Automatic Identification System (AIS) is a fairly new tool that dramatically changes the way we can track ships. An AIS transponder uses VHF frequencies, and broadcasts your own vessel's position, name, callsign, along with detailed parameters like length, beam, draft, and tonnage. It also broadcasts details of the current navigation system: speed, course, rate of turn, destination, and ETA. The transponder receives this same information from other ships, and either displays it on its own screen or emits it in an NMEA-standard data stream for use by chartplotters and other onboard nav gear. The positions and intentions of nearby vessels are available to you unambiguously and in real time.

Much has been written about AIS's design and technical background that won't be repeated here. The US Coast Guard Navigation Center has a great summary of AIS technical details. A few very economical AIS implementations bear particular attention.

The first is a software solution from COAA called Shipplotter, available for Microsoft Windows-compatible computers. Shipplotter uses the audio output of a VHF receiver, and the computer's sound card, to decode ship position reports. Ships are plotted on the PC's display, and a standard NMEA data stream is available on the computer's serial output. If your PC is on the net, you can share ship posiition reports with other Shipplotter users. This allows you to monitor traffic in the English Channel while you're in the middle of Iowa. Price for the Shipplotter software is Euro €25, plus VAT. You'll also need a marine-band capable VHF receiver, with discriminator output, and a Pentium-class PC. Maps for Shipplotter are available on this page.

As of October, 2006 I know of 4 companies providing receive-only AIS units. Many of these require a computer or other device to decode the data and display ship positions. These units seem like they would be just right for recreational boaters and hobbyists who don't want to sink thousands of dollars into a top-line AIS transponder/display. I have not tried any of these myself.

Milltech Marine sells the SmartRadio line of AIS receivers with one or two channel capability, one of which includes a built-in GPS. The lowest-priced receiver in the line has a list price of $249, but occasionally goes on sale. Milltech also sells a pre-made serial splitting cable to allow you to have both GPS and AIS data coming in on the same serial port. The SmartRadio line is a much-improved version of a product that was originally manufactured under the name of Shenzhen Yuantong Telecom Co. Ltd, Shenzhen, China. As of this writing I have owned an SR161 for about 2 weeks, and it is working beautifully in spite of my having put a poor antenna on it. It seems very solidly built, with a metal case, and worked straight out of the box. The images below were constructed using my SR161 from my home.

UK-based Nasa Marine has two interesting products for AIS. The first, the misnamed "AIS Radar", combines single-channel AIS receiver and an LCD display; 12v DC required, external GPS optional. Don't be fooled by the name; this unit is AIS only, not a radar. Street price seems to run just below 250 GBP. They also sell just the receiver package, without display, which accepts GPS NMEA input and provides serial AIS output. This "AIS Engine" seems to run about GBP 110-130. I have found retailers only in the UK, but most say they will ship internationally.

The German company Weatherdock Inc. makes the receive-only EasyAIS receiver: They have several distributors internationally. One is Yachtbits, in the UK., who has it listed at 169.36 UK pounds, ex-VAT.

Shine Micro sells the also-misnamed Radarplus SL161R, a two-channel receiver for US$999, or US$1249 with built-in GPS. Again, despite the marketing department's wishful thinking, this unit has nothing to do with radar.

Rose Point Navigation's Coastal Explorer software apparently supports AIS decoding and plotting, if you are willing to take along a Windows-based PC.

Finally, there are several websites that display live AIS reports, either locally or globally.

Known live AIS sites:

  • has a very impressive life display of San Francisco Bay
  • COAA's ShipPlotter software ( lets you create your own site, using only a networked PC and a radio receiver.
  • is the largest AIS sharing site I know of. Paid subscription required for live data; free registration required for time-delayed data. Strong coverage of Europe, and also some North American and 3 Asian ports.
  • Hobbyist Mike Fallon provides a free AIS feed of the English Channel, using one of the SmartRadio units and COAA's ShipPlotter software.
  • Shine Micro: coverage for the Canary Islands and USA (Pacific Northwest, California, Boston, Connecticut). Free registration required.

Seattle waterfront, made with Google Earth, using COAA Shipplotter's KML data feed and an SR 161 from Milltech Marine:

sailing weather and tide predictions

AIS Information