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Ais message decoding

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AIS message encoding and decoding. 100% pure Python. Supports AIVDM/AIVDO messages. Supports single messages, files and TCP/UDP sockets.

You can find the full documentation on readthedocs.


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This module contains functions to decode and parse Automatic Identification System (AIS) serial messages. For detailed information about AIS refer to the AIS standard.


I open to any form of idea to further improve this library. If you have an idea or a feature request - just open an issue. :-)

Migrating from v1 to v2

Version 2.0.0 of pyais had some breaking changes that were needed to improve the lib. While I tried to keep those breaking changes to a minimum, there are a few places that got changed.

  • msg.decode() does not return a pyais.messages.AISMessage instance anymore
    • instead an instance of pyais.messages.MessageTypeX is returned, where X is the type of the message (1-27)
  • in v1 you called decoded.content to get the decoded message as a dictionary - this is now decoded.asdict()

Typical example in v1

message = NMEAMessage(b"!AIVDM,1,1,,B,15M67FC000G?ufbE`FepT@3n00Sa,0*5C")
decoded = message.decode()
data = decoded.content

Typical example in v2

message = NMEAMessage(b"!AIVDM,1,1,,B,15M67FC000G?ufbE`FepT@3n00Sa,0*5C")
decoded = message.decode()
data = decoded.asdict()


The project is available at Pypi:

$ pip install pyais


U Decode a single part AIS message using decode()::

from pyais import decode

decoded = decode(b"!AIVDM,1,1,,B,15NG6V0P01G?cFhE`R2IU?wn28R>,0*05")

The decode() functions accepts a list of arguments: One argument for every part of a multipart message::

from pyais import decode

parts = [

# Decode a multipart message using decode
decoded = decode(*parts)

Also the decode() function accepts either strings or bytes::

from pyais import decode

decoded_b = decode(b"!AIVDM,1,1,,B,15NG6V0P01G?cFhE`R2IU?wn28R>,0*05")
decoded_s = decode("!AIVDM,1,1,,B,15NG6V0P01G?cFhE`R2IU?wn28R>,0*05")
assert decoded_b == decoded_s

Decode the message into a dictionary::

from pyais import decode

decoded = decode(b"!AIVDM,1,1,,B,15NG6V0P01G?cFhE`R2IU?wn28R>,0*05")
as_dict = decoded.asdict()

Read a file::

from import FileReaderStream

filename = "sample.ais"

for msg in FileReaderStream(filename):
    decoded = msg.decode()

Decode a stream of messages (e.g. a list or generator)::

from pyais import IterMessages

fake_stream = [
for msg in IterMessages(fake_stream):


It is also possible to encode messages.

:exclamation: Every message needs at least a single keyword argument: mmsi. All other fields have most likely default values.

Encode data using a dictionary

You can pass a dict that has a set of key-value pairs:

  • use from pyais.encode import encode_dict to import encode_dict method
  • it takes a dictionary of data and some NMEA specific kwargs and returns the NMEA 0183 encoded AIS sentence.
  • only keys known to each message are considered
    • other keys are simply omitted
    • you can get list of available keys by looking at pyais/
    • you can also call MessageType1.fields() to get a list of fields programmatically for each message
  • every message needs at least two keyword arguments:
    • mmsi the MMSI number to encode
    • type or msg_type the type of the message to encode (1-27)

NOTE: This method takes care of splitting large payloads (larger than 60 characters) into multiple sentences. With a total of 80 maximum chars excluding end of line per sentence, and 20 chars head + tail in the nmea 0183 carrier protocol, 60 chars remain for the actual payload. Therefore, it returns a list of messages.

from pyais.encode import encode_dict

data = {
    'course': 219.3,
    'lat': 37.802,
    'lon': -122.341,
    'mmsi': '366053209',
    'type': 1,
# This will create a type 1 message for the MMSI 366053209 with lat, lon and course values specified above
encoded = encode_dict(data, radio_channel="B", talker_id="AIVDM")[0]

Create a message directly

It is also possible to create messages directly and pass them to encode_payload.

from pyais.encode import MessageType5, encode_msg

payload = MessageType5.create(mmsi="123", shipname="Titanic", callsign="TITANIC", destination="New York")
encoded = encode_msg(payload)

Commandline utility

If you install the library a commandline utility is installed to your PATH. This commandline interface offers access to common actions like decoding single messages, reading from files or connection to sockets.

$ ais-decode --help
usage: ais-decode [-h] [-f [IN_FILE]] [-o OUT_FILE] {socket,single} ...

AIS message decoding. 100% pure Python.Supports AIVDM/AIVDO messages. Supports single messages, files and TCP/UDP sockets.rst.

positional arguments:

optional arguments:
  -h, --help            show this help message and exit
  -f [IN_FILE], --file [IN_FILE]
  -o OUT_FILE, --out-file OUT_FILE

Decode messages passed as arguments

Because there are some special characters in AIS messages, you need to pass the arguments wrapped in single quotes (''). Otherwise, you may encounter weird issues with the bash shell.

$ ais-decode single '!AIVDM,1,1,,A,15NPOOPP00o?b=bE`UNv4?w428D;,0*24'
{'type': 1, 'repeat': 0, 'mmsi': '367533950', 'status': <NavigationStatus.UnderWayUsingEngine: 0>, 'turn': -128, 'speed': 0.0, 'accuracy': True, 'lon': -122.40823166666667, 'lat': 37.808418333333336, 'course': 360.0, 'heading': 511, 'second': 34, 'maneuver': <ManeuverIndicator.NotAvailable: 0>, 'raim': True, 'radio': 34059}

Decode messages from stdin

The program reads content from STDIN by default. So you can use it like grep:

$ cat tests/ais_test_messages | ais-decode
{'type': 1, 'repeat': 0, 'mmsi': '227006760', 'status': <NavigationStatus.UnderWayUsingEngine: 0>, 'turn': -128, 'speed': 0.0, 'accuracy': False, 'lon': 0.13138, 'lat': 49.47557666666667, 'course': 36.7, 'heading': 511, 'second': 14, 'maneuver': <ManeuverIndicator.NotAvailable: 0>, 'raim': False, 'radio': 22136}
{'type': 1, 'repeat': 0, 'mmsi': '205448890', 'status': <NavigationStatus.UnderWayUsingEngine: 0>, 'turn': -128, 'speed': 0.0, 'accuracy': True, 'lon': 4.419441666666667, 'lat': 51.237658333333336, 'course': 63.300000000000004, 'heading': 511, 'second': 15, 'maneuver': <ManeuverIndicator.NotAvailable: 0>, 'raim': True, 'radio': 2248}
{'type': 1, 'repeat': 0, 'mmsi': '000786434', 'status': <NavigationStatus.UnderWayUsingEngine: 0>, 'turn': -128, 'speed': 1.6, 'accuracy': True, 'lon': 5.320033333333333, 'lat': 51.967036666666665, 'course': 112.0, 'heading': 511, 'second': 15, 'maneuver': <ManeuverIndicator.NoSpecialManeuver: 1>, 'raim': False, 'radio': 153208}

It is possible to read from a file by using the -f option

$ ais-decode -f tests/ais_test_messages 
{'type': 1, 'repeat': 0, 'mmsi': '227006760', 'status': <NavigationStatus.UnderWayUsingEngine: 0>, 'turn': -128, 'speed': 0.0, 'accuracy': False, 'lon': 0.13138, 'lat': 49.47557666666667, 'course': 36.7, 'heading': 511, 'second': 14, 'maneuver': <ManeuverIndicator.NotAvailable: 0>, 'raim': False, 'radio': 22136}
{'type': 1, 'repeat': 0, 'mmsi': '205448890', 'status': <NavigationStatus.UnderWayUsingEngine: 0>, 'turn': -128, 'speed': 0.0, 'accuracy': True, 'lon': 4.419441666666667, 'lat': 51.237658333333336, 'course': 63.300000000000004, 'heading': 511, 'second': 15, 'maneuver': <ManeuverIndicator.NotAvailable: 0>, 'raim': True, 'radio': 2248}

Decode from socket

By default the program will open a UDP socket

$ ais-decode socket localhost 12345

but you can also connect to a TCP socket by setting the -t tcp parameter.

$ ais-decode socket localhost 12345 -t tcp

Write content to file

By default, the program writes it's output to STDOUT. But you can write to a file by passing the -o option. You need to add this option before invoking any of the subcommands, due to the way argparse parses it's arguments.

$ ais-decode -o /tmp/some_file.tmp single '!AIVDM,2,1,1,A,538CQ>02A;h?D9QC800pu8@T>0P4l9E8L0000017Ah:;;5r50Ahm5;C0,0*07' '!AIVDM,2,2,1,A,F@V@00000000000,2*35' 

# This is same as redirecting the output to a file

$ ais-decode single '!AIVDM,2,1,1,A,538CQ>02A;h?D9QC800pu8@T>0P4l9E8L0000017Ah:;;5r50Ahm5;C0,0*07' '!AIVDM,2,2,1,A,F@V@00000000000,2*35' > /tmp/file

I also wrote a blog post about AIS decoding and this lib.

Performance Considerations

You may refer to the Code Review Stack Exchange question . After a some research I decided to use the bitarray module as foundation. This module uses a C extension under the hood and has a nice user interface in Python. Performance is also great. Decoding this sample with roughly 85k messages takes less than 6 seconds on my machine. For comparison, the C++ based libais module parses the same file in ~ 2 seconds.


This module is a private project of mine and does not claim to be complete. I try to improve and extend it, but there may be bugs. If you find such a bug feel free to submit an issue or even better create a pull-request. :-)


Currently, this module is able to decode most message types. There are only a few exceptions. These are messages that only occur in very rare cases and that you will probably never observe. The module was able to completely decode a 4 hour stream with real-time data from San Francisco Bay Area without any errors or problems. If you find a bug or missing feature, please create an issue.

Known Issues

During installation, you may encounter problems due to missing header files. The error looks like this:


    bitarray/_bitarray.c:13:10: fatal error: Python.h: No such file or directory
       13 | #include "Python.h"
          |          ^~~~~~~~~~
    compilation terminated.
    error: command 'x86_64-linux-gnu-gcc' failed with exit status 1


In order to solve this issue, you need to install header files and static libraries for python dev:

$ sudo apt install python3-dev

For developers

After you cloned the repo head into the pyais base directory.

Then install all dependencies:

$ pip install .[test]

Make sure that all tests pass and that there aren't any issues:

$ make test

Now you are ready to start developing on the project! Don't forget to add tests for every new change or feature!

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