About seven years ago I designed an information system for law enforcement along Europe's waterways and seaports: AQUATRACK. I then equipped the remote receiving stations with a AIS receiver OEM module from SRT Marine Systems. An excellent product. The phenomenon of AIS fascinated me so much that I was curious if I could make a building block as a hobby project for a fraction of the cost. A search on the internet led me to a nice initiative, the dAISy project which turned out to be based on an EZRadioPRO ISM Band receiver chip: the Si4362 from Silicon Labs and a simple 8-bit MSP430 processor from Texas Instruments. Because I am a little more comfortable on ARM processors and also have the development tools for it, I'd rewrote the source code for a Cortex M0 chip, the STM32F042K6 from STMicroelectronics to be precise. I also made some print circuit board designs for this.
For the backgrounds of AIS I would like to refer to some links at the bottom of this article, but the essence is that the GPS position of the ship on which the transponder is installed is broadcast periodically on one of two channels reserved for this purpose in the maritime VHF band. 161.975Mhz and 162.025Mhz to be exact. Depending on the speed of the ship, that varies between once every three minutes (when anchored) to once every 2 seconds at speeds above 23 knots. Ships in the area can receive these signals directly and display the position on a map. In that sense it is a "poor man" equivalent of the radar albeit providing much more information than position but also heading, speed, vessel name, purpose of use, dimensions etc..
In order to be able to receive all surrounding ships, both AIS frequencies will have to be monitored alternately or simultaneously. This article concerns the first version of the PCB and has - just like the dAISy - a single Si4362 receiver on board that "hops" at a fixed interval between the two channels. In a second version of the print space has been created for a second receiver chip and there is real simultaneous reception.
The Si4362 is a receiver chip that works between 140 and 960 Mhz. It is pin-compatible with the Si4463, which is identical in terms of receiving part, but also has a transmitter on board. The latter chip is produced in much larger volumes and is available both per piece or as a building block on AliExpress for little money.
The purpose of my AIS board (5x5cm) is to eventually place it (in an even smaller size) directly or in the vicinity of the receiving antenna so that a coax cable can be omitted. For that reason, in addition to the USB, an RS485/RS422 output has also been provided and a MAX3483 transceiver has been placed on the PCB. For power supply and data transfer, a simple two wire pair signal cable a cheap RS485/USB adapter at the other end of the cable is sufficient. The result is truly "digital" AIS-antenna. And the advantages are: much more freedom in the installation and the possibility to bridge tens of meters without any signal loss. If you opt for USB and placement directly at the PC or laptop, the mounting of the MAX3483 can simply be omitted.
Besides a lack of time, the reason why I let the project slip a bit is that I found this receiver less sensitive than the professional receivers I had at my disposal at that time. In an attempt to improve this, I made some space in the print layout beforehand for extra RF preselection (two bandpass filters with a pHEMT mosfet in between), but that didn't help much. The thing costs a fraction of a professional receiver and is certainly usable, but also within the dAISy user group the somewhat less sensitivity was a recurring topic of discussion and real progress has not been made as far as I know. I still have the idea that the problem is not caused by the Si4362 (the specifications are quite good) but should be solved by a better algorithm of the digital signal processing in the Si4362 and possibly shifting some of the DSP work to the ARM Processor. I simply lack the knowledge to create a better GMSK demodulation with Matlab and implement it in both components. However, the preliminary work that has been done with the porting of the software to the much more powerful ARM processor should give it every chance. For those who want to pick it up; please keep me informed!
By the way, when this article was written, I delved into it again and that has led to some nice insights about the LC balun that Silicon Labs recommends in its Application Notes, but can be very unfortunate for the AIS channels in the VHF band due to blocking from strong signals in the FM broadcast band. I have devoted an article to it and am curious about the experiences of others.
The original AISy project was started on Energia 43oh but that forum is now dormant. The original source code was then published under CC BY-NC-SA Creative Commons Attribution-NonCommercial- ShareAlike license and can be found at o Github.
Below you can find both schematics, the print layouts, 3d PCB animations and some photos. It also contains some examples of transformers with different footprints, the location for the TO72 transistor sockets, the ring cores and 'double aperture cores' (pig's noses), powder iron cores and air coils at the entrance.
Schematic
Downloads
