Automatic Identification System (AIS) messages are transmitted over radio waves. The system was designed, in part, so that vessels could “see” the positions of nearby ships on a monitor and avoid collisions. These radio signals are received by satellites and used for many monitoring purposes. Each boat can broadcast a distinct message as often as every two seconds. We receive more than 20 million AIS messages a day. When we get them, they’re just strings of code. The code below is one AIS message from one boat.
So what does it all mean? When you see a vessel track in the Global Fishing Watch map, every lighted point represents an individual AIS message. The messages we receive are fed into our computer model which evaluates each message in relation to surrounding messages to identify individual tracks and determine which points along the track represent likely fishing activity and which represent activity likely to be non-fishing such as transiting. In order to develop and refine the model’s algorithm, our team needs to be able to read AIS messages, so our analyst Kurt Schwehr wrote a program that parses them into something readable. This is what a raw AIS message such as the one above looks like in “translation.”
“timestamp”: “2016-10-07 00:07:29 UTC”,
This is a location message. A number of fields may not be obvious, but your can easily pick out the MMSI number, the date and time, and a speed of 1.5 knots. A latitude of -26.39 (26.39 degrees South) and longitude 62.75 degrees (62.75 degrees East) places the vessel in the Indian Ocean to the east of Madagascar.
This message is part of the track shown above. Go to it here on the Global Fishing Watch map. Scroll out to see the whole track, or zoom in and move your cursor over the track while monitoring the latitude and longitude in the lower right of the screen to find the point on the map that matches this signal.
In addition to location messages such as the one shown here, vessels also broadcast messages that provide identification. Sent less frequently, these messages include the vessel’s call sign as well as its name, which is how it appears in the GFW toolbar when you click a vessel’s track, and what type of activity it is engaged in, which helps us identify which vessels are fishing.