Digital Selective Calling (DSC)
A short digital 'call' used by ships and coast stations, including distress alerts, as part of the global maritime safety system.
What does it sound like?
A brief, fast two-tone warble lasting a few seconds.
Who uses it?
Ships and coast stations worldwide.
Where and when can I hear it?
On shortwave and medium wave, 2187.5, 4207.5, 6312, 8414.5, 12577 and 16804.5 kHz; on VHF, channel 70 (156.525 MHz).
Hear it live
- University of Twente WebSDR (Enschede, the Netherlands) · 8,413 kHz USB
Short bursts of a few seconds at unpredictable times; the channel can be silent for long stretches. The 8414.5 kHz channel is centered in this link's passband.
These are free public receivers run by other people; we only link to them. You'll hear the signal only if it's on the air right now and conditions carry it to that receiver, so if it's quiet, try another time. Links last reviewed 2026-09-28.
The science
On medium and shortwave, DSC uses two tones 170 Hz apart sent at 100 baud; on VHF channel 70 it uses audio tones of 1300 and 2100 Hz at 1200 baud. Each character is a 10-bit code, seven information bits plus three error-detecting check bits, and every character is sent twice with a time delay, so a receiver can rebuild a call even if a burst of noise wipes out one copy. A distress alert carries the vessel's Maritime Mobile Service Identity (MMSI) and, when a GPS is connected, its position.
Why use it?
DSC lets a ship instantly send an automatically formatted distress alert instead of relying on a spoken mayday being heard. The Coast Guard also compares it to the dial and bell of a telephone: radios can call and ring each other, so no one has to sit listening to a speaker.
Lore
Under the SOLAS convention, DSC-equipped radios became required on February 1, 1999, on all passenger ships and most other ships of 300 gross tons and up on international voyages. The Coast Guard urges boaters, in the strongest terms, to connect a GPS to their DSC radio, saying it may save their life in a distress situation.
What do the decoded parts mean?
- Caller and called MMSI: The nine-digit identities of the calling and called stations.
- Category: Routine, safety, urgency or distress.
- Nature and position: For distress alerts: what happened and where.
Easily confused with
- AIS (ship positions), from the same family