JT65, devel­oped and released in late 2003, is intend­ed for extreme­ly weak but slow­ly vary­ing sig­nals, such as those found on tro­poscat­ter or Earth-Moon-Earth (EME, or “moon­bounce”) paths.  It can decode sig­nals many deci­bels below the noise floor in a 2500 Hz band (note that SNR in a 2500 Hz band is approx­i­mate­ly 28 dB low­er than SNR in a 4 Hz band, which is clos­er to the chan­nel band­width of an indi­vid­ual JT65 tone), and can often allow ama­teurs to suc­cess­ful­ly exchange con­tact infor­ma­tion with­out sig­nals being audi­ble to the human ear. Like the oth­er modes, mul­ti­ple-fre­quen­cy shift key­ing is employed; unlike the oth­er modes, mes­sages are trans­mit­ted as atom­ic units after being com­pressed and then encod­ed with a process known as for­ward error cor­rec­tion (or “FEC”). The FEC adds redun­dan­cy to the data, such that all of a mes­sage may be suc­cess­ful­ly recov­ered even if some bits are not received by the receiv­er. (The par­tic­u­lar code used for JT65 is Reed-Solomon.) Because of this FEC process, mes­sages are either decod­ed cor­rect­ly or not decod­ed at all, with very high prob­a­bil­i­ty. After mes­sages are encod­ed, they are trans­mit­ted using MFSK with 65 tones.

Oper­a­tors have also begun using the JT65 mode for con­tacts on the HF bands, often using QRP (very low trans­mit pow­er); while the mode was not orig­i­nal­ly intend­ed for such use, its pop­u­lar­i­ty has result­ed in sev­er­al new fea­tures being added to WSJT in order to facil­i­tate HF oper­a­tion.


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