Digital selective calling

[3] However for automated monitoring a second, receive-only antenna is often needed (especially on non-commercial leisure boats) since a separate tuner is used apart from the main one;[4][5] this is separate from programming radios to monitor user-defined DSC frequencies (which would use the main antenna).

[6][7] When sending a distress signal, the DSC device will at minimum include the ship's MMSI number.

It may also include the coordinates if radio is connected to GPS system and, if necessary, the channel for the following radiotelephony or radiotelex messages.

In the former, a distress signal is sent on one band and the system will wait up to four minutes for a DSC acknowledgment from a coast station.

As this requires retuning the antenna for each sending, without waiting for an acknowledgment, a multi-frequency attempt should only be done if there are only a few minutes until the ship's batteries are under water.

These are "abandoning ship", "fire or explosion", "flooding", "collision", "grounding", "listing", "sinking", "disabled and adrift", "piracy or attack" and "man overboard".

The sending device will then both stop repeating the alert, and tune to the designated channel for the distress message to be sent.

On both class A and D you can be directed to Ch 06, 08, 72, 77 or other simplex channel for the follow-up RT (radio telephony [voice]) call.

[6][7][13] The DSC is a synchronous system using characters composed from a ten-bit error detecting code.