Distress
When sending a distress signal, the DSC device will at minimum include the ship's MMSI number. It will also include the coordinates if available and, if necessary, the channel for the following radiotelephony or radiotelex messages. The distress can be sent either as a single-frequency or multi-frequency attempt. 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. If none is received, it will repeat the distress alert up to five times. In a multi-frequency attempt, the distress signal is sent on the MF and all the HF distress frequencies in turn. 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. As the distress message can only be sent on one of the bands, many ships and coast stations may be listening to a band without the message, and will after five minutes relay the distress signal to a coast station.
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". 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. Operators are required to cancel falsely sent distress alerts with a transmission on the channel designated by the distress signal.
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 of coast station range or do not receive a acknowledgment, are required to relay the distress alert by any means to land.
Read more about this topic: Digital Selective Calling
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