Digital selective calling (DSC) equipment
Message anatomy
A DSC message contains format, address, category, nature of distress or communication priority, position and time, telecommand, and end-of-sequence. Formats include individual, all ships, group, and geographic area. Addresses use Maritime Mobile Service Identities (MMSIs): a ship is nine digits with a maritime identification digit (MID); a coast station begins 00; a group begins 0; a SAR aircraft begins 111. VHF DSC uses channel 70 only; MF/HF DSC uses the dedicated distress and calling frequencies. Class A equipment supports full GMDSS functions; Class B is limited.
Installation and operation
Correct MMSI entry is fundamental. The terminal needs a position and UTC source, usually GNSS, so distress alerts carry accurate coordinates. Self-test checks the display, memory, and transmitter at low power but does not replace an on-air test. Watchkeeping is automatic on the distress frequencies. The distress button is protected against accidental operation and typically requires a deliberate press and hold. A DSC distress alert should be followed by voice or NBDP as indicated by the telecommand.
Worked example
An operator wants to call a coast station for a routine test. Enter the coast station MMSI beginning 00, select routine category, choose the working channel or telecommand, and transmit on the non-distress DSC frequency. Do not use the distress button or the distress frequency. If the coast station offers a DSC test facility, use the specified test format. The reasoning: DSC addressing and priority are separate; a routine call must not raise a distress or urgency flag, and the working frequency is assigned after the DSC call.
Traps
Do not press the distress button to test. Do not speak on channel 70. Do not enter a group MMSI as the ship MMSI. Do not leave position input manual if GNSS is available. Do not confuse a distress relay with a distress alert. Do not assume a Class B VHF DSC satisfies an A1 carriage requirement without checking approval. Do not ignore the five-second distress activation delay; releasing early cancels the alert.