AIS transponders act as conduits for information about a vessel, gathering both dynamic data about a ship, such as position, course and speed, and pseudo-static data, such as vessel name, call sign and cargo. This information is broadcast digitally over designated VHF radio channels for any listening station to monitor.
Other AIS transponders or receivers in the area can receive the data, instantly locating and identifying nearby ships. Not only is the presence of the ship known, but also its heading, speed, length, beam, etc. And since each vessel’s name and call sign are included, bridge-to-bridge communications are improved.
The control protocol for AIS transmissions supports over 2,000 simultaneous transmitters sharing the allocated frequencies, automatically insuring service will not degrade in congested areas.
AIS permits direct and certain identification for collision-avoidance calls and maneuvers. No other system provides the same situational awareness in real time.
If this were the entire story, however, its overall value might be debatable. Instead, advances in electronics and communications have taken the usefulness of AIS well beyond its original conception.
Advances in electronics generally, and computers in particular, have found broad application in ship instrumentation. Navigation equipment especially has benefited through migration to computer-based platforms. The non-specific algorithmic nature of computers is well suited to the data-intense, graphically portrayed electronic chart display and information system (ECDIS).
ECDIS evolved from simple chart-display programs, eventually incorporating data from such disparate sources as GPS position sensors, gyros, logs and anemometers. As such, they integrated a ship’s real-time location with relevant chart details into an on-demand and readily understood display.
Next generation ECDIS included live radar data providing situational awareness in a unified display. One shortcoming of this technology is the inability to conclusively identify any individual target while multiple contacts are present. In congested areas or in times of limited visibility, this uncertainty increases unnecessary radio traffic and maneuvering response time, and could ultimately contribute to a higher risk of collision.
AIS equipment addresses this shortcoming directly by providing an obvious correlation between a target and its relevant information. But as an independent display, it could actually increase confusion through its unique interface and by distraction from the immediately evolving situation. This concern was raised during the U.S. Coast Guard’s comment period for its original AIS regulations.
Now state-of-the-art integrated navigation systems include data from a ship’s AIS transponder. In this manner, the AIS equipment is analogous to a GPS receiver. Latitude and longitude previously had to be read from a separate GPS display and mentally or manually incorporated into navigation work. Today virtually all electronic charting displays treat the GPS data as just another automatically displayed sensor input.
Similarly, integrated navigation systems display AIS information in graphical form, allowing the mariner to “see” ships that might otherwise be hidden around corners or shadowed behind larger radar targets. And since course and speed are also known, continuous radar contact is not required for predicted track and CPA calculations.
Steve Beuth, captain of Dan Moore, a research vessel operated by Cape Fear Community College in Wilmington, N.C., considers an integrated navigation system to be the most important addition to bridge equipment on a new build. “I want to see a console that offers everything – including weather data – in one display,” Beuth said.
Because AIS must meet international standards of performance and compatibility, it has achieved acceptance in vessel traffic services (VTS) worldwide.
AIS transponders, by their broadcast operation, allow port authorities to identify and track approaching vessels.
Commercially available, shore-based AIS receivers enable the establishment of VTS in ports that could not otherwise have mandated a proprietary standard for AIS equipment or technologies.
State security agencies also benefit from the same AIS technology. Ship movements can be tracked as soon as a vessel is within range of a receiving station.
The AIS standard includes facilities for a “long-range function,” which essentially ties the AIS transmission to an automatic MF/HF radio or satellite communication system. Fleet managers can use this function for real time fleet tracking.
A second tier of AIS service has also been established. Companies are gathering AIS broadcast data either from existing port authorities or through independent networks of receivers. These data are then served publicly over the Internet through subscription or free services. Interested persons, organizations and even countries can subscribe to obtain a worldwide view of real-time ship movements. Free services are more regionalized, but still real time.
In spite of broad implementation, AIS has shortcomings limiting its potential.
AIS transponders are relatively expensive. Although mandated by the IMO for ships on international voyages, they are not uniformly required on domestic commercial vessels. Nor is this technology likely to quickly “trickle down” to smaller commercial or recreational vessels. It is not feasible, therefore, to rely solely on AIS for situational awareness.
Similarly, even on international voyages, there are still plenty of vessels – especially in developing nations – that barely have functioning navigation lights, let alone more complex equipment like VHF radios and GPS receivers. AIS will not penetrate these markets because of the lack of perceived value. Affected ships will remain invisible to the AIS network.
Then, too, there are security concerns associated with the availability of AIS broadcasts. In areas of high piracy, shipmasters may minimize their visual and electronic footprints to reduce the likelihood of attack. Well-intentioned officers or crew could disable AIS transponders in certain areas.
Public availability of AIS data is also considered a security threat by some since ships’ positions, cargos and destinations are all part of AIS broadcasts. Previously free access to some AIS Web sites has been restricted to subscribers-only as a result. (See PM #91, Page 8.)
AIS has quickly gained global support. Tangible benefits to ship safety, port operations and security have been realized. Still, like any aid to navigation, mariners must use this system responsibly, always cognizant of its inherent limitations. Ã.‚¬.
Jeff Williams is an electrical engineer and holds a U.S. Coast Guard 100-ton master’s license.
