DSC is, basically, a paging system that is used to automate distress alerts sent over terrestrial (i.e.: non-satellite) VHF, MF and HF marine radio systems.
The DSC system’s digital processing techniques, combined with the relatively narrow receiver bandwidths used, provide a DSC signal with resistance to noise and fading over the radio path. This result in increased range compared with radiotelephone transmissions.
DSC is used to establish initial contact between stations. Following an alert by DSC, communications are normally carried out by radiotelephone or Narrow Band Direct Printing (NBDP – radio telex).
Rather than just indicate that the sending station is in distress, the DSC system allows a great deal more information to be transmitted, including:
- the priority of the call – DISTRESS, URGENCY, SAFETY or ROUTINE;
- the address – ie: all ships or a single ship/station;
- the identification of the ship in distress;
- the position of the ship in distress; and
- the nature of the distress.
DSC may also be used to call another vessel of a Coast Radio Station to establish communication by voice or NBDP.
The ITU has allocated a DSC distress and safety channel in the MF, each of the HF and the VHF marine radio bands. These are:
VHF DSC DISTRESS AND SAFETY CHANNEL: VHF marine channel 70
MF/HF DSC DISTRESS AND SAFETY CHANNELS: 2187.5 4207.5 6312.0 8414.5 12577.0 16804.5 (kHz).
The DSC is normally a part of the VHF and/or MF/HF communication set, but can be supplied separately.