Due to my job I am very interested in the merchant shipping world. One of the websites I like a lot is www.marinetraffic.com. This website shows shipping traffic all over the world, which it collects using the AIS data which every ship over 300t gross tonnage has to broadcast. Volunteers all over the world receive this data, and forward it to the website so it can be used to show the information. (More info: Class A and Class B fact sheets)
Commercial AIS receivers are normally expensive, and not easily available. A lot of people use modified maritime radios or scanners with a hardware decoder and a PC to collect and transmit the data. When investigating the possibility to build my own decoder I came across the website of Alex Scafidas where he describes the design of an AIS decoder based on an ATMega8 microcontroller and a CMX GMSK decoder IC. It looked very promising so I decided to use that design as a basis for my integrated receiver / decoder. I found a suitable receiver module at Radiometrix in the UK, the NRX1 . For network connectivity I decided to use a Lantronix Xport module (datasheet) which are regularly available at eBay for not too much…
I bought the required decoder chip, the CMX589A (datasheet) at eBay, together with the Xport module. I was lucky and spent only $25 for the both of them. The Xport is also available at Mouser or Digikey, but for around $45-$50 each, so it is worthwhile checking eBay! The CMX589 is not available at Mouser or Digikey, so the only option to get them is eBay. Unfortunately the only version of the CMX589 which is available at eBay is the SMD (24 pin TSSOP) version. The DIP version would be a lot easier to use, but it would take up a lot more PCB real estate. Since I am not “afraid” of SMD components it is not a problem to use these.
I created a prototype to fit into a tin plated box, and it has been running wonderfully for some time now. Since a tin plated box is not the nicest enclosure I decided to spend some time and make the design to fit a nice case (Case #1111 at rfsupplier.com) which I still had laying around. The end result is a very nice layout which allows the connection of an external receiver if desired,. As voltage regulator I decided to use a switching regulator of Recom. These regulator has as a very big advantage of staying very cool during operation, even when the input voltage is >12V.
I have not build this design yet, but since it is based on my prototype with some small errors removed I am positive that the design will work flawlessly. I have built the design as published and it works perfectly!
A note about the Xport modules from eBay; The modules I bought for $15 were incorrectly flashed. The symptom is a not properly working webinterface. I have solved this by downloading the firmware updating tool from the Lantronix website, together with the latest firmware. The instructions how to flash are included with the firmware files. However, they are incorrect! The instructions tell you to flash the web interface to WEB4, but it should be WEB1!!
Top view of PCB
Bottom view of PCB
Finished AIS receiver
XPort user guide
XPort configuration sheet
Source code + hex files from Alex Scafidas