MAIANA™: The Open Source AIS Transponder
I started this project around 2015 with the intention of experimenting and eventually building something for my own boat. I was not impressed with commercial AIS class B transponders. They seemed bloated, expensive and some of them were particularly power-hungry. So I set out to create a lean and mean design.
In 2018 I installed this system on my boat. It is still working fine after 2 years.
I have refined the design somewhat, and it now has an "official" name, borrowed from a Pacific atoll in the Kiribati islands. I do intend to trademark this, in the same way that "Arduino" is a trademark that identifies the original rather than the many clones that inevitably emerged.
Overall description
Hardware
The main difference between this design and nearly every commercial transponder is that it's a standalone unit. It contains all of its radios and antennas and thus only needs a power + data cable to connect to the cabin. The PCBA is 1" wide so it fits inside 1" PVC pipe, which I used as the antenna base. The GNSS receiver and antenna are on the board:
On the hardware side, the design is based on two Silicon Labs 4463 transceiver ICs and an STM32L432KBU6 microcontroller. The GNSS is a Telit SE873 (7x7mm module) and relies on a Johanson ceramic SMD antenna. It usually takes about a minute to acquire a fix outdoors. The transmitter output is 2 Watts (+33dBm) and it has a verified range of over 10 nautical miles with a vanilla telescopic antenna (< 3dBi).
The unit runs on 12V and exposes a 3.3V UART for connecting to the rest of the boat's system. The UART continuously sends GPS and AIS NMEA0183 data while listening for CLI commands. Persistent station data (MMSI, call sign, name, dimensions, etc) is stored on a 1Kbit EEPROM and is provisioned via the CLI. The unit implements SOTDMA synchronization based on the GNSS 1 PPS signal and UTC, but it is a class B, so it will not attempt to reserve slots. It will just transmit independently and autonomously, at the schedule permitted for such transponders.
If station data is not provisioned, the device will simply run as a receiver and never transmit. In addition, there is a "TX OFF" signal in the latest design which will disable transmission if it is pulled low externally (say via a rocker switch in the control panel).
The circuit draws about 45mA from 12V in RX mode, and spikes up to 600 mA during transmission (for about 30 milliseconds).
The latest design (not the one pictured above) uses an RJ45 connector, as Ethernet cable is cheap, widely available and offers enough signals to instrument controls such as "TX OFF". I will include a reference design for a control box that I'm working on but every boat is different, so your mileage will absolutely vary.
Software
The firmware is an Eclipse CDT project that you should be able to import and build. It has a BSP architecture so you need to tweak bsp.hpp or define one of the required symbols in the preprocessor to build for different board revisions. It contains snippets of STM32Cube generated code, but is not structured around it.
Building the unit
This is going to be difficult for all but the most technically advanced.
The board features mostly surface mounted components, with 5 QFNs, a few SOT-363s and all 0603 passives. Unless you're skilled with stencils and reflow, you won't likely be able to build it. So I am going to make it available as part of a kit (probably on tindie.com). The kit will include a 95% finished PCBA as well as the VHF antenna, enclosure and sealing components. The board will be programmed and tested, and the antenna will be perfectly matched. The cable and whatever lies on the other side of it will be the end user's responsibility, as every boat is different.
License
CAD and firmware are licensed under GPLV3. I chose the most "copyleft" license possible to discourage commercial entities from ripping this off and then "close-sourcing" it. So don't try anything fishy, because I will find out and then ... well, let's just say you'll be buying me another boat and I have a particular one in mind ;)
If you're a tinkerer and want to further this design while adhering to the GPL, more power to you! Build it, sell it, give it away, do whatever you want to get it out there. You cannot, however, use the name in your forked designs, as it will be trademarked.
Concluding thoughts
The commercial AIS industry has not exactly been successful in the low-cost recreational boating market, and with prices like $500-$800 for a unit that needs external GPS (LOL) and VHF antennas that's hardly a surprise. I hope to change that. More on that later ...