Editing Digital selective calling (section)

==Distress==
When sending a distress signal, the DSC device will at minimum include the ship's [[MMSI]] number. It may also include the [[geographic coordinate system|coordinates]] if radio is connected to GPS system and, if necessary, the channel for the following [[radiotelephony]] or [[radiotelex]] messages.<ref name=b37 /> 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 [[antenna tuner|retuning]] the [[antenna (radio)|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.<ref name=b38>Bréhaut (2009): 38</ref>

Distress calls can be both non designated and designated. The latter allows one of ten predefined designations to be sent along with the distress signal. These are "abandoning ship", "fire or explosion", "flooding", "[[ship collision|collision]]", "grounding", "listing", "sinking", "disabled and adrift", "piracy or attack" and "[[man overboard]]".<ref name=b38 /> 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.<ref name=b40>Bréhaut (2009): 40</ref> Operators are required to cancel falsely sent distress alerts with a transmission on the channel designated by the distress signal.<ref name=b44>Bréhaut (2009): 44</ref>

A coast station which receives a DSC distress alert will wait 2.75 minutes prior to sending an acknowledgment to allow other vessels in the nearby area to receive the alert. This helps shore stations narrow down where a vessel without GPS is located. 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 coast station range or do not receive an acknowledgment, are required to relay the distress alert by any means to land.<ref name=b40 />