Just curious about "...is posted to the Internet by volunteers who have receivers located in fixed positions (often far from water)." I noticed AIS data showing in the Antarctic, are there volunteers there? It seems they would have better things to do than to be feeding data into the Internet. Curious how that works?
There is one volunteer receiving station at Palmer Station, Antarctica (country code AQ). At the time of this writing, it has been feeding data only about 20% of the time. Not bad for Internet connectivity from Antarctica.
You can see a full list of the receiving stations (sorted by country) here: Stations Status & Performance - AIS Marine Traffic
Note that at any given time, about one quarter of the stations are offline, and that only one station - a test system run by the system operator (probably a few feet away from the marinetraffic server) has 100% availability.
A description of how all this works (in answer to your question) is posted here: Cover your Area on the Live Ships Map
. As you will see, the data is uploaded by unpaid volunteers - often using scanners bought at Radio Shack. Those scanners, by the way, can only receive one of the two AIS channels at a time, so 50% of the AIS message traffic can be lost. Have I poked enough holes in the reliability of this service for collision avoidance? I am using a dedicated dual-channel AIS transponder (the same one I use while underway), but I am in the minority.
I am participating as a volunteer because it gives me an effortless way to monitor the health of my AIS transponder when the boat is at the dock (marinetraffic provides some nice graphs of my receiver's performance), while simultaneously performing a public service by sharing the data. But I'm a Ham radio guy, and that sort of stuff turns me on.
As an side, for anyone curious about the maximum receiving range of an inexpensive AIS transponder with a simple store-bought masthead antenna 40 feet above the water: my record so far is 188 nautical miles - a fluke probably due to atmospheric ducting or meteor trail bounce. My "solid" reliable range for receiving Class A transponders (the Big Boats) is 30 miles.
AIS has saved my bacon more than once already! I just happened to have a camera rolling during one encounter with a Pilot Boat. You can see it here:
The video was shot with a fisheye lens - the boat is MUCH closer than it appears. I was able to call the boat specifically by name and get it to turn and slow down - when all I could see was a bow with lots of spray coming right at me. There's no chance I would have known its name until I read it off the stern ... post-collision. My chartplotter computed that the boat would have gone right through my hull and sounded the CPA alarm, and without that AIS collision alarm, I wouldn't have seen it until much later because it was hidden behind the jib. Previously, it had been on a parallel crossing course but then turned to a conflicting heading while behind my jib. Note the shadow from my mainsail: the sun was almost directly in the pilot boat helmsman's eyes. Had there been any delay at all (such as you'd get using an iPhone), my boat could have been reduced to floatsom.