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Ais message decoding

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AIS message 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. :-)


The project is available at Pypi:

$ pip install pyais


Using this module is easy. If you want to parse a file, that contains AIS messages, just copy the following code and replace filename with your desired filename.

from pyais import FileReaderStream

filename = "sample.ais"

for msg in FileReaderStream(filename):
    decoded_message = msg.decode()
    ais_content = decoded_message.content

It is possible to directly convert messages into JSON.

from pyais import TCPStream

for msg in TCPStream(''):
    json_data = msg.decode().to_json()

You can also parse a single message encoded as bytes or from a string:

from pyais import NMEAMessage, decode_msg

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

# or newer 

msg = decode_msg("!AIVDM,1,1,,A,403Ovl@000Htt<tSF0l4Q@100`Pq,0*28")
msg = decode_msg(b"!AIVDM,1,1,,A,403Ovl@000Htt<tSF0l4Q@100`Pq,0*28")

See the example folder for more examples.

Another common use case is the reception of messages via UDP. This lib comes with an UDPStream class that enables just that. This stream class also handles out-of-order delivery of messages, which can occur when using UDP.

from import UDPStream

host = ""
port = 55555

for msg in UDPStream(host, port):
    # do something with it

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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