Digital Selective Calling (DSC) Operations
DSC questions test the alert contents, the correct distress frequency, the acknowledgement rules, and your ability to avoid and cancel false alerts.
Frequencies and MMSI
VHF Ch 70 (156.525 MHz); MF 2187.5 kHz; HF 4207.5, 6312, 8414.5, 12577 and 16804.5 kHz. MMSI: ship 9 digits with a MID; coast station 00MIDxxxxx; group 0MIDxxxxx. Ch 70 carries no voice.
Distress alert contents
Format = distress; MMSI; nature of distress; position and time (from GNSS); subsequent communication (radiotelephony); and, where fitted, the type of distress. Send on one distress frequency, then prepare to follow up by radiotelephony on the associated frequency (Ch 16, 2182 kHz or the HF voice pair).
Acknowledgements
Coast stations acknowledge a DSC distress alert by DSC. Ship stations should not acknowledge a DSC distress alert by DSC - they acknowledge by radiotelephony on the distress traffic frequency, and only when appropriate. If no acknowledgement arrives, the accepted GOC guidance is to allow about 1 minute on VHF and 3 minutes on MF/HF before retransmitting or using another means.
Relays, tests and cancellation
A distress relay (all ships or selective) is sent only when the distressed station cannot transmit, or when a station not in distress has information and no acknowledgement has been heard. Routine test calls use the test format to a coast station, never the distress format. A false alert is cancelled by a DSC "distress alert cancellation" to all ships plus a radiotelephony announcement, and the coast station/RCC is informed.
Worked example
You see a DSC distress alert on 2187.5 kHz from a vessel 40 nm away; no coast station acknowledgement appears. Action: note MMSI, position and nature; keep 2182 kHz watch; acknowledge by radiotelephony on 2182 kHz; inform the nearest coast station; stand by. Do not send a DSC acknowledgement from your ship.
Traps
- Sending a DSC distress alert on 8414.5 kHz and omitting the radiotelephony follow-up.
- Using Ch 70 for voice or for routine calling.
- Sending a distress relay when the alert is already acknowledged and assistance is under way.
- Leaving a false alert uncancelled "because the RCC will see it anyway".