ExAIS

ExAIS is an Elixir library for decoding AIS (Automatic Identification System) data transmitted by vessels via NMEA 0183 v4.0 format sentences.

Features

• Decode standard NMEA 0183 v4.0 AIS sentences (AIVDM/AIVDO)
• Support for AIS tag blocks commonly used by satellite AIS providers (e.g., Spire)
• Handle multi-part message groups and fragmented messages
• GenServer-based decoder for concurrent message processing
• Telemetry integration for monitoring decoder performance
• Built-in checksum validation
• Support for filtering specific AIS message types

Installation

Add exais to your list of dependencies in mix.exs:

def deps do
  [
    {:exais, "~> 0.2.6"}
  ]
end

Usage

Basic Decoding

Decode a single AIS message:

# Simple NMEA sentence
ExAIS.Decoder.decode_nmea("!AIVDM,1,1,,B,15M67FC000G?ufbE`FepT@3n00Sa,0*5C", [1, 2, 3])

# With satellite AIS tag block
message = "\\c:1503079500*55\\!AIVDM,1,1,,B,C6:b0Kh09b3t1K4ChsS2FK008NL>`2CT@2N000000000S4h8S400,0*50"
ExAIS.Decoder.decode_message(message, %{groups: %{}, latest: DateTime.utc_now()}, ExAIS.Data.Ais.all_msg_types())

Using the Decoder GenServer

The library provides a GenServer-based decoder for processing streams of AIS messages:

# Start a decoder
decoder_opts = %{
  id: :my_decoder,
  module: ExAIS.Decoder,
  name: :my_decoder,
  batch_size: 10_000,
  processor: :my_processor,
  supervisor: MyApp.TaskSupervisor,
  msg_types: [1, 2, 3]  # Optional: filter specific message types
}

{:ok, pid} = ExAIS.Decoder.start_link(decoder_opts)

# Send messages for decoding
GenServer.cast(:my_decoder, {:decode, list_of_messages})

Tag Block Support

ExAIS handles tag blocks commonly used in satellite AIS feeds:

# Tag block format: \tag1:value1,tag2:value2*checksum\!AIVDM,...
# Supported tags:
# - c: Unix timestamp (seconds or microseconds)
# - s: Source (e.g., "terrestrial", "satellite")
# - p: Provider identifier
# - g: Group message identifier (format: "n-total-id")
# - q: Quality indicator
# - t: Text string

Message Groups

ExAIS automatically handles grouped messages that span multiple sentences:

# Multi-part messages are reassembled automatically
# Group tag format: g:1-2-12345 (message 1 of 2, group ID 12345)

Supported AIS Message Types

The library supports decoding of various AIS message types. By default, all message types are processed, but you can filter to specific types when initializing a decoder.

Development

# Install dependencies
mix deps.get

# Run tests
mix test

# Run tests with coverage
mix coveralls

# Format code
mix format

# Run static analysis
mix credo

# Generate documentation
mix docs

License

MIT License - see LICENSE for details.

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.