The following procedures were adapted from the International Telecommunications Union, Radiocommunications Study Group Recommendation M.541, Operational Procedures for the use of Digital Selective-Calling (DSC) Equipment in the Maritime Mobile Service.

Operating procedures may vary somewhat among different radios, depending upon how the radio is designed, how the radio's software is configured, and how the DSC processor is connected to the radio transceiver.

Transmission of DSC Distress Alert

A distress alert should be transmitted if, in the opinion of the Master, the ship or a person is in distress and requires immediate assistance.

A DSC distress alert should as far as possible include the ship's last known position and the time (in UTC) when it was valid. The position and the time may be included automatically by the ship's navigational equipment or may be inserted manually.

The DSC distress alert is transmitted as follows:

1. tune the transmitter to the DSC distress channel (2 187.5 kHz on MF, channel 70 on VHF)
2. if time permits, key in or select on the DSC equipment keyboard
   • the nature of distress,
   • the ship's last known position (latitude and longitude),
   • the time (in UTC) the position was valid,
   • type of subsequent distress communication (telephony),

   in accordance with the DSC equipment manufacturer's instructions;

3. transmit the DSC distress alert
4. prepare for the subsequent distress traffic by tuning the transmitter and the radiotelephony receiver to the distress traffic channel in the same band, i.e. 2 182 kHz on MF, channel 16 on VHF, while waiting for the DSC distress acknowledgment.

NOTE - Some maritime MF radiotelephony transmitters shall be tuned to a frequency 1700 Hz lower than 2187.5 kHz, i.e. 2185.8 kHz, in order to transmit the DSC alert on 2187.5 kHz.

Actions on Receipt of a Distress Alert

Ships receiving a DSC distress alert from another ship should normally not acknowledge the alert by DSC since acknowledgment of a DSC distress alert by use of DSC is normally made by coast stations only.

Only if no other station seems to have received the DSC distress alert, and the transmission of the DSC distress alert continues, the ship should acknowledge the DSC distress alert by use of DSC to terminate the call. The ship should then, in addition, inform a coast station or a coast earth station by any practicable means.

Ships receiving a DSC distress alert from another ship should also defer the acknowledgment of the distress alert by radiotelephony for a short interval, if the ship is within an area covered by one or more coast stations, in order to give the coast station time to acknowledge the DSC distress alert first.

Ships receiving a DSC distress alert from another ship shall:

1. watch for the reception of a distress acknowledgment on the distress channel (2187.5 kHz on MF and channel 70 on VHF);
2. prepare for receiving the subsequent distress communication by tuning the radiotelephony receiver to the distress traffic frequency in the same band in which the DSC distress alert was received, i.e. 2182 kHz on MF , channel 16 on VHF;
3. acknowledge the receipt of the distress alert by transmitting the following by radiotelephony on the distress traffic frequency in the same band in which the DSC distress alert was received, i.e. 2182 kHz on MF, channel 16 on VHF:
   • "MAYDAY",
   • the 9-digit identity of the ship in distress, repeated 3 times,
   • "this is",
   • the 9-digit identity or the call sign or other identification of own ship, repeated 3 times,
   • "RECEIVED MAYDAY".

NOTE - Ships out of range of a distress event or not able to assist should only acknowledge if no other station appears to acknowledge the receipt of the DSC distress alert.

Distress Traffic

On receipt of a DSC distress acknowledgment the ship in distress should commence the distress traffic by radiotelephony on the distress traffic frequency (2182 kHz on MF, channel 16 on VHF) as follows:

1. "MAYDAY",
2. "this is",
3. the 9-digit identity and the call sign or other identification of the ship,
4. the ship's position in latitude and longitude or other reference to a known geographical location,
5. the nature of distress and assistance wanted,
6. any other information which might facilitate the rescue.

Transmission of a DSC Distress Relay Alert

A ship knowing that another ship is in distress shall transmit a DSC distress relay alert if

• the ship in distress is not itself able to transmit the distress alert,
• the Master of the ship considers that further help is necessary.

The DSC distress relay alert is transmitted as follows:

1. tune the transmitter to the DSC distress channel (2187.5 kHz on MF, channel 70 on VHF),
2. select the distress relay call format on the DSC equipment, key in or select on the DSC equipment keyboard:
   • All Ships Call or the 9-digit identity of the appropriate coast station,
   • the 9-digit identity of the ship in distress, if known,
   • the nature of distress,
   • the latest position of the ship in distress, if known,
   • the time (in UTC) the position was valid (if known),
   • type of subsequent distress communication (telephony);
   • transmit the DSC distress relay call,
3. prepare for the subsequent distress traffic by tuning the transmitter and the radiotelephony receiver to the distress traffic channel in the same band, i.e. 2182 kHz on MF and channel 16 on VHF, while waiting for the DSC distress acknowledgment.

Acknowledgment of a DSC Distress Relay Alert Received From a Coast Station

Coast stations, after having received and acknowledged a DSC distress alert, may if necessary, retransmit the information received as a DSC distress relay call, addressed to all ships, all ships in a specific geographical area, a group of ships or a specific ship.

Ships receiving a distress relay call transmitted by a coast station shall not use DSC to acknowledge the call, but should acknowledge the receipt of the call by radiotelephony on the distress traffic channel in the same band in which the relay call was received, i.e. 2182 kHz on MF, channel 16 on VHF.

Acknowledge the receipt of the distress alert by transmitting the following by radiotelephony on the distress traffic frequency in the same band in which the DSC distress relay alert was received:

1. "MAYDAY",
2. the 9-digit identity or the call sign or other identification of the calling coast station,
3. "this is",
4. the 9-digit identity or call sign or other identification of own ship,
5. "RECEIVED MAYDAY".

Acknowledgment of a DSC Distress Relay Alert Received From Another Ship

Ships receiving a distress relay alert from another ship shall follow the same procedure as for acknowledgment of a distress alert, given above.

Cancellation of an Inadvertent Distress Alert (Distress Call)

A station transmitting an inadvertent distress alert shall cancel the distress alert using the following procedure:

1. Immediately transmit a DSC "distress acknowledgment" in accordance with Recommendation ITU-R M.493, 8.3.1 e.g. with own ship's MMSI inserted as identification of ship in distress. (NOTE: This feature is not yet generally available on DSC-equipped radios)
2. Cancel the distress alert aurally over the telephony distress traffic channel associated with each DSC channel on which the "distress call" was transmitted.
3. Monitor the telephony distress traffic channel associated with the DSC channel on which the distress was transmitted, and respond to any communications concerning that distress alert as appropriate.