Also known as: DSC
DSC (Digital Selective Calling) is a maritime protocol for calling specific stations and broadcasting distress alerts. Part of the Global Maritime Distress and Safety System (GMDSS), it sends short FSK data bursts on VHF channel 70 (156.525 MHz) and on MF/HF distress frequencies. Rather than keep a crew listening on a voice channel, DSC lets a radio automatically send or receive a digitally addressed call — including a one-button distress alert carrying the vessel’s identity and position — and only then draw attention to it.1
Overview
A DSC message carries the sender’s and, for selective calls, the recipient’s MMSI, a category (routine, safety, urgency, distress), a message content or nature of distress field, an optional position and time, and a means of continuing the exchange (e.g. the working channel to switch to). A distress alert automatically conveys the vessel identity and, if a GPS receiver is interfaced, its position and the nature of the emergency, then repeats until acknowledged by a coast station. DSC complements AIS on the safety side and is the alerting layer that tells stations when to listen on the marine VHF voice channels.
Physically, VHF DSC uses fast FSK (FFSK) at 1200 baud with mark and space tones of 1300 and 2100 Hz, sent as a phase-continuous audio-frequency shift. MF/HF DSC uses narrow-shift FSK at 100 baud. The protocol has no cyclic redundancy check; instead it relies on time-diversity error control — every character is transmitted twice, once in a forward-position stream and again, delayed, in a receive-position stream, and each 10-bit symbol carries a parity count of its four zero bits. A decoder can therefore correct or flag most single-burst errors by comparing the two copies.
Technical characteristics
| Property | Value |
|---|---|
| Band | VHF Ch 70 (156.525 MHz) + MF/HF DSC frequencies |
| Modulation | FFSK (VHF, 1200 baud) / narrow FSK (MF/HF, 100 baud) |
| Symbol | 10 bits: 7 data + 3-bit zero-count parity |
| Error control | Symbol repetition (time diversity) + parity |
| Addressing | 9-digit MMSI, plus group and all-ships calls |
| Categories | Routine, safety, urgency, distress |
History
DSC was standardised by the ITU in Recommendation ITU-R M.493 and adopted as the calling and distress-alerting core of the GMDSS, which the IMO phased in through the 1990s (fully effective from 1 February 1999). It replaced the older manual watch on distress voice frequencies and, on MF, the human-monitored 2182 kHz and Morse 500 kHz watches, with automated digital alerting that guarantees a distress call reaches a coast station and other nearby vessels.12
Deployment
DSC-capable radios are mandatory on SOLAS vessels and standard on modern recreational VHF sets, where a dedicated red distress button triggers the alert. Coast stations, rescue coordination centres, and other ships maintain an automatic watch on channel 70, so the system works alongside EPIRB 406 beacons and AIS-SART to form the GMDSS alerting chain. Because the bursts are short and unencrypted, DSC traffic is readily received and logged by shore-based SDR listeners.
Decoding it with GopherTrunk
GopherTrunk tunes channel 70 (or an MF/HF DSC frequency), demodulates the FFSK to a symbol stream, aligns to the dot/phasing pattern, applies the symbol-repetition and zero-count error control, and decodes the format, MMSI, category, and position fields into a readable DSC message. Because DSC is unencrypted and the modulation is simple, it is one of the maritime protocols GopherTrunk decodes end to end, alongside AIS. See the DSC decoder page.
Sources
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Digital selective calling — Wikipedia, for the maritime DSC protocol within GMDSS, VHF channel 70 signalling, MMSI addressing, symbol repetition, and distress alerting. ↩ ↩2
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Recommendation ITU-R M.493 — International Telecommunication Union, the primary standard defining the DSC message format, symbol structure, error control, and call categories. ↩