How AIS Ship Tracking Works

The AIS System

The ship tracks shown on our San Francisco Bay ship tracking page come from a system called AIS (Automatic Identification System). Starting in 2002, all new commercial ships over 300 gross tons, and all new passenger vessels, are required to include AIS transponders. The requirement for existing ships has been gradually phased in.

The AIS system transmits information encoded on two VHF channels, at 161.975 MHz 162.025 MHz. Transmissions use 9600 bit per second GMSK FM modulation and HDLC packet protocols. Each ship transmits during one of 2,250 time slots, so all ships can share the two channels. The two channels provide redundancy and some level of protection from interference. Transmission range is basically line-of-sight, like VHF voice traffic. Typical range is 20 nautical miles.

The AIS transponder receives information from the ship's other navigational instruments. Position, course, and speed over ground information is typically provided by a GPS receiver. Additional information can be provided by other instruments. The ship's officers must enter information such as ship name, ship type, ship dimensions, and estimated time of arrival. This information is not always entered correctly and is sometimes out of date, which accounts for the peculiar information occasionally seen in the ship information panel on our ship tracking page.

Ships that are moored or at anchor are required to transmit their position information at least every 3 minutes. Ships moving at up to 14 kts must transmit their position every 10 seconds; at up to 23 kts, every 6 seconds; and at faster speeds, every 2 seconds. In addition, static information (such as the ship's name, destination, and estimated time of arrival) is transmitted separately every 6 minutes.

BoatingSF.com's AIS System

BoatingSF.com's San Francisco Bay ship tracking page displays information received from a SR161 receiver, supplied by Milltech Marine. The receiver is located in the Berkeley Hills. It connects to a serial device server, which converts the RS-232 signal to Ethernet and acts as a TCP/IP server.

Our Web server maintains a TCP/IP socket connection to the serial device server and processes the data stream. The messages are in a packed-binary format represented as a series of 6-bit ASCII characters; a typical message looks like !AIVDM,1,1,,B,13M@DngP00OlOf8LD3p4H9CH04hl,0*00. Custom PHP software on our Web server decodes the messages and stores the AIS reports in a MySQL database.

Every five minutes, software running on the Web server analyzes the last hour of AIS reports and creates an XML file that summarizes these reports. When you load our San Francisco Bay ship tracking page, a Flash program reads this XML file from the server and displays the animated ship positions. To reduce the amount of data that must be sent, the server provides position reports on a one-minute interval, and the Flash program interpolates from these positions.

You can also view near-real-time positions by clicking the Real Time button below the chart. The positions will be updated every 5 seconds, with a 2-3 minute delay.

Resources for More Information on AIS

U.S. Coast Guard technical summary of AIS
AIS information site designed for ship operators, with lots of useful information and links to articles and vendors.
IALA specification for the AIS system. This document explains the various message formats, ship type codes, and other technical details.
IEC specification describing the NMEA message format encoding the AIS information. This is the primary technical specification for the AIS system. Unfortunately, it is not available without payment of a fee; the cost is 52 CHF (Swiss Francs), or about $42. But you probably don't need it; the IALA document listed above should give you everything you need.
Yachting Magazine article on AIS
AIS Live provides AIS tracks for many locations around the world (the service is free for positions with a randomized time delay; a subscription is required for real-time reports).
Digital Ship magazine has an article in its April 2006 issue on the security controversy surrounding AIS availability on the Web. (This link is to a PDF of the entire issue of the magazine; the AIS article is on page 25.)

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