This standard specifies digital on-board vessel communication equipment by SCPC (Single Channel Per Carrier) method by Four-level frequency shift keying (4FSK) as modulation method among the digital communication methods in the 400MHz band specified in Article 45-3-7 of Ordinance Regulating Radio Equipment and Article 2, Paragraph 1, Item 40-3 of Ordinance for Enforcement of the Radio Act.

This digital on-board vessel communication equipment is composed of a base type equipment and a portable type equipment, and communicates between the base type and the portable type or between the portable type and the portable type (via the base type or directly).

In ITU-R Recommendation M.1174-3 on-board vessel communication equipment, channel intervals of 12.5kHz and 6.25kHz for the 4FSK digital system are specified. The frequency allocation of 6.25kHz is only permitted in Japan from the viewpoint of effective frequency utilization, therefore this standard specifies the modulation method of 4FSK and the channel interval of 6.25kHz.