Also known as: SITOR, Simplex teleprinting over radio, NAVTEX mode, AMTOR
SITOR — Simplex Teleprinting Over Radio — is an HF maritime radiotelex mode that adds error correction to teleprinter traffic over shortwave.1 Where plain RTTY sends characters with no way to catch a corrupted bit, SITOR is built on the 7-bit CCIR 476 code, a constant-ratio code in which every valid character has exactly four mark bits and three space bits — so any bit flipped in transit breaks the 4:3 ratio and is instantly flagged as an error. It is transmitted as frequency-shift keying on HF.2
How it works
The foundation is the constant-ratio CCIR 476 alphabet. By fixing the number of marks and spaces in every symbol, the code turns simple bit errors into detectable violations: a receiver that gets a character with the wrong mark/space count knows it is corrupt without needing a checksum. SITOR then uses that detection differently in its two modes.
SITOR-A is an ARQ (Automatic Repeat reQuest) mode for a two-way link between a specific pair of stations. The sender transmits short blocks of characters and pauses; the receiver checks each block and sends back a brief acknowledgement, and any block that fails the ratio test is re-requested and sent again. The two stations alternate in a tight handshake, so throughput drops on a poor channel but the delivered text is essentially error-free. This is the mode used for point-to-point radiotelex traffic.
SITOR-B is a forward-error-correction mode for one-way broadcast, where there is no return path to request repeats. Instead of asking again, the transmitter sends each character twice, separated in time — a form of time diversity. A receiver that gets a corrupt copy of a character can fall back on the delayed second copy, so short fades and noise bursts rarely take out both. Because it needs no acknowledgement, one transmitter can serve any number of listeners at once.
Family and relatives
SITOR spawned a family. NAVTEX, the international maritime safety broadcast service on 518 kHz, is simply SITOR-B carrying navigational warnings, weather, and search-and-rescue notices to any ship in range. AMTOR (Amateur Teleprinting Over Radio) is the amateur-radio adaptation of SITOR, using the same CCIR 476 code and the same ARQ/FEC pair of modes. All of them descend from the commercial and maritime radiotelex networks that carried teleprinter traffic over HF before satellite links, and they remain in service for the robustness that constant-ratio coding gives on a noisy shortwave channel.
Decoding it with GopherTrunk
SITOR is an HF, narrowband FSK mode, well outside GopherTrunk’s VHF/UHF land-mobile trunking focus, so GopherTrunk does not decode it. It is documented here because an SDR listener sweeping the marine HF bands will encounter its distinctive sound — the chirping, paused “chirp-chirp-pause” of SITOR-A’s handshake, or the steady stream of SITOR-B/NAVTEX — and because decoding it needs only a general-coverage receiver and a soundcard decoder such as those that read NAVTEX on 518 kHz. Recognising SITOR’s two modes by ear (handshaking pauses versus continuous broadcast) tells you immediately whether you are hearing a two-station radiotelex link or a one-way safety broadcast.
Sources
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SITOR — Wikipedia, for the CCIR 476 constant-ratio code, the SITOR-A ARQ and SITOR-B FEC modes, and the NAVTEX/AMTOR relationships. ↩
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Radioteletype — Wikipedia, for HF radiotelex context and how error-corrected teleprinter modes work over shortwave. ↩