Containerized AIS Decoding with RTL-SDR Dongle

It's been about 6 months since I started receiving AIS signals using a RTL-SDR Blog v3 dongle. My station is decoding between 35 and 70 ships at any given time, depending on time of day and how many ships there are in San Francisco Bay.

The RTL-SDR Blog v3 dongle is a really inexpensive software-defined receiver. The "official" version costs about $25, and knockoff versions can be had for $10 or less. However, there's a reason why knockoff versions are less than half price, they use very cheap components and drift like crazy. I have several friends who bought the cheap version only to find that they just don't work, then purchased another knockoff that also didn't work. So save yourself the hassle and just buy a real one.

The very old laptop I was running rtl-ais and kplex on was starting to die, with several unknown kernel panics over a few days. I purchased a new shack computer, but wanting to keep the base OS clean, I decided to play around with containerizing rtl-ais and kplex.

Containerization is a ...

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AIS Decoding with an RTL-SDR Blog v3 Dongle

Update July 2021: Run rtl-ais and kplex in a docker container.

At the conclusion of my last AIS blog post, I noted that the AIS receiving station I had set up was not sustainable for the long term. It was using a full hardware radio, and the software was running in a Windows 7 virtual machine on my personal laptop. Since AIS decoding is not super useful only when my laptop is powered on, a new station setup was needed.

One option would be buying a dedicated Raspberry Pi with a dAISy Hat receiver. It's a dual-channel receiver that spits out UART serial NMEA frames to the Raspberry Pi, and costs $70, plus a Pi for $35. But that's a lot of money for something that I don't really get any benefit from.

One Pandemic Positive is that I have a lot of free time on my hands, so I thought about receiving AIS signals using equipment I already own. I have an extra RTL-SDR Blog v3 receiver, and after searching the internets I found the rtl-ais project, which seemed ...

Decoding AIS Signals for Marine Monitoring

Automatic Identification System is a system for boats to broadcast their location. This is very helpful in congested waters, such as in the San Francisco Bay, where many ferries, huge container ships, and small pleasure craft are trying to avoid each other. Onboard digital chart plotters can show nearby ships, their heading, speed, and expected position a few minutes from now. Early AIS transmitters were very expensive, but the new generation of Class B transmitters are actually pretty inexpensive.

AIS operates on two frequencies, Marine VHF channels 87B and 88B. These are the high-side duplex frequencies of channel 87 and 88, and they are 161.975 and 162.025 MHz. The data link layer is 9600 baud GMSK, packet length of 256 bytes, which works out to 26.6 millisecond packet time including some guard time. Each AIS "frame" is 1 minute long, so within each frame there can be 2250 time slots (per frequency). With two channels available, a lot of positions can be transmitted every minute.

The boats use a self-organizing time division multiple access (SOTDMA) scheme, which ...

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