// Status=review

JT65 was designed for making minimal QSOs via EME (``moon-bounce'') on
the VHF and UHF bands. A detailed description of the protocol and its
implementation in program _WSJT_ was published in {jt65protocol} for
September-October, 2005. Briefly stated, JT65 uses 60 s T/R sequences
and carefully structured messages. Standard messages are compressed so
that two callsigns and a grid locator can be transmitted in just 71
information bits.  A 72^nd^ bit serves as a flag to indicate that a
message consists of arbitrary text (up to 13 characters) instead of
callsigns and a grid locator.  Special formats allow other information
such as add-on callsign prefixes (e.g., ZA/K1ABC) or numerical signal
reports (in dB) to be substituted for the grid locator. The basic aim
is to compress the most common messages used for minimally valid QSOs
into a minimum fixed number of bits. After compression, a Reed Solomon
(63,12) error-control code converts 72-bit user messages into
sequences of 63 six-bit channel symbols.

JT65 requires tight synchronization of time and frequency between
transmitting and receiving stations. Each transmission is divided into
126 contiguous time intervals or symbols of length 4096/11025 = 0.372
s. Within each interval the waveform is a constant-amplitude sinusoid
at one of 65 pre-defined frequencies. Frequency steps between
intervals are accomplished in a phase-continuous manner. Half of the
channel symbols are devoted to a pseudo-random synchronizing vector
interleaved with the encoded information symbols. The sync vector
allows calibration of time and frequency offsets between transmitter
and receiver. A transmission nominally begins at t = 1 s after the
start of a UTC minute and finishes at t = 47.8 seconds. The
synchronizing tone is at 11025 × 472/4096 = 1270.46 Hz, and is normally
sent in each interval having a “1” in the following pseudo-random
sequence:

 100110001111110101000101100100011100111101101111000110101011001
 101010100100000011000000011010010110101010011001001000011111111

Encoded user information is transmitted during the 63 intervals not
used for the sync tone. Each channel symbol generates a tone at
frequency 11025 × 472/4096 + 11025/4096 × (N+2) × m, where N is the
value of the six-bit symbol, 0 ≤ N ≤ 63, and m is 1, 2, or 4 for JT65
sub-modes A, B, or C.  Sub-mode JT65A is always used at HF.

For EME (but, conventionally, not on the HF bands) the signal report
OOO is sometimes used instead of numerical signal reports. It is
conveyed by reversing sync and data positions in the transmitted
sequence.  Shorthand messages for RO, RRR, and 73 dispense with the
sync vector entirely and use time intervals of 16384/11025 = 1.486 s
for pairs of alternating tones. The lower frequency is always 1270.46
Hz, the same as that of the sync tone, and the frequency separation is
110250/4096 = 26.92 Hz multiplied by n × m, with n = 2, 3, 4 for the
messages RO, RRR, and 73.
