15 May 2015

SITOR FEC (SITOR-B)


The SITOR-FEC mode is used in broadcast applications where a return channel is not required or unavailable. Two sub-modes are in use: General broadcasting to a number of stations and selective addressing of only one station. SITOR-FEC transmissions are used in maritime radio communication to distribute traffic lists, news, safety and weather information (Navtex and DSC-GMDSS). Amateurs also use the FEC mode for calls to all (CQ calls). The selective FEC is used when the receiving station must operate under "radio silence" conditions and hence cannot transmit.


In most cases SITOR-FEC transmissions uses a rate of 100 Baud using the standard shift of 170 Hz. SITOR recasts ITA2 into a 7 bit code called CCIR 476. Each 7-bit character in CCIR 476 has 4 marks (ones) and 3 spaces (zeros). Each character is transmitted twice with an interval of 35 bits. In the case of error-free reception of both characters, the character is displayed on the screen. No character is displayed if both characters are received in error.

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