DEFINITION AND SPECIFICATION OF THE FSK441 ENCODING SCHEME USED IN WSJT

By Joe Taylor, K1JT July 29, 2001

WSJT is the name of a computer program. It stands for "Weak Signal communications, by K1JT", and plans call for the program to include more than one communication mode. The first mode implemented, and the one that has given WSJT its wide initial popularity, is called FSK441. It is designed for high speed meteor scatter (HSMS) communication using "pings" reflected from the underdense ionization trails of random meteors at about 100 km height. Such pings are typically a few dB above the noise and last for anywhere from ten to a few hundred milliseconds. 

FSK441 uses Frequency Shift Keying at a baud rate of 441 Hz. Four distinct tones are used, namely 882, 1323, 1764, and 2205 Hz. Each encoded character uses three tone intervals and therefore requires 3/441 seconds (approximatelt 2.3 ms) for its transmission. FSK441 accommodates a potential "alphabet" of 48 characters. The present encoding scheme uses 43 of these characters, the same ones used in the PUA-43 alphabet. The present encoding of each character is defined in the table below. The four tones are labeled 0-3 for the tones 882 through 2205 Hz, in increasing order. The four possible "single-tone" encodings, namely 000, 111, 222, and 333, are reserved in WSJT for special use as shorthand messages. (These characters sent repeatedly amount to pure single-frequency carriers, and their pings are easily recognized by the software and by the human ear.) The present definition of the shorthand messages is respectively "R26", "R27", "RRR", and "73". These messages are very widely used in North American HSMS communications. 

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