JT65A

JT65A is a weak-signal digital mode that allows you to pull great DX out from under the noise on the high frequency spectrum. The JT65A communications protocol was conceived and first implemented by Joe Taylor, K1JT. Joe, a Professor Emeritus of physics at Princeton University, shares a Nobel Prize with Russell Alan Hulse (ex-WB2LAV) for the discovery of the first pulsar in a binary system as well as the first confirmation of the existence of gravitational radiation in the amount and with the properties first predicted by Albert Einstein. Joe has contributed to the amateur radio community in much the same way, changing the playing field for weak-signal operation.

JT65A is actually a "sub-mode" of Joe's original JT65 protocol, which he designed to optimize EME contacts on the HF and VHF bands. JT65 includes error-correcting features that make it very robust, even with signals much too weak to be heard. It was later realized that this protocol, with some adaptation, would also be very usable for terrestrial HF communications.

JT65A, 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.

JT65A test sample

 

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