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.
The procedures for DSC communication on HF are - with some additions described below - equal to the corresponding procedures for DSC communications on MF/HF.
Transmission of DSC Distress Alert
DSC distress alert should be sent to coast stations - e.g. in A3 and A4 sea areas on HF - and on MF and/or VHF to other ships in the vicinity.
The DSC distress alert should as far as possible include the ship's last known position and the time (in UTC) it was valid. If the position and time is not inserted automatically from the ship's navigational equipment, it should be inserted manually.
Ship-to-shore Distress Alert
Choice of HF band
Propagation characteristics of HF radio waves for the actual season and time of the day should be taken into account when choosing HF bands for transmission of DSC distress alert.
As a general rule the DSC distress channel in the 8 MHZ maritime band (8414.5 kHz) may in many cases be an appropriate first choice.
Transmission of the DSC distress alert in more than one HF band will normally increase the probability of successful reception of the alert by coast stations.
DSC distress alert may be sent on a number of HF bands in two different ways:
a) either by transmitting the DSC distress alert on one HF band, and waiting a few minutes for receiving acknowledgment by a coast station;
if no acknowledgment is received within 3 min, the process is repeated by transmitting the DSC distress alert on another appropriate HF band etc.;
b) or by transmitting the DSC distress alert at a number of HF bands with no, or only very short, pauses between the calls, without waiting for acknowledgment between the calls.
It is recommended to follow procedure a) in all cases, where time permits to do so; this will make it easier to choose the appropriate HF band for commencement of the subsequent communication with the coast station on the corresponding distress traffic channel.
Transmitting the DSC Alert;
- tune the transmitter to the chosen HF DSC distress channel (4207.5, 6312, 8414.5, 12577, 16804.5 kHz);
- follow the instructions for keying in or selection of relevant information on the DSC equipment keyboard as described earlier;
- transmit the DSC distress alert.
NOTE: Ship-to-ship distress alert should normally be made on MF and/or VHF, using the procedures for transmission of DSC distress alert on MF/HF described earlier.
- Some maritime HF transmitters shall be tuned to a frequency 1700 Hz lower than the DSC frequencies given above in order to transmit the DSC alert on the correct frequency.
In special cases, for example in tropical zones, transmission of DSC distress alert on HF may, in addition to ship-to-shore alerting, also be useful for ship-to-ship alerting.
Preparation for the subsequent distress traffic
After having transmitted the DSC distress alert on appropriate DSC distress channels (HF, MF and/or VHF), prepare for the subsequent distress traffic by tuning the radiocommunication set(s) (HF, MF and/or VHF as appropriate) to the corresponding distress traffic channel(s).
If method b) described above has been used for transmission of DSC distress alert on a number of HF bands:
- take into account in which HF band(s) acknowledgment has been successfully received from a coast station;
- if acknowledgments have been received on more than one HF band, commence the transmission of distress traffic on one of these bands, but if no response is received from a coast station then the other bands should be used in turn.
The distress traffic frequencies are:
Telephony: 4125 6215 8291 12290 16420 Telex: 4177.5 6268 8376.5 12520 16695
Telephony: 2182 Telex: 2174.5
VHF: Channel 16 (156.800 MHZ).
The procedures described earlier are used when the distress traffic on MF/HF is carried out by radiotelephony.
The following procedures shall be used in cases where" the distress traffic on MF/HF is carried out by radiotelex:
- The forward error correcting (FEC) mode shall be used unless specifically requested to do otherwise;
- all messages shall be preceded by:
- at least one carriage return,
- line feed,
- one letter shift,
- the distress signal "MAYDAY";
- The ship in distress should commence the distress telex traffic on the appropriate distress telex traffic channel as follows:
- carriage return, line feed, letter shift, the distress signal "MAYDAY",
- "this is",
- the 9-digit identity and call sign or other identification of the ship,
- the ship's position if not included in the DSC distress alert,
- the nature of distress,
- any other information which might facilitate the rescue.
Actions on Reception of a DSC Distress Alert on HF From Another Ship
Ships receiving a DSC distress alert on HF from another ship shall not acknowledge the alert, but should:
- watch for reception of a DSC distress acknowledgment from a coast station;
- while waiting for reception of a DSC distress acknowledgment from a coast station:
prepare for reception of the subsequent distress communication by tuning the HF radiocommunication set (transmitter and receiver) to the relevant distress traffic channel in the same HF band in which the DSC distress alert was received, observing the following conditions:
- if radiotelephony mode was indicated in the DSC alert, the HF radiocommunication set should be tuned to the radiotelephony distress traffic channel in the HF band concerned;
- if telex mode was indicated in the DSC alert, the HF radiocommunication set should be tuned to the radiotelex distress traffic channel in the HF band concerned. Ships able to do so should additionally watch the corresponding radiotelephony distress channel;
- if the DSC distress alert was received on more than one HF band, the radiocommunication set should be tuned to the relevant distress traffic channel in the HF band considered to be the best one in the actual case. If the DSC distress alert was received successfully on the 8 MHZ band, this band may in many cases be an appropriate first choice;
- if no distress traffic is received on the HF channel within 1 to 2 min, tune the HF radiocommunication set to the relevant distress traffic channel in another HF band deemed appropriate in the actual case;
- if no DSC distress acknowledgment is received from a coast station within 3 min, and no distress communication is observed going on between a coast station and the ship in distress:
- transmit a DSC distress relay alert,
- inform a Rescue Coordination Center via appropriate radiocommunications means.
Transmission of DSC distress relay alert
In case it is considered appropriate to transmit a DSC distress relay alert:
- considering the actual situation, decide in which frequency bands (MF, VHF, HF) DSC distress relay alert(s) should be transmitted, taking into account ship-to-ship alerting (MF, VHF) and ship-to-shore alerting;
- tune the transmitter(s) to the relevant DSC distress channel, following the procedures described above;
- follow the instructions for keying in or selection of call format and relevant information on the DSC equipment keyboard as described earlier;
- transmit the DSC distress relay alert.
Acknowledgment of a HF DSC distress relay alert received from a coast station
Ships receiving a DSC distress relay alert from a coast station on HF, addressed to all ships within a specified area, should NOT acknowledge the receipt of the relay alert by DSC, but by radiotelephony on the telephony distress traffic channel in the same band(s) in which the DSC distress relay alert was received.
Transmission of urgency messages on HF should normally be addressed:
- either to all ships within a specified geographical area,
- or to a specific coast station.
Announcement of the urgency message is carried out by transmission of a DSC call with category urgency on the appropriate DSC distress channel.
The transmission of the urgency message itself on HF is carried out by radiotelephony or radiotelex on the appropriate distress traffic channel in the same band in which the DSC announcement was transmitted.
Transmission of DSC announcement of an urgency message on HF
- choose the HF band considered to be the most appropriate, taking into account propagation characteristics for HF radio waves at the actual season and time of the day; the 8 MHZ band may in many cases be an appropriate first choice;
- tune the HF transmitter to the DSC distress channel in the chosen HF band;
- key in or select call format for either geographical area call or individual call on the DSC equipment, as appropriate;
- in case of area call, key in specification of the relevant geographical area;
- follow the instructions for keying in or selection of relevant information on the DSC equipment keyboard as described earlier, including type of communication in which the urgency message will be transmitted (radiotelephony or radiotelex);
- transmit the DSC call; and
- if the DSC call is addressed to a specific coast station, wait for DSC acknowledgment from the coast station. If acknowledgment is not received within a few minutes, repeat the DSC call on another HF frequency deemed appropriate.
Transmission of the urgency message and subsequent action
- tune the HF transmitter to the distress traffic channel (telephony or telex) indicated in the DSC announcement;
- if the urgency message is to be transmitted using radiotelephony, follow the procedure described in 2. 1;
- if the urgency message is to be transmitted by radiotelex, the following procedure shall be used:
- use the forward error correcting (FEC) mode unless the message is addressed to a single station whose radiotelex identity number is known;
- commence the telex message by:
- at least one carriage return, line feed, one letter shift,
- the urgency signal "PAN PAN",
- "this is",
- the 9-digit identity of the ship and the call sign or other identification of the ship,
- the text of the urgency message.
Announcement and transmission of urgency messages addressed to all HF equipped ships within a specified area may be repeated on a number of HF bands as deemed appropriate in the actual situation.
Reception of an urgency message
Ships receiving a DSC urgency call announcing an urgency message shall NOT acknowledge the receipt of the DSC call, but should tune the radiocommunication receiver to the frequency and communication mode indicated in the DSC call for receiving the message.
The procedures for transmission of DSC safety announcement and for transmission of the safety message are the same as for urgency messages, described for Urgency, except that:
- in the DSC announcement, the category SAFETY shall be used,
- in the safety message, the safety signal "SECURITE" shall be used instead of the urgency signal "PAN PAN".
Public correspondence on HF
The procedures for DSC communication for public correspondence on HF are the same as for MF.
Propagation characteristics should be taken into account when making DSC communication on HF.
International and national HF DSC channels different from those used for DSC for distress and safety purposes are used for DSC for public correspondence.
Ships calling a HF coast station by DSC for public correspondence should preferably use the coast station's national DSC calling channel.
Testing the equipment used for distress and safety on HF
The procedure for testing the ship's equipment used for DSC distress, urgency and safety calls on HF by transmitting DSC test calls on HF DSC distress channels is the same as for testing on the MF DSC distress frequency 2187.5 Hz.