AIS: CLass-A versus Class-B: Power, Network, Intervals, Data

Postby **jimh** » Thu Jun 25, 2026 11:01 am

In the Automatic Identification System there are two distinct groups of AIS devices, Class-A and Class-B. Larger vessels such as commercial ships will typically employ AIS Class-A devices. Smaller vessels such as recreational boats of modest size, will typically have AIS Class-B devices.

The Class A and Class B devices differ in several realms, including the power of the transmitter, the method of joining the self-organized network, the interval between transmissions, and the amount of data transmitted

Power Level and Self-Organizing Protocol

Class-A
Class-A AIS devices transmit at 12.5-Watts of output power, which can be further enhanced by antenna gain and higher antenna height from associated larger size, all contributing to an expecting operating range with other vessels of 20 to 30-nautical miles. Class-A devices alway use the Self-Organizing TIme Division Multiple Access (SOTDMA) protocol, which tends to ensure these vessels will always be able to find and reserve an open transmitting slot in a busy network. The SOTDMA protocol permits an AIS vessel to reserve a future time slot for its next transmission, thereby insuring a continual flow of data from that vessel to the self-organized network.

Class-B
Class-B AIS devices have two sub-categories: standard Class-B and an enhanced Class-B often denoted as Class-B+.

The standard Class-B device operates at 2-Watts, a reduction in power compared to Class-A of -8 dB. This results in Class-B vessels typically havinh a shorter expected operating range of perhaps 8 to 10-miles at best. Class-B devices also use a different method of joining the self-organizing network, called the Carrier Sense Time Division Multiple Access or CSTDMA protocol. Each Class-B device acts in a "polite" manner, and before it transmits in a particular slot, it first listens for a short time to make sure no other vessel is using that slot (or "Listen before Talk"). If no other signals is detected, the Class-B device then transmits, but uses a shorter message format. There is no reservation of a future time slot for subsequent transmissions.

The enhanced Class-B device (Class-B+) operates at 5-watts, a power level compared to Class-A that is -4 dB lower, but compared to Class-B is stronger by 4 dB, so essentially its power is halfway between Class-A and Class-B, typically giving an operating range of 8 to 15-miles. The enhanced Class-B also uses the more sophisticated SOTDMA protocol, permitting it to reserve a future time slot.

Intervals of Transmission and Data Transmitted

There are significant differences among the AIS classes for the frequency at which they will transmit data, and the amount of data they will transmit. Further, the speed of the vessel affects the frequency of transmission, with greater vessel speed producing more frequent transmissions.

Class-A

Class-A devices transmit Messages 1, 2, and 3 at the following intervals based on their speed or course change:

every 2-seconds if speed is greater than 23-knots (nautical-miles-per-hour)
every 3.33-seconds if a course change greater than 5-degrees has occurred;
every 6-seconds if speed is greater-than-14 to 23-knots
every 10-seconds if speed is 2 to 14-knots
every three minutes at anchor or moored or if speed is 2-knots or less

Messages 1 includes:

MMSI
Navigational Status
Rate of Turn
Speed over ground
Position Accuracy
Latitude
Longtitude
Course over ground
True Heading
Time stamp
Special maneuver indicator
Receiver autonomous integrity monitoring flag
Communication state

In addition, Class-A devices can transmit Message 27, the Long-range AIS Broadcast Message, intended to aid reception by satellite monitoring receivers.

Navigation Status Categories
There are 16 possible navigation status categories that can be indicated by Class-A AIS devices :

under way using engine
at anchor
not under command
restricted maneuverability
constrained by draft
moored
aground
engaged in fishing
under way sailing
(reserved for for future admendments)
(reserved for future amendments)
power-driven vessel towing astern
power-riven vessel pushing ahead or towing alongside
(reserved for future use)
AIS-SART, MOB-AIS, EPIRB-AIS
undefined (also use for AIS-SART, MOB-AIS, EPIRB-AIS

Class-A devices also every 6-minutes send Message 5, Static and Voyage Reporting data, which includes:

MMSI
IMO number
Call sign
Ship name
Ship type
Dimensions
Static draft
Destination
ETA
EPFS type
Data terminal availability

Class-B SOTDMA

Class-B SOTDMA devices transmit Message 18 as follows:

every 5-seconds if speed is greater than 23-knots;
every 15-seconds if speed is 14 to 23-knots;
every 30-seconds if speed is 2 to 14-knots;
every 3-minutes if speed is less than 2-knots.

Message 18 is similar to Message 1, but omits Rate of turn, and Navigational Status. However it adds several flags specific to Class-B devices to indicate the particular device capabilities.

Message 18 data includes

MMSI
SOG
Position Accuracy Index
Longitude
Latitidue
Course over ground
True heading
Time stamp
Several one-bit flag indicators:
- Class-B unit flag (SOTDMA or CS)
- Class B display flag
- Class B DSC flag
- Capable of operating in upper 535 kHz of marine band
- Class B Message 22 flag
- Mode flag
- Receiver autonomous integrity monitoring flag
- Communication state selector flag
SOTDMA Commication state

Class-B SOTDMA will transmit every 6-minutes Messages 24A and 24B, which includes

MMSI
Call sign
Ship name
Ship type
Dimensions
EPFS type
Data terminal availability
Vendor ID

Class-B CSTDMA
Class-B CSTDMA device transmit Message 18 (see above) every 30-seconds (± 4-seconds) and subject to slot availability, and reduce to every 3-minutes if speed is 2-knots or less.

Class-B CSTDMA devices also transmit Messages 24A and 24B every 6-minutes (see above).