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{{Short description|System for transmitting text messages}}
'''SITOR''' ([[Simplex communication|SImplex]] Teletype Over Radio) is a system for transmitting text messages. It was developed in the 1960s by [[Koninklijke TNT Post]] as an improvement over [[radioteletype]] (RTTY). Although it uses the same [[frequency-shift keying]] (FSK) modulation used by regular RTTY, SITOR uses [[error detection]], redundancy, and/or retransmission to improve reliability. 

There are two SITOR modes:
* SITOR-A is used for [[point-to-point (network topology)|point to point]] links. SITOR-A uses [[automatic repeat request]] (ARQ) to gain reliability. If the receiver detects an error, it requests a retransmission.
* SITOR-B is used for [[Broadcasting (computing)|broadcast]] links. SITOR-B transmits each character in a message twice to gain reliability. If the receiver detects an error in the first character, it uses the copy.  If both characters are garbled, the receiver won't know what was sent.
* SITOR-B by definition uses [[forward error correction]] (FEC), versus ARQ for SITOR-A.

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&nbsp;Hz frequency shift. The high frequency is a mark, and the low frequency is a space.<ref>{{Harvnb|Schetgen|1995|p=???}}</ref>

==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).{{Citation needed|date=April 2020}}

[[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 HTTP is layered on top of TCP.)

[[Amateur radio]] uses SITOR but calls it [[AMTOR]], AMateur Teleprinting Over Radio.<ref>{{Harvnb|Schetgen|1995|p=12.20}}</ref>  AMTOR-A is SITOR-A.  AMTOR-B (also called AMTOR-FEC) is SITOR-B.<ref>{{Harvtxt|Schetgen|1995|p=12.21}} claims groups of 5 characters are sent, but it is probably in error.</ref>  In 1991, an AMTOR extension was described that includes lower case and other printable ASCII characters.<ref>{{Harvnb|Schetgen|1995|pp=12.18&ndash;12.19}}</ref>

==Technical Details==

===Character set===
RTTY uses the ITA2 ([[Baudot code]]) character code. ITA2 is a five bit code with 32&nbsp;possible [[Code point|code points]]. Four code points are used for null ({{sc|BLANK}}), space ({{sc|SPACE}}), carriage return ({{sc|CR}}), and line feed ({{sc|LF}}). Two code points are used for a letter shift ({{sc|LTRS}}) or a figure shift ({{sc|FIGS}}). The remaining 26&nbsp;code points are used for characters in the letters and figures sets. Consequently, ITA2 can represent 2×26&nbsp;= 52&nbsp;additional characters.<ref>ITA2 does not assign all 52&nbsp;characters: The figure shift for {{sc|D}} is a checked box or [[cross pattée|pattée cross]] ({{math| ✠}}). The figure shifts for {{sc|F}}, {{sc|G}}, and {{sc|H}} are not assigned but should not cause errors. See {{nobr|ITU-R M 625-3}} page&nbsp;5. U.S. teletypes assign all 52. Figure shift for {{sc|D}} is "$", for {{sc|F}} is "!", for {{sc|G}} is "&", and for {{sc|H}} is "#". In AMTOR, {{sc|BLANK}} is sometimes used to toggle between the CCIR&nbsp;476 character set and Blank Code extension sets so lower case and other characters can be represented. See ASCII-over-AMTOR at {{Harvtxt|Schetgen|1995|pp=12.18&ndash;12.19.}}</ref>

SITOR recasts ITA2 into a 7&nbsp;bit code called [[CCIR 476]]. Each 7&nbsp;bit character in CCIR&nbsp;476 has 4&nbsp;marks (ones) and 3&nbsp;spaces (zeros).<ref>{{Harvnb|Schetgen|1995|pp=12.20&ndash;12.21, Table&nbsp;30.41 at 30.38}}</ref> Each valid character code has a [[Hamming distance]] of at least&nbsp;2 from every other character. A single-bit error will disrupt the balance of marks and spaces; a second bit error may (or may not) bring the count back to 4&nbsp;marks and 3&nbsp;spaces. Consequently, the CCIR&nbsp;476 alphabet is guaranteed to detect all single bit errors within a character.

===SITOR control characters===
The number of valid binary code values in CCIR&nbsp;476 is the number of ways to choose 4&nbsp;marks for 7&nbsp;bit positions, and the number can be calculated using the [[binomial coefficient]]: <math>\ \textstyle \binom{7}{3} = \binom{7}{4} = 35\ .</math> Thus CCIR&nbsp;476 has 3&nbsp;additional code points over ITA2. SITOR uses the additional code points for idle, phasing, and repeat requests.<ref>{{harvnb|Schetgen|1995|p=12.21}}</ref> In addition, some of the ordinary characters are reused as control signals.<ref>{{nobr|CCIR Rec. 625}} ''Service Information Signals''.</ref>

:{| class="wikitable"
|-
! Code<br/>(bits&nbsp;6543210)
! mode&nbsp;A<br/>(ARQ)
! mode&nbsp;B<br/>(FEC)
|-
| 1100101 ["L"]
| Control signal&nbsp;1 (CS1)
|
|-
| 1101010 [{{sc|NUL}}]
| Control signal&nbsp;2 (CS2)
|
|-
| 1011001 ["N"]
| Control signal&nbsp;3 (CS3)
|
|-
| 0110101 ["G"]
| Control signal&nbsp;4 (CS4)
|
|-
| 1101001 ["H"]
| Control signal&nbsp;5 (CS5)
|
|-
| 0110011
| Idle signal {{mvar|β}}
| Idle signal {{mvar|β}}
|-
| 0001111
| Idle signal {{mvar|α}}
| Phasing signal&nbsp;1
Idle signal {{mvar|α}}
|-
| 1100110
| Signal repetition (RQ)
| Phasing signal&nbsp;2
|}

===SITOR-A===
Transmission in synchronous frames of 450&nbsp;ms. Three characters are transmitted by the Information Sending Station (ISS), which takes 210&nbsp;ms. The ISS then waits 240&nbsp;ms for a response. The Information Receiving Station (IRS) receives the three characters and checks that they each have four marks and three 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 three characters or transmits the next three characters.

===SITOR-B===
{{listen|filename=NMC 8425.ogg|title=SITOR idling pattern|description=Listen to a sample of a SITOR idler pattern, from the U.S. Coast Guard station NMC ([[Point Reyes]]). The letterns "NMC" are also transmitted in Morse.}}

SITOR has an aurally easy to identify idling pattern. [[Synchronization burst]]s 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).

==See also==
* [[AMTOR]]
* [[NAVTEX]]
* [[Radioteletype]]

==References==
{{reflist|30em}}
* {{Citation |editor-last=Schetgen |editor-first=Robert |title=The ARRL Handbook for Radio Amateurs |year=1995 |edition=Seventy-Third (1996) |publisher=The American Radio Relay League |location=Newington, CT |isbn=0-87259-173-5 |url-access=registration |url=https://archive.org/details/arrlhandbookforr00amer }}

==External links==
* [http://www.ominous-valve.com/sitor.html Spectrogram of SITOR-B idle pattern, with description]
* [http://weather.gov/om/marine/hfsitor.htm US Coast Guard SITOR schedules (NOAA)]
* [http://www.sigidwiki.com/wiki/SITOR-A Signal Identification Guide SITOR-A]
* [http://www.sigidwiki.com/wiki/SITOR-B Signal Identification Guide SITOR-B]


[[Category:Quantized radio modulation modes]]