DSC

Digital Selective Calling (DSC) is a technique defined in ITU-R Recommendation M.493 that uses pre-formatted digital data bursts to initiate, acknowledge, and terminate radio communications. In the distress context, a DSC distress alert automatically transmits the vessel's MMSI, the nature of the distress (e.g. sinking, fire, man overboard), the GNSS-derived position, and the UTC time of the position fix, enabling receiving stations and Rescue Co-ordination Centres (RCCs) to respond without voice contact. Dedicated DSC distress frequencies are prescribed by the ITU Radio Regulations and referenced in SOLAS Chapter IV: VHF Channel 70 (156.525 MHz) for Sea Area A1; 2187.5 kHz (MF) for Sea Area A2; and six HF bands including 4207.5 kHz, 6312 kHz, 8414.5 kHz, 12577 kHz, and 16804.5 kHz for Sea Areas A3 and A4 HF operations. Shore stations and vessels maintain continuous DSC watch on the appropriate frequencies for their operational area. DSC controllers must be connected to a GNSS receiver to ensure the distress alert contains a current position. SOLAS Chapter IV, Regulation 7 specifies the watch-keeping requirements, and IMO Resolution A.803(19) sets Performance Standards for DSC equipment in the MF/HF bands, complemented by Resolution A.804(19) for VHF DSC controllers.