New AIS Class B SOTDMA Will Operate on Four Channels

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
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New AIS Class B SOTDMA Will Operate on Four Channels

Postby jimh » Thu Aug 19, 2021 10:30 am

The Automatic Identification System (AIS) was initially allocated exclusive use of two dedicated channels in the VHF Marine Band, at 161.975 MHz (AIS 1) and 162.025 MHz (AIS 2). At the 2012 World Radio Conference, two additional channels were allocated for AIS use: 156.775 MHz (AIS 3) and 156.825 MHz (AIS 4). These channels were added to enhance long-range (satellite receiver) tracking of ship movement. Note that previously these channels were designated as Channels 75 and 76, were not permitted to be used by ships or shore stations, and acted as empty channels adjacent to the international calling channel 16, 156.800-MHz to prevent interference to the primary channel.

The latest generation of AIS Class A and Class B SOTDMA devices will be transmitting on all four channels. This suggests that an antenna for those devices must have a suitably wide VSWR bandwidth covering the range from 156.975-MHz to 162.025-MHz. Antennas previously marketed to recreational boats as "AIS-tuned" antenna were generally optimized for the 162-MHz channels. The VSWR bandwidth of these antenna may not be sufficiently wide to operate on the two new channels near 156.8-MHz.

More information on the new AIS channels is given in a prior article:

Long Range AIS Channels