SwiftAIS lets you turn your computer into a mobile AIS receiver using cheap, simple hardware! Utilizing any SDR supported by SoapySDR, this software will demodulate AIS signals from vessels within range and output them as NMEA 0183 sentences. These sentences can be plugged into AIS decoders to reveal what information a vessel is broadcasting.
Here's an example NMEA sentence captured in Scituate, MA:
!AIVDM,1,1,,A,E>k`HC0VTah9QTb:Pb2h0ab0P00=N97j<4dDP00000<020,4*6C
And here's the output when decoded on https://ccgibbons.com/ais:
(This site is a work in progress! Some rarer message types can cause errors when displaying output. If you find one, let me know! connor@ccgibbons.com)
- A machine running macOS or Linux
- Xcode Command Line Tools
- SoapySDR installed. If using RTL-TCP, you will also need SoapyRTLTCP.
- On macOS, this can be installed with 'brew install SoapySDR'
- On Linux, you will need libsoapysdr-dev and soapysdr-tools. Run: 'sudo apt-get install -y libsoapysdr-dev soapysdr-tools pkg-config'
- Any SDR supported by SoapySDR (RTL-SDR included!) & an antenna (antennas designed for the VHF range are ideal, but not strictly required)
- Be located near marine activity (Reception range limited by VHF propagation. Inland users are likely out of range.)
Run:
curl -L -o SwiftAIS.zip \
https://github.com/ConnorGibbons/SwiftAIS/releases/download/1.0.0/SwiftAIS.zip
unzip SwiftAIS.zip
cd SwiftAIS
Then, if you have an RTL-SDR connected via USB:
./SwiftAIS -n
Alternatively, if you have a device running rtl_tcp:
./SwiftAIS -n -di <IP>:<Port>
To run an offline decoding on the included sample file, run:
./SwiftAIS -ot Samples/sample_161992000Hz_96000Hz.wav 161992000 96000 -n
You should see:
!AIVDM,1,1,,A,B52icIh00Ng42sV2Gf7Q3wtQnDiJ,0*5F
!AIVDM,1,1,,A,H52k1?ALPU@F0<520TT00000000,2*3B
!AIVDM,1,1,,A,B52d3RP00>g41U62GVwQ3wuQnDEr,0*71
!AIVDM,1,1,,B,E>k`HC0VTah9QTb:Pb2h0ab0P00=N97j<4dDP00000<020,4*6F
!AIVDM,1,1,,B,H52d3RTUCBD9qtr000000018413t,0*1E
!AIVDM,1,1,,B,B52ndgh006g3owV2G=SQ3whQjFjJ,0*72
Along with output about the time it took to prepare & process the data. On an M2 Macbook Air, the times are 709.678333 and 349.079542 ms respectively.
| Argument | Description |
|---|---|
-h |
Shows help message containing argument info. |
-d [Directory Path] |
Enables extra output for debugging (demodulation time, errors, etc.) Saves a .aisDebug file to specified directory for each failed demod attempt. |
-ot [File Path, Int, Int] |
Runs the demodulator on a prerecorded .wav file (must be in 16-bit interleaved IQ format). First argument is file path, then center frequency of recording, then sample rate. |
-n |
Prints valid NMEA packets to the console when received |
-agc |
Enables Automatic Gain Control on the RTL-SDR |
-b [Int] |
Controls the bandwidth of the RTL-SDR |
-di [Int/IP:Port] |
Choose device index of your RTL-SDR (useful if multiple are present). If IP:Port is entered, SwiftAIS will connect to an rtl_tcp server. Example: -di 127.0.0.1:1234 |
-tcp [Int] |
Makes SwiftAIS act as a TCP server, broadcasting NMEA 0183 packets. Enter port (1-65535). Example: -tcp 50100 |
-ec [Int] |
Enables error correction up to a defined number of bits. Max allowed is 3. This is experimental, it can cause false corrections. |
-s [File Path] |
Saves NMEA 0183 output text to a specified file. |
-rs [Port] |
Rebroadcasts rtl_tcp data via this port allowing for visualization in an SDR frontend (SDR++, GQRX, etc.) |
SwiftAIS can take in .wav files for decoding offline. This allows for use with old captures or without an SDR. Use the -ot argument, with subsequent arguments as follows:
A path (absolute or relative) to the .wav file for input. Note that it must be in 16-bit interleaved raw IQ format to be processed. Audio recordings from SDR frontends will not work. SDR frontends will refer to the proper format as recording 'baseband' or raw IQ.
The frequency that was tuned to during the recording, in Hz. If recorded with an SDR frontend like SDR++, this should be in the filename. Ex. 'baseband_161665000Hz_16-03-37_31-05-2025.wav'
The sample rate of the recording. On macOS, you can right click the recording in finder, click 'Get Info', and see the sample rate listed in kHz. 1 kHz = 1000 Hz. Important note: the sample rate must be an integer multiple of 48,000 Hz. (96,000; 240,000; etc.)
SwiftAIS -ot '/Users/connorgibbons/recordings/baseband_161665000Hz_16-03-37_31-05-2025.wav' 161665000 240000
Future plans include a Swift-based decoder and live ship visualizer for the NMEA output.
Some AIS messages are too long for the NMEA 0183 82-character maximum and need to be split across multiple sentences. SwiftAIS has been updated to split long messages and assign them sequence IDs.
Many factors (noise, distance, etc.) can cause errors while receiving the packet. When they occur, they are detected by the failure of a CRC (Cyclic Redundancy Check). SwiftAIS will not output messages with an incorrect CRC value. With the -ec launch argument, SwiftAIS will use a heuristic to attempt recovering a packet by flipping most-likely candidate bits and re-evaluating the CRC. During limited testing, this feature is able to recover packets, but it should be noted that false corrections are unlikely yet possible. AIS CRCs are only 16 bits long, and there are many inputs that could produce an identical CRC value -- therefore this method is capable of finding incorrect flips that still produce a "valid" seeming packet. Typically it is very clear if this occured, as the position reported could be far out of feasible range. This feature is experimental.
SwiftAIS now has support for networking with regards to both input and output. TCP-based input allows you to specify an IP and Port of a device running rtl_tcp, and recieve samples remotely. SwiftAIS can also act as a TCP server, outputting NMEA 0183 sentences on a desired port. This allows for connecting chartplotters such as OpenCPN to display captured AIS data live.
If you have any issues or suggestions, please reach out: connor@ccgibbons.com
Testing for this tool is limited and it is not certified by any official bodies. It should not be relied upon as a navigational aid or safety tool. SwiftAIS is created for hobbyist/educational use only.