AIS

We were eastbound in the Strait of Juan de Fuca, enjoying a smooth ride on the last night of what had been a relatively calm nine-day positioning up the coast from Cabo San Lucas. Near the Port Angeles pilot station, the mate came up to relieve me as we passed two tugs fairly close, but we had plenty of clearance and in a few minutes all the vessels would be safely on their separate ways. After the mate took the watch, I went back to my quarters and there on the desk was my phone, with a text message from my mother: "You're getting awfully close to the James T. Quigg!" Somewhere in Texas, Mom was watching my progress thanks to the Automatic Identification System (AIS) installed on most ships.

How It Works. AIS is a digital VHF radio system, which transmits data about a particular ship to AIS units on other ships. Most transceivers on commercial vessels have range up to fifty miles over the surface, but this can be enhanced in high traffic areas by repeaters on land or in Earth orbit.

An AIS transceiver has three types of information: static information about a vessel, such as its length, draft, and beam; passage-specific information such as destination and cargo; and always-changing information such as speed and course. The first two types are programmed into the unit by the vessel's navigators, the last time comes from interfaces with other bridge instruments such as GPS units. When an AIS unit on one vessel receives a signal from the unit on another vessel, it calculates even more information, such as CPA (closest point of approach) and time to CPA.

Uses. When AIS was first being developed, it was seen mainly as a collision avoidance and communication aid. In the rush of anti-terrorist legislation passed after the September 11, 2001 attacks, AIS was adopted as a security measure and its implementation moved up. Today it is used by vessel traffic systems, commercial vessels, and even recreational boats to identify vessels and coordinate their movements. AIS is frequently tied into a vessel's electronic chart, radar, or both to help with ease of identification and collision avoidance.

Concerns. For a bridge officer, the AIS can be just one more piece of equipment to be inaccurate, fail, or set off alarms at inopportune times. Especially if used with electronic charts, it can be a source of "fixation," or the use of one instrument (often with a brightly-glowing screen) to the exclusion of others. I once heard an Alaska pilot on a cruise ship repeatedly hailing another vessel by the wrong name because the other vessel's AIS was misprogrammed; before AIS he would simply have hailed the vessel by it's course and position if he couldn't otherwise identify it.

Another concern is the public availability of AIS information. Mom was able to see the vessels close to me through a website that collects and displays AIS information worldwide, and if Mom can see it so can pirates and terrorists. To my knowledge there is no case of a pirate or other criminal using AIS information to facilitate a crime against a vessel, but it's probably only a matter of time.

For more on AIS, check out the US Coast Guard's Navigation Center website here.

Read Professional Mariner columnist Capt. Kelly Sweeney's concerns about AIS here.

PMY: Power & Motoryacht writer Tim Bartlett makes the case for recreational boaters using AIS here.