WSJT is a com­put­er pro­gram used for weak-sig­nal radio com­mu­ni­ca­tion between ama­teur radio oper­a­tors. The pro­gram was ini­tial­ly writ­ten by Joe Tay­lor, K1JT, but is now open source and is devel­oped by a small team. The dig­i­tal sig­nal pro­cess­ing tech­niques in WSJT make it sub­stan­tial­ly eas­i­er for ama­teur radio oper­a­tors to employ eso­teric prop­a­ga­tion modes, such as high speed mete­or scat­ter and moon­bounce.

WSJT’s com­mu­ni­ca­tion modes can be divid­ed into fast and slow modes. While fast modes send char­ac­ter-by-char­ac­ter with­out error cor­rec­tion, the slow modes aim to opti­mize for min­i­mal QSO (high-pow­er) use. As of WSJT10, sup­port­ed fast modes are JTMS, FSK441, ISCAT, and JT6M, and the slow modes are JT65 and JT4WSJT‑X 1.8 addi­tion­al­ly imple­ments the “slow” JT9, FT8, and QRA64. Some modes have derived sub­modes with larg­er tone spac­ing.  Two oth­er modes, WSPR and Echo are includ­ed for mea­sur­ing prop­a­ga­tion and test­ing moon bounce echo.

From Wikipedia

WSJT‑X imple­ments com­mu­ni­ca­tion pro­to­cols or “modes” called FT8, JT4, JT9, JT65, QRA64, ISCAT, MSK144, and WSPR, as well as one called Echo for detect­ing and mea­sur­ing your own radio sig­nals reflect­ed from the Moon.  These modes were all designed for mak­ing reli­able, con­firmed QSOs under extreme weak-sig­nal con­di­tions.  All but ISCAT use near­ly iden­ti­cal mes­sage struc­ture and “source encod­ing,” the effi­cient com­pres­sion of stan­dard mes­sages used to make min­i­mal QSOs.  JT65 and QRA64 were designed for EME (“moon­bounce”) on the VHF/UHF bands; JT65 has also proved very pop­u­lar and effec­tive for world­wide QRP com­mu­ni­ca­tion at HFJT9 is opti­mized for the LF, MF, and HF bands.  It  is about 2 dB more sen­si­tive than JT65 while using less than 10% of the band­width.  With either JT9 or JT65, world-wide QSOs are pos­si­ble with pow­er lev­els of a few watts and com­pro­mise anten­nas.  JT4 and QRA64 are opti­mized for EME on the VHF and high­er bands, and espe­cial­ly the microwave bands from 2.3 to 24 GHz.  FT8 is oper­a­tional­ly sim­i­lar to JT65 but is much faster, using T/R cycles only 15 s long. MSK144 is used for Mete­or Scat­ter on the VHF bands. Final­ly, as described more ful­ly on its own page, WSPR mode imple­ments a pro­to­col designed for prob­ing poten­tial prop­a­ga­tion paths with low-pow­er trans­mis­sions.  WSPR is now ful­ly imple­ment­ed with­in WSJT‑X, includ­ing auto­mat­ic band-hop­ping.


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