Wednesday, June 24, 2015

SITOR and AMTOR

I had no idea this existed. SITOR (SImplex Teletype Over Radio) is a radio system for broadcasting text messages. Apparently Twitter isn't far reaching enough. Though it uses the same frequency-shift keying (FSK) modulation as a regular radio teletype, SITOR is more advanced. It has redundancy and error detections protocols built-in. The national weather service, NOAA and the U.S. Coast Guard (USCG) both  still use the system. If you ever scanned past  the signal on shortwave you might mistake it for crickets.

NOAA uses 24 transmitters at 7 cost guard stations to communicate it's weather messages. Originally developed as a commercial maritime radio service.. this is all that's left. It sends sends 7-bit characters as a bit stream at rate of 100 baud. At 100 bits per second that works out to 10 milliseconds per bit, or 70 milliseconds per character. The bitstream is FSK (frequency-shift key) modulated with a 170 Hz frequency shift. There are two SITOR modes:
SITOR-A is used for 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 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.


Actually in 2007 the USCG was freely admitting the system was becoming hard to maintain and parts hard to find. Specifically an unnamed official said the system had "exceeded it's life expectancy." Desipte that assessment 8 years ago, Amateur radio still uses SITOR under the name AMTOR, AMateur Teleleprinting Over Radio