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Here explained that the field 4 is a sequential message ID for multi-sentence messages. What does it means? What is the difference between field 4 and field 3?

panoet
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2 Answers2

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From the website:

Field 2 (1 in this example) is the count of fragments in the currently accumulating message. The payload size of each sentence is limited by NMEA 0183’s 82-character maximum, so it is sometimes required to split a payload over several fragment sentences.

Field 3 (1 in this example) is the fragment number of this sentence. It will be one-based. A sentence with a fragment count of 1 and a fragment number of 1 is complete in itself.

Field 4 (empty in this example) is a sequential message ID for multi-sentence messages.

Consider a message sent using 2 fragments:

  • Field 2 will be 2, the number of fragments.
  • Field 3 will be 1 for the first fragment, and 2 for the second
  • Field 4 can be any digit between 0 and 9, but will be common for both messages. This number is incremented for each new multi-fragment message sent by a vessel. It allows the decoding program to match together fragments that belong to the same message.

Here's one other example:

!AIVDM,2,1,8,A,56;OaD02B8EL990b221`P4v1T4pN0HDpN2222216HHN>B6U30A2hCDhD`888,0*4D
!AIVDM,2,2,8,A,88888888880,2*2C

The message has 2 parts (from field #2), we can see fragments 1 and 2 (from field #3), and the message_id for both fragments of this message is 8 (from field #4).

From the same vessels, I had previously received message ID 7, and right before this message ID 6, etc.

Nicolas
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  • Clear answer. Do you ever found message id (field 4) value of more than 10? – panoet Oct 30 '19 at 14:30
  • From a quick lookup in a large amount of AIS antenna logs, I've seen all numbers between 0 and 9 includes, but never more. – Nicolas Oct 31 '19 at 15:12
  • I have observed that the sequence number does not appear to be incremented by the sending transponder (vessel) alone but is incremented for any vessel i.e. vessel A: seq 0, vessel B: seg 1, vessel C: seq 2, vessel B: seq 3, vessel C: seq 4 etc. – Guy Oct 06 '20 at 17:41
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maybe it's to late for answer this now, but it could help others... This field 4, the sequential message id, runs from 0 to 9 and then start over 0. It's incremented each time a new multi-sentence message is generated, this is why the vessel A is seq 0, vessel B seq 1, etc because the receiver creates the NMEA messages and each multi-sentence message outputted increases this number no matter the transmitter (the Vessel)

Gustavo Gonzalez
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  • As it’s currently written, your answer is unclear. Please [edit] to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers [in the help center](/help/how-to-answer). – Community Apr 30 '23 at 12:15