Procedures to cancel false distress alerts vary for the various GMDSS equipments. Given below are the procedures for the various equipments.
VHF
a) switch off transmitter immediately
b) switch equipment on and set to Channel 16
c) make broadcast to “All Stations” giving the ship’s name, call sign and DSC number, and cancel the false distress alert.
Example:
All Stations, All Stations, All Stations
This is NAME, CALL SIGN,
DSC NUMBER, POSITION.
Cancel my distress alert of
DATE, TIME UTC,
= Master NAME, CALL SIGN,
DSC NUMBER, DATE, TIME UTC.
MF
a) switch off equipment immediately.
b) switch equipment on and tune for radiotelephony transmission on 2,182 kHz
c) make broadcast to “All Stations” giving the ship’s name, call sign and DSC number, and cancel the false distress alert.
Example:
All Stations, All Stations, All Stations
This is NAME, CALL SIGN,
DSC NUMBER, POSITION.
Cancel my distress alert of
DATE, TIME UTC,
= Master NAME, CALL SIGN,
DSC NUMBER, DATE, TIME UTC.
HF
Exactly as for MF, but the alert must be cancelled on all the frequency bands on which it was transmitted. Hence, the transmitter should be tuned consecutively to the radiotelephony distress frequencies in the 4, 6, 8, 12 and 16 MHz bands, as necessary.
Inmarsat-C
Notify the appropriate RCC to cancel the alert by sending a distress priority message via the same CES through which the false distress alert was sent.
Example of message
NAME, CALL SIGN, IDENTITY NUMBER,
POSITION,
Cancel my Inmarsat-C distress
alert of DATE, TIME UTC
= Master +
EPIRB
If for any reason an EPIRB is activated accidentally, the ship should contact the nearest coast station or an appropriate coast earth station or RCC and cancel the distress alert.
Notwithstanding the above, ships may use any means available to them to inform the appropriate authorities that a false distress alert has been transmitted and should be cancelled.
No action will normally be taken against any ship or mariner for reporting and cancelling a false distress alert. However, in view of the serious consequences of false alerts, and the strict ban on their transmission, Governments may prosecute in cases of repeated violations.
Note : Appropriate signals should precede these messages in accordance with the ITU Radio Regulations chapter NIX.
This answer applies only when the false alert is detected during transmission.
Ref: Navcen USCG website
