AIS (ship positions)
Automatic position beacons that ships send so others can see them.
What does it sound like?
Short, harsh data bursts.
Who uses it?
Commercial ships and many recreational boats.
Where and when can I hear it?
161.975 and 162.025 MHz.
The science
AIS transmits at 9600 bits per second using Gaussian minimum-shift keying (GMSK), a smooth form of frequency shifting that keeps the signal compact. Each of its two channels, 161.975 and 162.025 MHz, divides every minute into 2,250 time slots, and ships use self-organizing time division multiple access (SOTDMA): each station works out its own transmission schedule from recent channel traffic and announces in advance the slots it will move to, so the network schedules itself with no master station. A standard Class A position report is 168 bits long and gives position to a ten-thousandth of a minute, speed to a tenth of a knot and course to a tenth of a degree.
Why use it?
AIS works like a transponder for ships: it automatically sends each ship's identity, position, course, speed and other safety information to shore stations and other ships, and receives theirs. The self-organizing slot scheme lets the system handle well over 4,500 reports per minute with no central controller.
Lore
The IMO adopted the AIS carriage requirement in 2000, and it applied to all covered ships by December 31, 2004: ships of 300 gross tonnage and upward on international voyages, cargo ships of 500 gross tonnage and upward on domestic voyages, and all passenger ships. Its two channels, now called AIS 1 and AIS 2, were formerly known as marine VHF channels 87B and 88B.
What do the decoded parts mean?
- MMSI: The ship's nine-digit radio identity.
- Position, speed, course: Where the ship is and where it's heading.
- Name and type: Sent in separate messages from time to time.
Easily confused with
- Digital Selective Calling (DSC): DSC is slower, on marine calling channels