AIS Decoder

The automatic identification system (AIS) is an automatic Tracking system used on ships and by vessel traffic services (VTS). When satellites are used to detect AIS signatures, the term Satellite-AIS (S-AIS) is used. AIS information supplements marine radar, which continues to be the primary method of collision avoidance for water transport.

References:

• Gov:
  • https://www.navcen.uscg.gov/?pageName=AISMessagesA
• Gpsd:
  • http://catb.org/gpsd/AIVDM.html [best doc]
• OpenCPN:
  • https://github.com/OpenCPN/OpenCPN [file: AIS_Bitstring.cpp]
    • http://fossies.org/linux/misc/gpsd-3.11.tar.gz/gpsd-3.11/test/sample.aivdm
• online AIS decoder:
  • http://www.maritec.co.za/tools/aisvdmvdodecoding/
  • http://www.maritec.co.za/aisvdmvdodecoding1.php
• Wikipedia:
  • https://en.wikipedia.org/wiki/Automatic_identification_system
• OpenCPN
  • Github: https://github.com/OpenCPN/OpenCPN/blob/45504f35c005ed1ffcd117c67797279da42d53ae/src/AIS_Decoder.cpp
• http://arundale.com/docs/ais/ais_decoder.html

AIS MESSAGES

1. Position[0] format: !AIVDM, identifies this as an AIVDM packet (AIS format).
2. Position[1] message_count: Messages counter (number of messages), sometimes the ais messages will be split over several messages.
3. Position[2] message_id:
4. Position[3] sequence_id: Sequential message, the current message number
5. Position[4] channel: vhf channel A/B
6. Position[5] payload: the ais data itself
7. Position[6] size: number of bits required to fill the data

<format>,<message count>,<message id>,<sequence id>,<channel A/B>,<data>,<fill bits>

Important notes:

• Ais payload is represented in a 6bits encoded string

API

• constructor(AIS_Messages, safeMode)
  • AIS_Messages: Array of ais messages.
  • safeMode: set to false by default. when true the module will never throw an error (silent mode).
• getResults() - return a collection of the parse messages
• private decode(input: Array<any>, session: any): void - decode the raw ais messages
• private validateRawMessage(input: string): boolean - validate if the raw messages

Running example:

• npm run example

Example:

// new Decoder(Array<AIS_Messages>, safeMode<Boolean>)
import { Decoder } from '../lib/index';
const aisMessages:Array<string> = [
  '!AIVDM,1,1,,A,400TcdiuiT7VDR>3nIfr6>i00000,0*78',
  '!AIVDM,2,1,0,A,58wt8Ui`g??r21`7S=:22058<v05Htp000000015>8OA;0sk,0*7B',
  '!AIVDM,2,2,0,A,eQ8823mDm3kP00000000000,2*5D',
  '!AIVDM,2,1,0,A,58wt8Ui`g??r21`7S=:22058<v05Htp000000015>8OA;0sk,0*7B ',
  '!AIVDM,2,2,0,A,eQ8823mDm3kP00000000000,2*5D',
];
const safeMode = false;
const aisDecoder_ex2 = new Decoder(aisMessages, safeMode);

// results will hold a collection of the parsed ais messages
const results = aisDecoder_ex2.getResults();

TODO:

• Tests (tests are always good)
• Add unit and e2e tests with fixtures
• Parse Latitude and Longitude for Type 27

Bugs & PR's

• If you find any bug please feel free to open an issue. PR are always welcome.