AIS is not displayed

[CarCode]

Are there any receivers available to receive such messages at a recreational boat? I don't need a transponder, a receiver is sufficient.

[transmitterdan]

What exactly does it mean “satelllite AIS”? Does it mean AIS that only a satellite should listen too? Does it mean only a satellite will transmit that message type?

[Maelruan]

As indicated in earlier posts, I used an SDR dongle, an early version in fact, tuned to 156.775 & 156.825 mhz, to receive all type 27 AIS Messages mentioned. The antenna & dongle are the same as used for all other AIS messages received on the two 162~ mhz channels. Normally I use AISRec to control the dongle and that sends the raw signals to OpenCPN by UDP. When it became clear that OpenCPN was not decoding type 27, I tested those signals with Arundale's Aisdecoder which decoded them successfully. They were received by AISRec which passed them to Aisdecoder. The image I posted shows these messages then being piped from Aisdecoder to OpenCPN. At no stage was the internet involved.

[transmitterdan]

My question is who transmits these message? Boats or satellites?

[Maelruan]

Type 27 messages are transmitted by vessels. A Google search produces quite an amount of info about them, e.g.

"According to the relevant ITU recommendation, ITU-R M.1371-5, AIS Message 27 is:

...primarily intended for long-range detection of AIS Class A and Class B “SO” equipped vessels (typically by satellite). This message has a similar content to Messages 1, 2 and 3, but the total number of bits has been compressed to allow for increased propagation delays associated with long-range detection."

There is a satellite constellation which monitors them, and it would be great to find the downlink.

[bdbcat]

All...
The simple answer is:
"OpenCPN v5 does not decode Type 27 messages"


Source code reference: AIS_Decoder.cpp:1685


Accepting Feature Requests on Flyspray.....


Thanks for the heads up
Dave

[bcn]

From the literature retrieved, vessels stations are not expected to receive and evaluate Type 27 messages.


A comprehensive article about S-AIS (satellite AIS) and underlying motivations:
https://www.google.com/url?sa=t&rct=...TytI_It5z-Wmux


One more article about S-AIS and the results from using the additional AIS-3 and AIS-4 frequencies combined with Type 27 messages
https://link.springer.com/article/10...67-019-00245-z

with an observation:

However, the use of the AIS3 and AIS4 channels is still not widespread, with only 17,531 out of at least 68,161 vessels detected using the AIS3 and AIS4 channels versus the AIS1 and AIS2 channels, as shown in Table 2. In 2014, the number of AIS3 and AIS4 reporting vessels was estimated to less than 2 300 using an ASR100 type receiver on the International Space Station [22, 28]. With a linear adoption rate, the full fleet adoption of the AIS3 and AIS4 channels is not expected for another 14 years.

Actual Class A transceivers will transmit Type 27 and in the case of Class B only SO-TDMA units will do if enabled. From the documentation I've found transmission of Type 27 messages from Class B SO-TDMA units is "off" by default.

[transmitterdan]

It seems unlikely that any vessel would transmit only type 27. So any vessel transmitting 27 would also be expected to transmit other longer messages with same info as type 27 making decode of type 27 redundant. Is this thinking wrong?

[bcn]

This is correct.
A vessel will always transmit on AIS-1 and AIS-2 with the know and implemented messages.


Type-27 messages will be transmitted on top of those on AIS-3 and AIS-4.
Some papers state that only when not receiving base station Type-4 messages, means when offshore. I have not seen evidence in the papers if this has been enforced finally.
It looks that not all implementations are correct and don't send a shortened message as defined for 27. As precaution Type 27 messages shall be truncated always after 96 bits when decoding. The rest of the payload is garbage then.

[Maelruan]

Originally Posted by bdbcat

All...
The simple answer is:
"OpenCPN v5 does not decode Type 27 messages"
Source code reference: AIS_Decoder.cpp:1685
Accepting Feature Requests on Flyspray.....
Thanks for the heads up
Dave

Brilliant!!!
Thanks bdbcat!!