AIS NMEA Utility

AIS NMEA Payload Decoder

Decode AIS NMEA !AIVDM and !AIVDO sentences or raw AIS 6-bit armored payloads. Inspect message type, MMSI, repeat indicator, vessel position, speed over ground, course over ground, heading, timestamp and payload bits.

✓ !AIVDM ✓ !AIVDO ✓ AIS 6-bit ✓ MMSI ✓ Position ✓ SOG / COG

AIS

AIS Payload Decode

● Ready

AIS NMEA Sentence or Payload Paste a complete !AIVDM / !AIVDO sentence or enter only the AIS armored payload. For a raw payload, specify the fill-bit count below.

Raw Payload Fill Bits Used only when the input is a raw AIS payload. NMEA sentence fill bits are read automatically.

NMEA Checksum Validation

AIS payload encoding: each printable payload character represents six bits. The decoder first removes the AIS armoring, then reads fields according to the AIS message type.

AIS Decode Result Decoded

Decoded AIS Message —

Message Type —

Message Name —

MMSI —

Repeat Indicator —

Latitude —

Longitude —

Speed Over Ground —

Course Over Ground —

True Heading —

Timestamp —

Payload Bits —

Status —

NMEA Sentence Information —

AIS Field Breakdown —

6-bit Payload Decode —

Validation & Notes —

What Is an AIS NMEA Payload?

Automatic Identification System equipment commonly transports AIS messages inside NMEA sentences. Received vessel traffic is commonly represented by !AIVDM, while !AIVDO is commonly associated with information transmitted by the receiving vessel itself.

!AIVDM,1,1,,A,15Muq?P0000G?tRMD5MTDwvN0<0l,0*5C

Fields include:

Sentence type
Fragment count
Fragment number
Sequence ID
Radio channel
AIS payload
Fill bits
Checksum

AIS 6-bit Payload Encoding

The payload is an ASCII-armored representation of six-bit AIS data. Each payload character is converted into a value from 0 through 63 and then expanded into six binary bits.

Payload Character
      ↓
ASCII value
      ↓
AIS armoring conversion
      ↓
6-bit value
      ↓
Binary field stream

AIS Message Header

Most AIS messages begin with a common header containing the message type, repeat indicator and MMSI.

Bits	Length	Field
0–5	6	Message Type
6–7	2	Repeat Indicator
8–37	30	MMSI

AIS Message Types 1, 2 and 3

AIS message types 1, 2 and 3 are Class A position reports. They can contain navigation status, rate of turn, speed over ground, position accuracy, longitude, latitude, course over ground, true heading and UTC second.

Field	Bits
Message Type	0–5
MMSI	8–37
Navigation Status	38–41
Speed Over Ground	50–59
Longitude	61–88
Latitude	89–115
Course Over Ground	116–127
True Heading	128–136
UTC Second	137–142

AIS Message Type 18

Message type 18 is a standard Class B position report. It contains fields such as MMSI, speed, position accuracy, longitude, latitude, course, heading and timestamp.

Type 18 Position Fields

SOG:
Bits 46–55

Longitude:
Bits 57–84

Latitude:
Bits 85–111

COG:
Bits 112–123

Heading:
Bits 124–132

Timestamp:
Bits 133–138

How AIS Coordinates Are Decoded

AIS position reports commonly encode longitude and latitude as signed integers with a scale of 1/600000 degree.

Longitude =
Signed Longitude Field / 600000

Latitude =
Signed Latitude Field / 600000

AIS Speed Over Ground

For common position-report messages, speed over ground is represented in tenths of a knot.

Raw SOG:
123

Speed:
123 / 10

= 12.3 knots

AIS Course Over Ground

Course over ground is normally represented in tenths of a degree.

Raw COG:
2745

Course:
2745 / 10

= 274.5°

AIS MMSI

The Maritime Mobile Service Identity is a 30-bit field in the common AIS message header. It identifies the transmitting maritime station or vessel according to MMSI numbering conventions.

AIS NMEA Fragment Fields

NMEA Field	Meaning
1	Total number of fragments
2	Current fragment number
3	Sequential message ID
4	AIS radio channel
5	Encoded AIS payload
6	Fill-bit count

What Are AIS Fill Bits?

AIS armoring stores data in six-bit groups. When the underlying message does not end exactly on a six-bit boundary, unused bits are added at the end. The NMEA fill-bit field tells the decoder how many trailing bits must be removed.

AIS NMEA Checksum

The NMEA checksum is calculated by XORing the characters between the initial exclamation mark and the asterisk. The result is normally shown as a two-digit hexadecimal value after the asterisk.

!AIVDM,...*5C
 ↑          ↑
 start      checksum

Checksum =
XOR of characters between ! and *

AIS NMEA Payload Decoder FAQs

What is !AIVDM?

!AIVDM is an NMEA sentence format commonly used to carry AIS messages received from other stations.

What is !AIVDO?

!AIVDO is commonly used for AIS information associated with the transmitting or receiving vessel’s own AIS output.

Can I enter only the AIS payload?

Yes. Enter the armored payload directly and specify its fill-bit count.

What is MMSI in AIS?

MMSI stands for Maritime Mobile Service Identity and is carried in the common AIS message header.

How is AIS latitude decoded?

For common position-report messages, the signed latitude integer is divided by 600000 to obtain degrees.

How is AIS longitude decoded?

The signed longitude field is normally divided by 600000 to obtain longitude in degrees.

Does this decoder support multi-fragment messages?

This page decodes a single supplied payload. Multi-fragment NMEA messages must first be combined in fragment order before their payload can be interpreted as one AIS message.

Does the decoder validate NMEA checksums?

Yes. When a complete NMEA sentence includes a checksum, the tool can calculate and compare it with the transmitted checksum.