sometimes the above as far as 200 to 300 nm away. Again, this longer range is due to skip. We were picking up radio calls as clear as anything two weeks ago from Eden (south of Sydney - 200 nm) and Taree (north of Sydney - 160 nm).

[GILow]

Originally Posted by StuM

What doe USB stand for?


(I know, I know - you are talking about an RS-232 port )

Serial BUS, not Port.

[wholybee]

In rare instances, I have seen AIS targets over 300 miles away. I was just offshore of San Francisco, and could see multiple AIS targets leaving LA. I

Line of sight is the rule of thumb for consistent reliable reception. Rare atmospheric conditions can lead to much further contacts. I see 80 miles fairly often.

[Dsanduril]

Originally Posted by clownfishsydney

I have seen this said before (not just about Australia, but also that ships can retransmit) but have never seen any government or any other factual evidence/statement that this is true...

I absolutely can’t prove it’s true. I can say I have seen repeated position reports while in Australian waters, and that IME governments don’t tend to allow private parties to establish repeater stations. Finding out if a specific transmission has been repeated is baked into the AIS specification, figuring out how to get that information out of an AIS device may not be so simple.

Back to the OP’s question about a map, the ACMA license search function is pretty robust. Filtering on “client=Australian Maritime Safety Authority, minimum frequency = 161.975MHz, and maximum frequency = 162.025MHz” produces this map:



From this I can deduce that AMSA has AIS base stations at these locations that include transmit functions. Whether or not they are configured as repeaters (one, some, or all) I cannot say as I can’t find any specific information in that regard. If you dig through the AMSA Harradine Lists you can sometimes suss out information for a specific station, but it is time consuming and full of dead-ends.

[GILow]

Originally Posted by Dsanduril

I absolutely can’t prove it’s true. I can say I have seen repeated position reports while in Australian waters, and that IME governments don’t tend to allow private parties to establish repeater stations. Finding out if a specific transmission has been repeated is baked into the AIS specification, figuring out how to get that information out of an AIS device may not be so simple.



Back to the OP’s question about a map, the ACMA license search function is pretty robust. Filtering on “client=Australian Maritime Safety Authority, minimum frequency = 161.975MHz, and maximum frequency = 162.025MHz” produces this map:



Attachment 232062



From this I can deduce that AMSA has AIS base stations at these locations that include transmit functions. Whether or not they are configured as repeaters (one, some, or all) I cannot say as I can’t find any specific information in that regard. If you dig through the AMSA Harradine Lists you can sometimes suss out information for a specific station, but it is time consuming and full of dead-ends.

Great link, thank you. And at least two of those stations, if they were repeating, would account for the long range target reception the other day.

It might, as others have noted, have been simply a case of atmospheric conditions, but I am certainly going to have a closer look at the possibility of repeated signals just as soon as I can scrounge together the bits.

It’s a shame I will (should) be long gone from these waters before I get a chance to learn more.

[Dsanduril]

Because of the way AIS/NMEA compresses AIS messages it’s not the easiest thing (you have to look at everything bitwise or as 6-bit ASCIIj, but it can be done. I find the USCG AIS message list to be quite useful. Importantly for this discussion, the first 6 bits are the message type (once you get past the NMEA stuff at the front) you’re looking for 1,2,3, and 18 in general. The next two bits (so the upper two bits of the first byte) are the repeat bits.

[GILow]

Originally Posted by Dsanduril

Because of the way AIS/NMEA compresses AIS messages it’s not the easiest thing (you have to look at everything bitwise or as 6-bit ASCIIj, but it can be done. I find the USCG AIS message list to be quite useful. Importantly for this discussion, the first 6 bits are the message type (once you get past the NMEA stuff at the front) you’re looking for 1,2,3, and 18 in general. The next two bits (so the upper two bits of the first byte) are the repeat bits.

Messages 28 - 63 reserved for future use.

I like a bit of forward planning.

[HeinSdL]

I saw something similar some time ago in the Med, targets visible from a long way away on AIS. Particularly interesting was that with my position being somewhat east of the Balearic islands a big chunk of the Med to the south was visible, to the north not. So I figure the only explanation would be some kind of special atmospheric condition.

[clownfishsydney]

Originally Posted by Dsanduril

I absolutely can’t prove it’s true. I can say I have seen repeated position reports while in Australian waters, and that IME governments don’t tend to allow private parties to establish repeater stations. Finding out if a specific transmission has been repeated is baked into the AIS specification, figuring out how to get that information out of an AIS device may not be so simple.

Back to the OP’s question about a map, the ACMA license search function is pretty robust. Filtering on “client=Australian Maritime Safety Authority, minimum frequency = 161.975MHz, and maximum frequency = 162.025MHz” produces this map:

Attachment 232062

From this I can deduce that AMSA has AIS base stations at these locations that include transmit functions. Whether or not they are configured as repeaters (one, some, or all) I cannot say as I can’t find any specific information in that regard. If you dig through the AMSA Harradine Lists you can sometimes suss out information for a specific station, but it is time consuming and full of dead-ends.

I think you will find that these sites broadcast AIS for a fixed structure or structures. For example, many of the ones shown when filtered on the two AIS frequencies are lighthouses, others are buoys, at least two are oil rigs in Bass Strait, a few are pilot boarding ground buoys off Newcastle etc. They do not rebroadcast boats/ships AIS signals. A lot of these I have personal experience with seeing them on my AIS.