Which VHF channel to use for on-boat operations?

[paulears]

Well - in my office sits a Class A transceiver on 24/7 and I've been testing some AIS beacons and they have no output I have ever noticed in 156-157, just the two channels either side of 162MHz. I've double checked the manuals for my Icoms and 75 and 76 are not used for AIS services. I'll connect the antenna on the mast to the analyser tomorrow and see what the activity is on 75 and 76. Hopefully I'll detect some data and not voices. I'll report back.

[s/v Jedi]

Not here to fight over it but pretty sure you’re wrong. The UK is a full member of the ITU and must comply to the four AIS channels for worldwide use. The only regional changes are for additional channels, but every transceiver must work world wide and the satellites that listen for message 27 have world coverage, incl. Polar orbit.

Here is the ITU documentation, annex 4 is where it is: https://www.itu.int/dms_pubrec/itu-r...2-I!!PDF-E.pdf

Attached a screenshot from that.

[s/v Jedi]

Also, I recommend this book which can be found on the net as a free download. It is a complete work on AIS, with every aspect of satellite AIS explained in detail.

[paulears]

I'm not disagreeing - but simply pointing out there is absolutely nothing to stop marine users in the UK using those channels for ordinary comms - which would make a mess of the people using AIS on those channels. AIS and comms don't happily share channels. The band plan makes it plain that use of the 162 pair is AIS and of course ch 70 gets a mention - but all the manual for my kit seems to not mention AIS at all on the 156 frequencies. Seems a bit odd there's nothing in people's licences to warn them off?

[Auspicious]

Originally Posted by s/v Jedi

No, all SOTDMA transceivers, which is all class-A and the newish class-B+ transmit on channels 75 and 76. This is tx only, as it is just the ais satellites that listen, not shipstation transceivers.

You can find this info in the official channel list, under footnotes n and s: https://www.navcen.uscg.gov/?pageName=apps18

Looks like part of the ITU proposal but not approved or in place. All the satellite payloads I can find are still on 87B and 88B. I don't see any hardware even for the commercial market for 75 and 76. Seems like a work in progress that is some years away from activation.

[s/v Jedi]

Originally Posted by Auspicious

Looks like part of the ITU proposal but not approved or in place. All the satellite payloads I can find are still on 87B and 88B. I don't see any hardware even for the commercial market for 75 and 76. Seems like a work in progress that is some years away from activation.

I know it isn’t fun when you’re trying to hold an indefensible position because you’re really just digging a deeper hole.

First it was said that there was no AIS on ch. 75 and 76, now it just an experiment and years away from being produced. In reality the ITU document is from 2014 and all the manufacturers have complied soon afterward.

Yes you can find old transceivers that don’t have this yet but no it is not years away from activation because -all- sotdma based transceivers that are sold today have this and it is in full use.

Here is a good table with current class-B units being sold. You will find that every sotdma unit supports the satellite msg 27.
https://www.milltechmarine.com/custo...comparison.pdf

[s/v Jedi]

Just to make sure this isn’t missed, this is the footnote explaining the Sat-AIS Msg27 column

[paulears]

I said there was no AIS on these channels in a busy leisure and industrial port in England. This was a snapshot of activity, and I was not even aware AIS should have been there. We clearly have a number of international high tonnage vessels with Class A radios and a large amount of AIS on Class B units. Icom UK don’t seem to have any transponders with it, yet, and none of my radios on the shelves have any lockout from using those channels. In addition, the current UK licence does NOT require this.

I appreciate that we will see AIS being introduced on transponders in bus routed systems and stand alone devices like plotters. I suppose for a while short data bursts on rarely used comm channels might make it less useful, but until countries administrations protect the frequencies, it’s a system under test, in practical terms.

What I don’t know is if the usage I am seeing on the screen on 775 + 825 is people or a single vessel with the new kit. I will need to do more work on that. Clearly we have a new system under deployment, but it’s going very very slowly and will need time. The folk in the UK taking their tests use the same text book, and there is no mention of not using any frequency apart from 70 in it…….. yet.

[wingssail]

Originally Posted by paulears

I said there was no AIS on these channels in a busy leisure and industrial port in England. This was a snapshot of activity, and I was not even aware AIS should have been there. We clearly have a number of international high tonnage vessels with Class A radios and a large amount of AIS on Class B units. Icom UK don’t seem to have any transponders with it, yet, and none of my radios on the shelves have any lockout from using those channels. In addition, the current UK licence does NOT require this.

I appreciate that we will see AIS being introduced on transponders in bus routed systems and stand alone devices like plotters. I suppose for a while short data bursts on rarely used comm channels might make it less useful, but until countries administrations protect the frequencies, it’s a system under test, in practical terms.

What I don’t know is if the usage I am seeing on the screen on 775 + 825 is people or a single vessel with the new kit. I will need to do more work on that. Clearly we have a new system under deployment, but it’s going very very slowly and will need time. The folk in the UK taking their tests use the same text book, and there is no mention of not using any frequency apart from 70 in it…….. yet.

The reason why paulears might be unaware of AIS in 75 and 76 is the AIS msg 27 is deemed by the ITU as too short to interfere with human speech and therefore was placed on the Port Operations channels over the top of the previous, and still valid use of those channels. It's there, you just don't hear it (yet).

It's understandable that this is a confusing topic. Various countries (for example USA and Japan) are still publishing information such as this: "AIS transponders and receivers use two VHF radio frequencies: 161.975 MHz (AIS1, or channel 87B) and 162.025 MHz (AIS2, or channel 88B)."

And this regarding 75 & 76:

"Ch 75 & 76-PORT OPERATIONS https://www.fcc.gov/ship-radio-stations. Available for navigation-related port operations or ship movement only. Output power limited to 1 watt."

Yet elswhere internationally and even within the USCG it is recognized that Ch75 and Ch76 are authorized for AIS msg 27, although those channels are ALSO STILL allocated to Port Operations (confusing, huh?)

"Channels 75 and 76 are also allocated to the mobile-satellite service (Earth-to-space) for the reception of long-range AIS broadcast messages from ships (Message 27; see the most recent version of Recommendation ITU-R M.1371)." https://www.navcen.uscg.gov/?pageName=apps18

This dual use was justified by an ITU study which determined that the transmission of 17 ms messages repeated every three minutes with a power of 12.5 W did not cause any considerable interference in voice communications on channels 75, and 76, these were allocated for transmission of Satellite AIS messages. With the exception of AIS, the use of these channels (75 and 76) should be restricted to navigation-related communications only and all precautions should be taken to avoid harmful interference to channel 16 by limiting the output power to 1 W.

Interestingly enough, I found reference to msg27 also being legally transmitted on chan 87B and 88B

OH! Another thing. I read that, to reduce traffic on 75 & 76, msg 27 is not to be broadcast by ships which are within range of shore AIS stations. If your Class A unit is hearing a shore based AIS station, it shouldn't transmit msg 27 on ch75 and ch76 because, presumably, the shore station will be receiving you on ch87B and 88B. It's used while at sea away from shore.

[paulears]

This is quite interesting in that to the satellites the use of low power voice doesn’t interfere? I wonder how they handle the difference between F3E and the presumably FSK data burst? I went into the office and for the past 24 hours the two ‘new’ channels have been totally inactive. On Monday I’ll have another go.

[wingssail]

Originally Posted by paulears

This is quite interesting in that to the satellites the use of low power voice doesn’t interfere? I wonder how they handle the difference between F3E and the presumably FSK data burst? I went into the office and for the past 24 hours the two ‘new’ channels have been totally inactive. On Monday I’ll have another go.

No traffic on 75 & 76 on the weekends huh? Are the ships all gone from the English Channel?

Maybe the only thing going on on Ch75 & 76 are some port operations people who don't work on the weekend.

Maybe the ships are not sending any msg27 due to their proximity to the shore based AIS stations.

[Jammer]

Originally Posted by paulears

I wonder how they handle the difference between F3E and the presumably FSK data burst?

They ignore the F3E because nothing comes through the FSK decoder.


I don't know how these particular satellites are designed but most orbital receivers have many high-gain antennas each covering a fairly narrow geography. If the signals of interest are in e.g. the Pacific (or even the north Atlantic) it doesn't matter what people are transmitting in F3E from the straits of Dover because the straits of Dover are outside the sweet spot of the antenna.

[Auspicious]

Originally Posted by s/v Jedi

I know it isn’t fun when you’re trying to hold an indefensible position because you’re really just digging a deeper hole.

First it was said that there was no AIS on ch. 75 and 76, now it just an experiment and years away from being produced. In reality the ITU document is from 2014 and all the manufacturers have complied soon afterward.

You were right and I was wrong. My apologies. This has turned into quite a science project.

I found the frequency reallocations dating back just a few years and simply did not expect implementation to happen so quickly. There are so many moving parts with ITU and IMO recommendations being implemented in national laws and regulation and then manufacturers getting on board.

Here are some things I learned. One of the big challenges with satellite detection of AIS 1 (87B) and AIS 2 (88B) is that in many countries those frequencies are also allocated to land mobile applications. In the US that appears to mostly be railroads. 75 and 76 are allocated internationally to maritime service and only maritime service. The reasons are good: 156.775 MHz (75) and 156.825 (76) are guard channels for 156.800 (16). That's also why they are limited to 1W for their (current) primary allocation for port operations.

While SAT-AIS on 75 and 76 are still in early days they are indeed operational. Kongsberg makes model AIS 300 for Class A and indeed makes AIS x50 for satellite payloads. I don't see hardware from Orbcomm or Spire. I ran out of steam before getting to Harris and Raytheon.

Satellite AIS is all about asset tracking and not collision avoidance. Msg 27 is stripped down to the bare essentials, heavily compressed, and doesn't even include a time stamp.

Most S-AIS payloads on orbit are still 87B and 88B including the feeds to Marinetraffic. My data source here is a year old so that may be changing.

[paulears]

Don;t worry about the right and wrong - it's interesting to discuss. I didn't know about AIS on the other two channels being tricky with railways?

Until we started this, I had not even heard of message 27 - so I'm grateful for the steer. I've got an AIS unit in the office - I have a UK Testing and demonstration licence for marine stuff, so have a couple of MMSIs that are not my normal licence one - It's interesting to use my office unit on a fixed external antenna to check the net AIS websites, and often I will see my East Anglian Radio callsign replaced by just random digits and things like that on ship finder - but not on the others. One thing that really makes me jump is when I get a collision alert. One of the slipways is on the direct bearing to my office, and getting a collision alert from a WW2 torpedo boat is always fun! If you use the internet locators - you can see my office if you search for Lowestoft on the east coast of the UK - the lump that sticks out into the North Sea. I'm a leisure vessel as there is no code for a brick building that doesn't float!

On the timestamp front, I suppose that with a GPS, the messages and time marry up, so to speak enough for SaR usage.

[Auspicious]

Originally Posted by paulears

On the timestamp front, I suppose that with a GPS, the messages and time marry up, so to speak enough for SaR usage.

Found EAST ANGLIAN RADIO 235041071

With no time stamp, time of flight to satellite, latency there, time of flight to ground, latency there, S-AIS isn't the first choice for SAR. Nothing better than an EPIRB. According to ITU time gets applied at the display unit for message 27. In general that's many many seconds and often minutes. Marinetraffic has had latency of hours in some unusual circumstances. That's fun.