AIS Technical Notes
Picture a shipboard display with electronic chart data that includes a mark for every ship within VHF radio range. Then, add information that shows speed and heading. Each mark could reflect the actual size of the ship and report its latitude and longitude. If you selected the mark you could learn the ship's nave, course, speed, classification, and other information. Maneuvering information, closet point of approach, time to closest point of approach, and other target information would also be available as well as destination. In other words, you'd have much of the information that's available to the Vessel Traffic Service at your fingertips.
This information can be yours NOW with an inexpensive AIS--Automatic Identification System--receiver, navigation software that supports AIS display, and an onboard computer.
AIS is a system used by ships and vessel traffic systems to identify vessels at sea. Radio transponders are integrated with GPS systems and other navigation equipment to report vessel movement automatically Ships of greater than 300 gross tons are required to install AIS; other smaller commercial vessels are installing systems as well. Because a given area of ocean can get very crowded, vessels that are anchored or are moving slowly transmit far less frequently than those moving faster or changing course. Each transponder transmits on two redundant channels and can be "seen" for approximately 20 nautical miles if in a direct line of sight.
Recreational boaters are not required to have AIS transponders onboard. However, many are installing inexpensive receivers that integrate with navigation software to display AIS information on computer. It's a great aid for collision avoidance in waters crowded with big ships and provides information to supplement radar. However, it's not in and of itself a substitute for radar, particularly in open ocean.