AIS语句序列号字段4用途及与字段3的差异咨询

Hey there! Let's break down the AIS field 4 (multi-sentence message sequence number) and its differences from field 3 clearly.

When an AIS message is too large to fit into a single standard sentence, it gets split into multiple consecutive sentences. Field 4 exists only for these split multi-part messages—it acts as a "grouping and ordering tag" with the format n/m, where:

• n is the sequence number of the current sentence (starting from 1)
• m is the total number of sentences in this split message group

Its core job is to help the receiving device:

• Identify which sentences belong to the same original message
• Reassemble them in the correct order to get the complete, full-length message

For example, if you see 3/5 in field 4, that means this is the 3rd sentence out of 5 total that make up one full AIS message.

Let’s contrast them point by point:

• Purpose & Role
  • Field 3 is the Message ID—it’s a predefined code that tells the receiver what type of message this is (e.g., code 1-3 = position report, code 5 = static ship data, code 12 = safety-related broadcast). Every single AIS sentence has this field, regardless of whether it’s part of a split group.
  • Field 4 is the multi-part sequence marker—it only exists for split messages, and its sole purpose is to manage the reassembly of long messages. It has nothing to do with the message’s actual type.
• Presence
  • Field 3 is mandatory for all AIS sentences—you’ll never see an AIS sentence without it.
  • Field 4 is optional: it only appears when the original message is too big for one sentence. Single, complete AIS messages won’t have this field.
• Value Meaning
  • Field 3 uses fixed, standardized numeric codes (defined by the AIS protocol) that map to specific message types.
  • Field 4 uses a n/m format where the numbers are relative to the split message group—they don’t correspond to any predefined protocol codes, just the order and total count of the split set.

内容的提问来源于stack exchange，提问作者panoet