Digital Selective Calling

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A Sailor VHF DSC unit





Often, ships use separate VHF DSC and MF/HF DSC controllers. For VHF, DSC has its own dedicated Receiver for monitoring Channel 70, but uses the main VHF transceiver for transmission. However, for the user, the controller is often a single unit.[1] MF/HF DSC devices monitor multiple distress, urgency and safety bands in the 2, 4, 6, 8, 12 and 16 MHz bands. At minimum, controllers will monitor 2187.5 kHz and 8414.5 kHz and one more band.[3]



Distress calls can be both non designated and designated. The latter allows one of ten pre-defined designations to be sent along with the distress signal. These are "abandoning ship", "fire or explosion", "flooding", "collision", "grounding", "listing", "sinking", "disabled and adrift", "piracy or attack" and "man overboard".[4] To avoid false distress alerts, distress buttons normally have protective covers, often with a spring-loaded cover so two hands need to be used simultaneously. Alternatively, some devices have two-button systems.[5] Operators are required to cancel falsely sent distress alerts with a transmission on the channel designated by the distress signal.[6]

A coast station which receives a DSC distress alert will immediately send an acknowledgment. The sending device will then both stop repeating the alert, and tune to the designated channel for the distress message to be sent. Ships receiving a distress alert who are outside coast station range or do not receive an acknowledgment, are required to relay the distress alert by any means to land.[5]

Other priorities[edit]

Class A devices, used on commercial ships, have the ability to send distress, distress relay, all ships urgency, all ships safety, individual, group, geographic area and telephone alerts. Class D devices, used for most leisure vessels, can send distress, all ships urgency, all ships safety and individual on channels 06, 08, 72 and 77. The latter is only required to have one antenna and is thus not required to watch Channel 70 when in use. For routine alerts, which are used to establish communication with another station on a working channel, the receiver acknowledges to confirm that communication can be done on the appropriate channel.[7]

Technical details[edit]

The DSC is a synchronous system using characters composed from a ten-bit error detecting code. The bits are encoded using frequency shift keying at 100 Baud. For High Frequency and Medium Frequency two tones 170 Hz apart either side of the allocated frequency are used. For VHF the two frequencies used are 1300 and 2100 Hz. Each character is transmitted twice with a time delay. The detailed specification is published in the International Telecommunications Union recommendation ITU-R M.493, revision 13 being the most recent.[8]


  2. ^ Payne (2006): 4
  3. ^ a b Bréhaut (2009): 37
  4. ^ a b Bréhaut (2009): 38
  5. ^ a b Bréhaut (2009): 40
  6. ^ Bréhaut (2009): 44
  7. ^ Bréhaut (2009): 43
  8. ^ "Recommendation M.493: Digital selective-calling system for use in the maritime mobile service". International Telecommunications Union. 2010.