aiscat 0.68.0a6

Fast Python bindings for AIS-catcher's NMEA-to-JSON AIS decoder

aiscat

A fast, complete Python library for decoding AIS NMEA into structured dicts. Built on the AIS-catcher decoder; messages come out compatible with the AIS-catcher JSON format.

Why

Python has two established AIS decoders: pyais (pure-Python, MIT) and libais (C extension, Apache 2.0). Both have gaps:

• pyais is ~20× slower than what's achievable in C.
• libais's stream layer does not reassemble multi-part AIVDM — it silently drops every type-5 message (and other multi-part types).

aiscat closes both gaps by exposing AIS-catcher's mature decoder as a Python extension: full multi-part handling, lookup-table text fields (status_text, shiptype_text, …), country-code resolution, message 6/8 ASMs — everything AIS-catcher decodes — at C++ speed, in a dict shape that matches what AIS-catcher itself emits.

For live decode of a single station (~50 msg/s) any of these libraries is fast enough. aiscat earns its keep when throughput matters: replay of historical recordings, multi-receiver fan-in, batch analysis, and research workloads on millions to billions of messages.

Tradeoffs. aiscat is a C++ extension, so pip install either picks up a pre-built wheel for your platform or falls back to a source build that needs CMake and a C++11 compiler. The API is deliberately small — feed bytes, get dicts — without the configuration knobs, custom output formats, or filter chains pyais offers. And the licence is GPLv3 (inherited from AIS-catcher), so linking aiscat into a closed-source application makes that application GPL too. If any of those matter more to you than throughput, pyais is the right choice; aiscat is the right choice when you want the AIS-catcher JSON format produced quickly and reliably from Python.

Quickstart

pip install aiscat

import aiscat

dec = aiscat.Decoder()
dec.feed(b"!AIVDM,1,1,,A,15MgK45P3@G?fl0E`JbR0OwT0@MS,0*4E\r\n")
print(dec.next())
# {'type': 1, 'mmsi': 366730000, 'channel': 'A', 'rxuxtime': 1777739134.027,
#  'lat': 37.803802, 'lon': -122.392532, 'speed': 20.8, 'course': 51.3,
#  'status': 5, 'status_text': 'Moored', ...}

Benchmark

2,000,000 AIS messages (mixed types 1–4), Apple M-series, single thread:

Tool	Time	msg/s	µs/msg	vs fastest
AIS-catcher CLI (-r txt -o 5)	1.04s	1,922,000	0.52	1.00×
aiscat (this library)	1.05s	1,906,000	0.52	1.01×
gpsdecode (gpsd, BSD-2) — CLI	3.14s	636,000	1.57	3.02×
libais 0.17 (C, Apache 2.0)	5.33s	375,000	2.67	5.12×
pyais 3.0.0 (pure Python, MIT)	22.36s	89,000	11.18	21.49×

aiscat sits within 1% of the native AIS-catcher CLI despite producing rich Python dict objects (the CLI just stringifies JSON to stdout). It's 3× faster than gpsdecode, 5× faster than libais, and 21× faster than pyais.

The full benchmark — including the test-fixture generator — is in benchmarks/. Types 1–4 (single-line position reports / base station reports) are used because libais's stream API does not reassemble multi-part AIVDM groups; on a mixed corpus including type 5 (multi-part static data), libais silently drops every type-5 message. AIS-catcher, gpsdecode, and pyais handle multi-part correctly.

For perspective: a busy AIS shore station produces ~50 msg/s. Throughput matters when you're replaying recordings or aggregating dozens of receivers, not for live decode of a single antenna.

API

class Decoder:
    def __init__(self) -> None: ...
    def feed(self, data: bytes | bytearray | str) -> int: ...
    def next(self) -> dict | None: ...
    def pending(self) -> int: ...

def iter_decode(chunks: Iterable[bytes | str]) -> Iterator[dict]: ...

• feed(data) parses NMEA AIVDM/AIVDO sentences out of the buffer. Multipart messages are reassembled internally; partial chunks are buffered until completed. Returns the number of decoded messages waiting.
• next() pops one decoded message as a dict, or None if the queue is empty. Pop in a loop after each feed().
• The queue is unbounded — drain it after each feed, or memory grows.

# Streaming pattern
import aiscat
dec = aiscat.Decoder()
with open("session.nmea", "rb") as f:
    while chunk := f.read(65536):
        dec.feed(chunk)
        while (msg := dec.next()) is not None:
            handle(msg)

Output format

Each decoded message is a dict containing the AIS protocol fields plus a small reception envelope. AIS-catcher's internal "meta" fields that don't apply to a Python decoder (SDR signal levels, decoder version, the original NMEA echo, etc.) are suppressed.

Lookup-table values are decoded for you — status comes through as both the raw integer and status_text ("Moored", "Under way using engine", …); same for shiptype_text, aid_type_text, etc.

For the full field reference — every key, its unit, and the lookup tables — see the AIS-catcher JSON decoding documentation.

Type hints

Decoded messages are plain dicts at runtime — zero overhead. For static type-checking and IDE autocomplete, aiscat.types provides one TypedDict per AIS message type (1–28) plus an AISMessage union:

from aiscat import Decoder, AISMessage

def handle(msg: AISMessage) -> None:
    if msg["type"] == 1:
        # mypy narrows to Type1; knows msg["lat"] is float
        print(msg["mmsi"], msg["lat"], msg["lon"])

Licence

GPLv3, inherited from AIS-catcher. Linking aiscat into a closed-source application makes that application GPLv3.

If you need a permissive licence (BSD/MIT/Apache), use one of:

• pyais — pure-Python, MIT, the de facto standard for Python AIS work
• libais — older C extension, Apache 2.0

aiscat exists for users who want AIS-catcher's decoding (multipart handling, lookup tables, channel A/B disambiguation, country tables, message-type 6/8 ASMs) and can live with the licence.

Building from source

Requires CMake ≥ 3.15, a C++11 compiler, and Python ≥ 3.9 with development headers.

git clone https://github.com/jvde-github/AIS-catcher
cd AIS-catcher/python
pip install -e .

The build pulls a curated subset of AIS-catcher's source (Marine/, JSON/, Library/, Utilities/) — about 8K lines of C++ — and compiles them together with the binding into _core.cpython-*.so. Build takes ~10 seconds.

Status

Pre-1.0. The decoder itself is mature — it's the AIS-catcher decoder, battle-tested in shore-station and SDR deployments. The Python surface is new and may evolve — feedback welcome.

Versioning tracks AIS-catcher's: 0.68.x uses AIS-catcher v0.68's decoder. Patch releases bump independently for binding-only changes.