SITOR SITOR ( SImplex Teletype Over Radio ) is a system for transmitting text messages . Although it uses the same frequency - shift keying ( FSK ) modulation used by regular radioteletype ( RTTY ) , SITOR uses error detection , redundancy , and / or retransmission to improve reliability . There are two SITOR modes : SITOR sends 7 - bit characters as a bit stream at 100 baud ( which , in this case , is 100 bits per second , 10 milliseconds per bit , or 70 milliseconds per character ) . The bitstream is FSK modulated with a 170 Hz frequency shift . The high frequency is a mark , and the low frequency is a space . Applications . SITOR is used commonly on shortwave bands , where it is used to transmit maritime - related information such as weather forecasts and storm warnings . SITOR - B is used for narrow - band direct printing ( NBDP ) . NAVTEX marine weather and safety messages are broadcast using SITOR - B . The NAVTEX messages have a specific format that is interpreted by NAVTEX receivers . ( NAVTEX is layered on top of SITOR - B just as HTML is layered on top of TCP . ) Amateur radio uses SITOR but calls it AMTOR , AMateur Teleleprinting Over Radio . AMTOR - A is SITOR - A . AMTOR - B ( also called AMTOR - FEC ) is SITOR - B . In 1991 , an AMTOR extension was described that includes lower case and other printable ASCII characters . Technical Details . Character set . RTTY uses the ITA2 ( Baudot code ) character code . ITA2 is a five bit code with 32 possible code points . Four code points are used for null ( BLANK ) , space ( SPACE ) , carriage return ( CR ) , and line feed ( LF ) . Two code points are used for a letter shift ( LTRS ) or a figure shift ( FIGS ) . The remaining 26 code points are used for characters in the letters and figures sets . Consequently , ITA2 can represent 52 ( 2 * 26 ) additional characters . 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 valid character code has a Hamming distance of at least 2 from every other character . A one bit error will disrupt the balance of marks and spaces ; a second bit error may ( or may not ) bring the count back to 4 marks and 3 spaces . Consequently , the CCIR 476 alphabet is guaranteed to detect all single bit errors within a character . The number of valid characters in CCIR 476 is the number of ways to choose 4 marks for 7 bit positions , and the number can be calculated using the binomial coefficient : formula _ 1 . Thus CCIR 476 has 3 additional code points over ITA2 . SITOR uses the additional code points for idle , phasing , and repeat requests . . In addition , some of the ordinary characters are reused as Control Signals . ( CCIR Rec . 625 Service Information Signals . ) SITOR - A . Transmission in synchronous frames of 450ms . Three characters are transmitted by Information Sending Station ( ISS ) . Takes 210 milliseconds . The ISS then waits 240 ms for a response . Information Receiving Station ( IRS ) receives the three characters and checks that they each have 4 marks and 3 spaces . If they do , then the IRS transmits an acknowledgement . If they don ' t , then the IRS requests retransmission . At the beginning of the next frame , the ISS either retransmits the last 3 characters or the next three characters . SITOR - B . SITOR has an aurally easy to identify idling pattern . Synchronization bursts are transmitted every second or so and last for slightly more than one second . Every few sync bursts , a Morse identifier of three letters is transmitted ( for example NMO in Honolulu ) .