Older articles:

1. Argo Floats from the TS Golden Bear: Deployment and Data

   I recently read an article about how a "cold blob" was interrupting the Atlantic Meridional Overturning Circulation (AMOC). This is pretty interesting, as the AMOC takes heat from Florida and brings it to Europe, keeping Europe warmer during the winter. The fear is if this current stops, winters in Europe will become a lot colder than they currently are. Towards the end of the article, they talk about using ocean float data for in situ measurements. That jogged my memory, I actually helped to deploy these ocean floats a long time ago!

   In spring 2005, during my junior year at Cal Poly San Luis Obispo, I participated in the Cal Poly At Sea program. This partnership between Cal Poly and Cal Maritime invites Cal Poly students to take classes about the Training Ship Golden Bear for two months. The trip was great, we sailed from San Francisco to Hawaii, Suva, Noumea, Sydney, and Brisbane, with a stop off at Palmyra Atoll on the way back to Hawaii. Professors from Cal Poly taught real classes on environmental chemistry, biology, astronomy, and ...

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2. 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 ...

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3. 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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