ais-tools

Tools for encoding and decoding AIS messages

Uses https://github.com/schwehr/libais as the base decoder

Multi-sentence messages

The strategy for handing multi-sentence messages, such as AIS type 5, is to group the sentence parts into a single unit as early as possible in the processing chain. Ideally this happens at the AIS receiver or at the point when these messages are streaming in real-time and the tagblock with timestamp is added to the !AIVDM payload.

This can be done in a text stream by simply concatenating the sentence parts into a single line of text. In a JSON encoded message, this can also be done by providing a list in the nmea attribute.

For example:

the following two line message

!AIVDM,2,1,1,B,56:`@2h00001`dQP001`PDpMPTs7SH000000001@0000000000<000000000,0*3E
!AIVDM,2,2,1,B,00000000000,2*26

becomes

!AIVDM,2,1,1,B,56:`@2h00001`dQP001`PDpMPTs7SH000000001@0000000000<000000000,0*3E!AIVDM,2,2,1,B,00000000000,2*26

messages with tagblock are also concatenated including the tagblock so

\tagblock\!AIVDM_part_one
\tagblock\!AIVDM_part_two

becomes

\tagblock\!AIVDM_part_one\tagblock\!AIVDM_part_two

Install

$ pip install git+https://github.com/GlobalFishingWatch/ais-tools

Command line usage

$ ais-tools --help

Decode

Decode nmea in a file as json to stdout

$ ais-tools decode ./sample/sample.nmea

Add tagblock

Used to add a tagblock to AIVDM messages. this is intended to be used with a real time stream of messages as they are received, for instance from an AIS RF signal decoder of\r from a udp stream. The default action is to apply the current timestamp

$ echo '!AIVDM,1,1,,A,15NTES0P00J>tC4@@FOhMgvD0D0M,0*49' | ais-tools add-tagblock -s my-station

outputs something like

\\c:1577762601537,s:my-station,T:2019-12-30 22.23.21*5D\\!AIVDM,1,1,,A,15NTES0P00J>tC4@@FOhMgvD0D0M,0*49

Join Multipart

Match up multipart nmea messages

Takes a stream of nmea text lines and tries to find the matching parts of multi part messages which may not be adjacent in the stream and may come out of order.

Matched message parts will be concatenated together into a single line using join_multipart() All other messages will come out with no changes

$ ais-tools join-multipart ./sample/multi-part.nmea > joined.nmea

Chaining operations

To perform multiple operations on a stream of messages, use the pipe operator

$ ais-tools join-multipart ./sample/multi-part.nmea | ais-tools decode > decoded.nmea

Python usage

Decode NMEA

import json
from ais_tools.aivdm import AIVDM
from ais_tools.message import Message

nmea = [
    '\\c:1599239526500,s:sdr-experiments,T:2020-09-04 18.12.06*5D\\!AIVDM,1,1,,A,B>cSnNP00FVur7UaC7WQ3wS1jCJJ,0*73',
    '\\c:1599239587658,s:sdr-experiments,T:2020-09-04 18.13.07*58\\!AIVDM,1,1,,B,H>cSnNP@4eEL544000000000000,0*3E',
    '\\c:1599239644720,s:sdr-experiments,T:2020-09-04 18.14.04*5E\\!AIVDM,1,1,,B,H>cSnNTU7B=40058qpmjhh000004,0*31',
]

decoder = AIVDM()

for msg in Message.stream(nmea):
    msg = decoder.decode(msg)
    print(json.dumps(msg))

Developing

git clone https://github.com/SkyTruth/ais-tools
cd ais-tools
virtualenv venv
source venv/bin/activate
pip install -e .\[dev\]
flake8 . --exclude ./venv/ --max-line-length=127
pytest  --cov=./

GlobalFishingWatch / ais-tools