Description:
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 :
rectangular
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 :
no
Convolution code :
no
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 : 1
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).
