Digital Selective Calling - Equipment, MMSI, Alerting and Watchkeeping
What the exam tests here
DSC is a digital calling system that sends a formatted alert or call to a specific station, a group, or all stations. The exam tests frequencies, the MMSI structure, the distress alert sequence, acknowledgement rules and watchkeeping, and the correct treatment of a false alert.
Frequencies
VHF Ch70 (156.525 MHz) is DSC only and carries no voice. MF DSC is on 2187.5 kHz. HF DSC uses 4207.5, 6312, 8414.5, 12577 and 16804.5 kHz, with NBDP distress on 2174.5 kHz and the associated HF NBDP frequencies.
MMSI structure (nine digits)
- Ship station: MID + 6 digits.
- Coast station: 00 + MID + 4 digits.
- Group: 0 + MID + 5 digits.
- SAR aircraft: 111 + MID + 3 digits.
- AIS-SART: 970xxxxxx; MOB device: 972xxxxxx; AIS-EPIRB: 974xxxxxx.
- Craft associated with a parent ship: 98 + MID + 4 digits. Handheld VHF DSC: 8 + MID + 5 digits.
Distress alerting
Press and hold the distress button for about five seconds. The alert carries the MMSI, position, time and nature of distress (default "undesignated"). The equipment may repeat the alert until acknowledged. A coast station normally acknowledges by DSC and then by radiotelephony; a ship should not send a DSC acknowledgement to another ship's alert, but should acknowledge by voice after about five minutes.
Watchkeeping and testing
Continuous DSC watch is kept on Ch70 and, depending on sea area, on 2187.5 kHz and the HF bands. Never test on Ch70: use the built-in self-test or a routine call to a specific station.
Worked example
A watchkeeper presses the distress button accidentally. Correct action: do not switch off, immediately transmit on Ch16 to the nearest coast station - "This is <name, call sign, MMSI>, I have transmitted a false distress alert at , please cancel" - and enter it in the radio log.
Traps
- Holding the button for one second; nothing is transmitted.
- Entering a stale position or wrong UTC time.
- Testing on Ch70 or using it for routine calls.