Open Source AIS Transponder (Class B)
This is the first ever (that I'm aware of) attempt at creating an open source AIS transponder. The prototype device based on this code and designs is fully functional, but there is still a long way to go before it is a "compliant" unit.
I intend to publish a blog (linking to and from here) as the project evolves. This repository will host the latest Eagle CAD files as well as the source code for the microcontroller. The source code project requires Eclipse CDT, but I'm open to transitioning to something else if there is enough interest and participation.
On the hardware side, the design is based on two Silicon Labs 4463 transceiver ICs and an STM32F302CBT6 ARM Cortex M4 microcontroller. One of the SiLabs ICs acts as a transceiver, while the other IC works as a receiver only. In receiver mode, each IC tunes to a different channel. When a transmission is scheduled, the ICs swap channels if the transceiver is not listening on the next transmit channel. This configuration may be construed as a violation of the AIS specification, but it makes for a much simpler PCB layout and negates the need for a 3-position RF switch.
The GPS is a GlobalTop "LadyBird" unit, but any decent GPS module with NMEA and PPS output should work.
The RF board incorporates an external bandpass / LNA (NXP BGA2869) and a Skyworks 66100 front end (PA/switch).
The transmitter output is nominally 0.5Watts (+27dBm) and it has a verified range of 5 nautical miles with a vanilla telescopic antenna (< 3dBi).
Persistent station data (MMSI, call sign, name, dimensions, etc) is stored on a 1Kbit EEPROM attached to I2C1. Remarkably, it works fine with the MCU's internal pull-ups, but I updated the design to include external pull-up resistors on the SDA and SCL lines. The code should be modified if you choose to install those.
The circuit is powered entirely from a 5V connection (USB for now, but leaning against it long term). It draws 135 mA in RX mode, and spikes up to 350 mA during transmission at full power.
I intend to use a Raspberry Pi as the front end of the transceiver, as the unit is supposed to be mounted outside, directly connected to its own antenna. The Pi will act as a source of power, a WiFi Access Point, a NMEA distributor and a web server for configuration and software updates. All communication between the transponder and the Pi is done over a single serial port.