aiscat 0.68.2

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; messages come out compatible with the AIS-catcher JSON format.

Why

aiscat is built for fast, complete decoding of AIS NMEA from Python. Output dicts match the AIS-catcher JSON format field-for-field, so anything you've built around AIS-catcher's documented JSON output works the same when the source becomes a Python iterator.

Coverage is meant to be exhaustive: all standard message types (1–28), multi-part AIVDM reassembly, country-code resolution from MMSI prefix, lookup-table text fields (status_text, shiptype_text, …) populated from the same tables AIS-catcher uses, and decoding of binary application messages — type 6 and 8 ASMs across IMO, IALA, USA, and inland-waterway DAC/FID payloads (AtoN monitoring, meteo/hydro, route info, persons-on-board, and others).

For live decode of a single station (~50 msg/s) any decent AIS library 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 configuration knobs or custom output formats. And the licence is GPLv3 (inherited from AIS-catcher), so linking aiscat into a closed-source application makes that application GPL too; see the Licence section below for permissive alternatives.

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.

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, *, annotated: bool = False) -> 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], *, annotated: bool = False) -> 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.
• annotated=True wraps every scalar as {"value": x, "unit": ..., "description": ..., "text": ...} (the latter three included only when defined for that key) — useful for self-describing displays and field reference. See Annotated mode below.

# 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.

Annotated mode

Decoder(annotated=True) returns each scalar wrapped with its unit, description, and (where applicable) the human-readable text from the lookup table. Mirrors AIS-catcher's MSGFORMAT JSON_ANNOTATED (-o 6) output:

dec = aiscat.Decoder(annotated=True)
dec.feed(b"!AIVDM,1,1,,B,13`e;R001`PNcD2MH48KQq>P0@;5,0*63\n")
m = dec.next()
m["type"]    # {'value': 1, 'description': 'Message Type', 'text': 'Position report'}
m["status"]  # {'value': 0, 'description': 'Navigation Status', 'text': 'Under way using engine'}
m["speed"]   # {'value': 10.4, 'unit': 'knots', 'description': 'Speed over Ground (SOG)'}

The 28 message-type names, status enums, ship types, AtoN types, and other lookups all come from the same tables AIS-catcher uses, so what you see matches the C++ output exactly.

See examples/pretty_print.py for a complete consumer that turns annotated output into a readable table:

$ python examples/pretty_print.py '!AIVDM,1,1,,B,13`e;R001`PNcD2MH48KQq>P0@;5,0*63'
Message 1 — Position report
!AIVDM,1,1,,B,13`e;R001`PNcD2MH48KQq>P0@;5,0*63

Field          Value                                     Unit                Description
-------------  ----------------------------------------  ------------------  -------------------------------------------------------------------------
rxuxtime       1777804294.817 (2026-05-03 10:31:34 UTC)                      Host receive time (Unix epoch s).
channel        B                                                             VHF channel (A or B).
type           1 (Position report)                                           Message Type
repeat         0                                                             Repeat indicator (0..3; 3=do not repeat).
mmsi           244009864                                                     MMSI
status         0 (Under way using engine)                                    Navigation Status
status_text    Under way using engine                                        Navigation status text.
turn_unscaled  0                                                             Raw ROT field (-128..127; 128=N/A).
turn           0                                         degrees per minute  Rate of Turn (ROT)
speed          10.400001                                 knots               Speed over Ground (SOG)
accuracy       true                                                          Position accuracy (1=DGPS <10m; 0=GNSS >10m).
lon            6.701442                                  degrees             Longitude
lat            51.338295                                 degrees             Latitude
course         295.100006                                degrees             Course over Ground (COG)
heading        295                                       degrees             True Heading (HDG)
second         16                                                            UTC second (0..59; 60=N/A; 61=manual; 62=dead reckoning; 63=inoperative).
maneuver       0 (Not available (default))                                   Maneuver indicator.
power          false                                                         Transmit power flag (type 22; 0=high power, 1=low power).
raim           false                                                         RAIM flag (EPFD integrity monitoring in use).
radio          66245                                                         Radio status (19-bit SOTDMA/ITDMA state).
sync_state     0 (UTC direct)                                                TDMA sync state.
slot_timeout   4                                                             Frames until new slot (0=next).
slot_number    709                                                           TDMA slot number used.

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.