SITOR

Lore & Background

SITOR was developed in the 1960s by Koninklijke TNT Post to improve upon radioteletype (RTTY). While it uses the same FSK modulation as RTTY, SITOR incorporates error detection and correction. It recasts the ITA2 (Baudot) 5-bit code into a 7-bit code called CCIR 476, where each character has exactly four marks (ones) and three spaces (zeros), ensuring a Hamming distance of at least 2 between valid codes and guaranteeing detection of all single-bit errors.

SITOR operates in two modes. SITOR-A is used for point-to-point links and employs automatic repeat request (ARQ): the receiver checks each character and requests retransmission if an error is detected. SITOR-B is used for broadcast links and uses forward error correction (FEC) by transmitting each character twice; if the first copy is garbled, the receiver uses the second, but if both are garbled the character is lost. SITOR-B is also known as narrow-band direct printing (NBDP).

SITOR is commonly used on shortwave bands for maritime information such as weather forecasts and storm warnings. NAVTEX marine weather and safety messages are broadcast using SITOR-B, with NAVTEX layered on top of SITOR-B. In amateur radio, SITOR is called AMTOR (AMateur Teleprinting Over Radio), with AMTOR-A corresponding to SITOR-A and AMTOR-B (or AMTOR-FEC) to SITOR-B. An AMTOR extension described in 1991 added lowercase and other printable ASCII characters.

Reader's Guide

SITOR's significance lies in its practical enhancement of radioteletype for reliable text communication over noisy shortwave channels. By introducing error detection and correction—either through ARQ in SITOR-A or FEC in SITOR-B—it made maritime safety broadcasts and point-to-point messaging far more dependable than standard RTTY. The system's use of the CCIR 476 alphabet, with its built-in single-bit error detection, became a foundation for robust data transmission in environments where signal fading and interference are common.

Its legacy is most visible in maritime applications: SITOR-B is the basis for NAVTEX, which broadcasts weather and safety messages to ships worldwide. The amateur radio community adopted SITOR as AMTOR, extending its life into the 1990s with support for lowercase and ASCII characters. SITOR's design—synchronous framing, 100 baud rate, and 170 Hz FSK shift—remains a reference point for narrow-band digital modes. The system's idling pattern, with synchronization bursts and Morse identifiers, is aurally distinctive and aids in signal identification. SITOR thus represents a key step in the evolution from simple teletype to error-corrected digital communication on HF bands.

Did You Know?

• SITOR uses the CCIR 476 7-bit code, where each character has exactly four marks and three spaces, guaranteeing detection of all single-bit errors.
• SITOR-B transmits each character twice; if both copies are garbled, the receiver cannot determine what was sent.
• In amateur radio, SITOR is called AMTOR, and a 1991 extension added lowercase and other printable ASCII characters.