In September the U.S. Coast Guard warned Automatic Identification System (AIS) users from Virginia to Connecticut that they may be operating on the wrong frequencies. A series of broadcast commands issued inadvertently in late summer caused the error, potentially rendering AIS-equipped vessels unable to see or be seen by others.
No casualties resulted from the incident, which was resolved by the end of the month.
For redundancy, AIS is designed to work on multiple channels within the VHF-FM band. If the default frequencies (161.975 MHz/Channel 87B/2087 and 162.025 MHz/Channel 88B/2088) are compromised or unavailable, the Coast Guard can issue channel management commands that automatically switch vessel-based AIS devices to other channels. No user intervention is required in these changeovers.
Channel management commands affect defined regions between 20 and 200 nautical miles, and remain in AIS system memory for five weeks — or until the vessel moves more than 500 nautical miles from the region.
During a late-summer test of the Nationwide AIS, the Coast Guard accidentally issued four such commands, said Jorge Arroyo, AIS Regulatory Project Officer — two over land and two for vessel-based systems.
The over-land commands were non-issues, he said, and because one of the other two covered a defined region larger than 200 nautical miles, it was ignored by AIS systems that received it.
The fourth command, however, was sent to vessels in Virginia’s James and York rivers, upper Chesapeake and Delaware bays, the New Jersey shore and New York Harbor and approaches. While as many as 1,500 vessels could potentially have been affected by the command, fewer than a dozen actually were, Arroyo said.
The initial commands were issued using contractors’ equipment during a test of nationwide transmission capabilities for future use. By the time the Coast Guard realized its mistake, there was no immediate way to recall the commands. While it currently has listening stations around the country, just 10 stations are also capable of broadcast, he said.
“The irony is, that’s what we were testing,” Arroyo said. “But for now, we had to come up with an alternative. That alternative was Digital Selective Calling.”
DSC is integrated into all AIS units for just such an occasion. Beginning Sept. 1st, the Coast Guard broadcast hourly channel management commands switching AIS units in the same region to the default frequencies. These messages were broadcast on DSC Channel 70 to ensure all AIS units received them.
The initial command expired on its own in late September. Any affected systems not reached by the correction message automatically reverted to the default operating frequencies.
The Coast Guard asked AIS users to inform others who may be affected. Commands can be manually overridden through the units’ channel management functions.
“This was an unfortunate, unintended transmission,” Arroyo said. “Once we became aware of the problem, we did what we could to remedy it. No casualties occurred as a result of the incident. We’ve not had a single vessel owner even report the error to us.”
The incident remains under investigation, but the Coast Guard is working to change the procedures that caused it.
“A lot of good came out of all this,” he said. “We encountered ways we can improve the process.” Among expected changes is a reduction from the five-week shelf life of channel management commands to around five days, and more standardization about how different AIS manufacturers define geographic distances.