One of the potential upgrades that we had in mind for Rode Trip was an Automatic Identification System (AIS). AIS is a relatively new system that the Coast Guard requires for commercial traffic. It integrates a ship's GPS and VHF radio and transmits their name, position, direction of travel and current speed. The system also allows the ships to see other ships that are transmitting, hopefully preventing tanker/freighter collisions. These systems are really slick but come with a rather hefty price tag, both in initial cost and power consumption. To buy a full AIS system would have cost a minimum of $800. Many recreational boats buy a unit that can receive the commercial traffic signals, but doesn't transmit. We had our eye on one of these that would integrate with our laptop, which came in at a more manageable $300.
A couple of weeks ago for some reason I was thinking about a conversation that I had a couple of years ago with my friend Paul about software defined radios. I remember that he had been looking into these nifty little devices and receiving radio stations on his computer. A brainstorm hit me, the VHF signals are being broadcast by the big ships, I wonder if I could use a software defined radio to receive the signals. A quick search on the internet showed me that I wasn't the first one to think of this clever idea. In fact there is an entire blog dedicated to software defined radios with a very detailed post on building an software defined radio - AIS
. Even better it uses all freely available software.
There were only two pieces of hardware that I needed to purchase the SDR "dongle"
and an adapter
to connect it to our VHF antenna. I already had a PL-259 "T" fitting to allow the computer to share the mast top antenna with Rode Trip's VHF. The grand total for doing the project this way was $28!
After following the detailed instructions in the link above and connecting the VHF antenna to the laptop I had AIS targets on the laptop! You can see in this picture Rode Trip (the red boat) and two other ships (green triangles) that are at a nearby marina. In addition you can see the two ships names listed on the right side of the picture.
We are looking forward to using this on our future passages. It will be nice if we need to hail any commercial traffic to be able to hail them by name rather than by Lat/Lon coordinates.