The GMDSS (Global Maritime Distress and Safety System) is a DSC (Digital Selective Calling) system for the navy. To abstract, it permits for a ship:
      * to send out a distress alert or a safety message,
      * to take contact with une coast station.
    It exists a MF/HF system and an other in VHF. Both are decoded here.

   This mode is included into the software MULTIPSK by Patrick F6CTE, there is the description of this mode and its variants as well as the specifications coming from his software :
GMDSS (Global Maritime Distress and Safety System)
Click on the copy of the screen in order to listen to the corresponding audio signal.

Baud rate :
100 in MF/HF and 1200 in VHF. A character is composed of 10 bits, which allows to easily detect an error as the minimum number of bits is 7 (the bits 8, 9 and 10 are parity bits).
Modulation :
FSK two tones (« mark » and « space », "mark" high) with a shift between tones of 170 Hz in MF/HF and 800 Hz in VHF (1300/2100 Hz)
Reception mode : USB in MF/HF and FM in VHF
Character set :
the symbols (of 10 bits) between 00 and 99 correspond to numbers. The symbols between 100 and 127 are used to code service commands. There are no letters.
Shape of pulse :
Bandwidth :
about 800 Hz (due to rectangular shape) in MF/HF and about 3.5 Khz in VHF,
Demodulation :
non coherent
Synchronization :
automatic using the signal
Correction code :
Convolution code :
Parity code :
yes, thanks to a parity symbol taking into account all the information symbols of the message
Interleaving :
each symbol is repeated 5 positions later (that is, for example, 500 ms between 2 transmissions of the same symbol in MF/HF). This diversity in time allows the correction of an erroneous symbol. Its detection is easy due to the particular content of a symbol (10 bits for 127 symbols only).
Pmean/Ppeak :
Lowest S/N :
  about -5 dB in MF/HF and and about +8 dB in VHF (assement)

The mode is described in the recommendation ITU-R M 493-11 and, for the MF/HF system, looks like SITOR B (AMTOR FEC).

A message lasts 6 to 7 seconds for the MF/HF system and about ½ second for the VHF system. There are different types of message. A message is in general composed of a bit synchronisation sequence, a symbol synchronisation sequence, a format specifier, the called ship or coast station identifier, the "category" information (message priority), the identifier of the ship calling, a message with position included, an end of message and an error-check character.

The main frequencies are in MF/HF: 2182.0; 2187.5, 8414.5, 4207.5 kHz, 6312 kHz, 12577.0, 16804.5 Khz with a central AF frequency of 1700 Hz.

In VHF the main frequency is 156.525 MHz (channel 70).

Back to the list of modes included in MULTIPSK