Currently, there are two classes of AIS. For
commercial vessels greater than 300 tons
operating internationally and all vessels of 500
tons or more operating domestically, along
with many types of passenger vessels, there is
the original Class A AIS. Mandated by the
IMO’s (International Maritime Organization)
Maritime Safety Committee, all commercial
vessels meeting the criteria above are required
to have and operate a Class A AIS. Some estimates put the number of vessels required to
have a Class A AIS as high as 300,000 ships.
Class A units transmit at 12. 5 watts and
output detailed information about the vessel,
including dynamic information such as position, speed, current status and course with
rate of turn. Static information includes vessel
name, IMO number, MMSI number, and
dimensions including draught. Class A also
includes voyage-specific information that is
manually entered by crew such as destination
and ETA. This class transmits its information
every two seconds and has priority over the
subordinate Class B AIS.
Class B AIS is the fastest growing segment
of the AIS market. Prices have dropped as
popularity has increased and a greater variety
has entered the market. This class is now
being incorporated into VHF radios and integrated into GPS units and other onboard electronics. These units have also developed from
simple transceivers to sophisticated stand-alone units with their own displays and WiFi-capable transmissions to tablets and smart
phones. Transmitting at 2 watts compared to
the 12. 5 watts of the prioritized Class A units,
Class B units range out at about 10 miles (still
very good for collision avoidance, especially
for smaller, more maneuverable vessels). The
automatic transmissions of data are also less
Primarily designed for recreational vessels
and commercial vessels not required to carry a
Class A system, they offer a more economical
option that still allows for situational awareness aboard. The information they transmit
and receive is not as comprehensive as the
IMO-compliant vessels but still includes the
MMSI identifier, speed, course, heading and
time. They have proved useful for not only
collision avoidance but also as an aid to search
With the growing popularity of AIS, and
Class B AIS in particular, display screens on
vessels in some congested areas can become
At left are AIS chart overlays on a pair of MFDs. The
screenshot from Marine Traffic.com shows AIS transmissions from the always-busy English Channel.
Screen shot courtesy of Marine Traffic.com
MFD photo by Glenn Hayes
VDES—the VHF Data Exchange System—is where AIS will experience xpansion in the future. Due to the limitations of range and capacity of the current terrestrial-based AIS systems, there is a push on at an international
level to adopt the use of two new frequencies that would be used for a satellite
VHF data exchange of AIS information and application-specific messages.
AIS data and information transmitted terrestrially is currently limited to line
of sight VHF capability. If a maritime or navigation authority or Vessel Traffic
Service (VTS) wanted to gather data or transmit information to vessels outside of
the range of VHF it does not have many options.
The footprint a satellite can both receive from and transmit to space is a great
deal larger than its terrestrial counterpart. By expanding to a much larger footprint there are many more options to AIS usage beyond collision avoidance.
There are added benefits, such as increased security, improved search and rescue capabilities, greater fleet management, longer-range vessel traffic management and navigation information, along with increased VHF bandwidth for
application-specific messaging. VDES could also become one of core facilitating
elements for implementation of e-navigation and modernization of GMDSS.
Although most current equipment can be upgraded, a new generation of transceivers would likely be required to full take advantage of VDES.
Experts say that this next generation of AIS capability is years away from being
operable and available for general use. There are many international hurdles that
have to be crossed, along with careful planning of protocols and bandwidth designation and cash-strapped government agencies obtaining the necessary equipment to make it viable. Current collision avoidance AIS would remain on the
existing frequencies and be the priority, with VDES acting a supplementary system adding information and features. Although still in its infancy this new phase
of AIS has a promising future.
VHF Data Exchange System
Offering more options for AIS