Broadcast to Allied Merchant Ships

On the outbreak of World War II, the British Admiralty took over control from the GPO, and the embryo merchant ship broadcast system, called GBMS came into force.

After the fall of France, the Admiralty assumed control of all Allied merchant shipping which complied with British procedures.

When America entered the war, the world was divided into two strategic zones, the Admiralty being responsible for merchant shipping in one, and the United States Navy in the other.

[3] The GBMS organisation proved to be inadequate for the efficient clearance of traffic for a number of reasons, including poor coverage by wireless telegraphy (W/T) stations, obsolescent equipment, and many ships only able to listen at single or two operator periods.

[3] After the war, captured records showed that the German Kriegsmarine Beobachtungsdienst (Signals Intelligence Service) had been reading BAMS since the start of the conflict.

Within a zone, multiple stations synchronize using Spread spectrum frequency radio to provide complete coverage.

The Arranging Authority also ensures that Coastal stations under their control, transmit messages that are considered necessary, even if the originator does not specify such a transmission.

An AA may need to know the content of the BAMS dispatch but doesn't require forward routing via the station under its control.

Note: Should the merchant ship carry less than three radio operators, the fifth letter of the BAMS instructional group was to be replaced by Y or Z which were allotted the following meanings.

Corrections to errors in transmission of code groups took the form of a new plain language dispatch containing appropriate instructions regarding the changes.