pyais 2.3.0

Ais message decoding

pyais

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.

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

Acknowledgements

This project is a grateful recipient of the free Jetbrains Open Source sponsorship. Thank you. 🙇

General

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

Features/Improvements

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. :-)

Installation

The project is available at Pypi:

$ pip install pyais

Usage

There are many examples in the examples directory.

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")
print(decoded)

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

from pyais import decode

parts = [
    b"!AIVDM,2,1,4,A,55O0W7`00001L@gCWGA2uItLth@DqtL5@F22220j1h742t0Ht0000000,0*08",
    b"!AIVDM,2,2,4,A,000000000000000,2*20",
]

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

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()
print(as_dict)

Read a file::

from pyais.stream import FileReaderStream

filename = "sample.ais"

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

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

from pyais import IterMessages

fake_stream = [
    b"!AIVDM,1,1,,A,13HOI:0P0000VOHLCnHQKwvL05Ip,0*23",
    b"!AIVDM,1,1,,A,133sVfPP00PD>hRMDH@jNOvN20S8,0*7F",
    b"!AIVDM,1,1,,B,100h00PP0@PHFV`Mg5gTH?vNPUIp,0*3B",
    b"!AIVDM,1,1,,B,13eaJF0P00Qd388Eew6aagvH85Ip,0*45",
    b"!AIVDM,1,1,,A,14eGrSPP00ncMJTO5C6aBwvP2D0?,0*7A",
    b"!AIVDM,1,1,,A,15MrVH0000KH<:V:NtBLoqFP2H9:,0*2F",
]
for msg in IterMessages(fake_stream):
    print(msg.decode())

Encode

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/encode.py
  • 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)
print(encoded)

Under the hood

graph LR
    raw -->|"!AIVDM,1,1,,B,6B?n;be,2*4A"| nmea
    nmea[NMEASentence] -->|parse NMEA sentence layer| ais[AISSentence]
    ais -->|decode| final[AISMessage]

Decoding each AIS message is a three step process.

At first, the NMEA 0183 physical protocol layer is parsed. The NMEA layer is the outer protocol layer that is used by many different sentences/protocols for data transmission. Just like Ethernet can be used as a data link protocol to transfer data between nodes, the NMEA protocol can be used to transmit data between maritime equipment.

After the raw message was parsed into a NMEASentence, the inner protocol layer is parsed. While there are tons of different inner protocols that build upon NMEA, pyais currently only supports AIS sentences. Every AISSentence holds basic information about the AIS message like:

• the AIS message ID
• the number of fill bits required for ASCII6 encoding
• the fragment count and fragment number
• the actual AIS payload
• the sequence number

Finally, the AIS payload is decoded based on the AIS ID. There are 27 different types of top level messages that are identified by their AIS ID.

Gatehouse wrappers

Some AIS messages have so-called Gatehouse wrappers. These encapsulating messages contain extra information, such as time and checksums. Some readers also process these. See some more documentation here.

As an example, see the following, which is followed by a regular !AIVDM message

$PGHP,1,2020,12,31,23,59,58,239,0,0,0,1,2C*5B

Such messages are parsed by pyais only when using any of the classes from pyais.stream. e.g. FileReaderStream or TCPStream.

Such additional information can then be accessed by the .wrapper_msg of every NMEASentence. This attribute is None by default.

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.

Disclaimer

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. :-)

Coverage

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!