// Status=review

_WSJT-X_ is a computer program designed to facilitate basic amateur
radio communication using very weak signals. The first four letters in
the program name stand for "`**W**eak **S**ignal communication by
K1**JT**,`" while the suffix "`*-X*`" indicates that _WSJT-X_ started
as an extended branch of an earlier program, _WSJT_, first released in
2001.  Bill Somerville, G4WJS, and Steve Franke, K9AN, have been major
contributors to development of _WSJT-X_ since 2013 and 2015, respectively.

_WSJT-X_ Version {VERSION_MAJOR}.{VERSION_MINOR} offers twelve
different protocols or modes: *FST4*, *FT4*, *FT8*, *JT4*, *JT9*,
*JT65*, *QRA64*, *ISCAT*, *MSK144*, *WSPR*, *FST4W*, and *Echo*.  The
first seven are designed for making reliable QSOs under weak-signal
conditions. They use nearly identical message structure and source
encoding.  JT65 and QRA64 were designed for EME ("`moonbounce`") on
the VHF/UHF bands and have also proven very effective for worldwide
QRP communication on the HF bands.  QRA64 has some advantages over
JT65, including better performance for EME on the higher microwave
bands.  JT9 was originally designed for the HF and lower bands.  Its
submode JT9A is 1 dB more sensitive than JT65 while using less than
10% of the bandwidth.  JT4 offers a wide variety of tone spacings and
has proven highly effective for EME on microwave bands up to 24 GHz.
These four "`slow`" modes use one-minute timed sequences of
alternating transmission and reception, so a minimal QSO takes four to
six minutes — two or three transmissions by each station, one sending
in odd UTC minutes and the other even.  FT8 is operationally similar
but four times faster (15-second T/R sequences) and less sensitive by
a few dB.  FT4 is faster still (7.5 s T/R sequences) and especially
well-suited for radio contesting.  FST4 was added to _WSJT-X_ in
version 2.3.0.  It is intended especially for use on the LF and MF
bands, and already during its first few months of testing
intercontinental paths have been spanned many times on the 2200 and
630 m bands.  Further details can be found in the following section,
<<NEW_FEATURES,New Features in Version 2.3.0>>.  On the HF bands,
world-wide QSOs are possible with any of these modes using power
levels of a few watts (or even milliwatts) and compromise antennas.
On VHF bands and higher, QSOs are possible (by EME and other
propagation types) at signal levels 10 to 15 dB below those required
for CW.

*ISCAT*, *MSK144*, and optionally submodes *JT9E-H* are "`fast`"
protocols designed to take advantage of brief signal enhancements from
ionized meteor trails, aircraft scatter, and other types of scatter
propagation. These modes use timed sequences of 5, 10, 15, or 30 s
duration.  User messages are transmitted repeatedly at high rate (up
to 250 characters per second for MSK144) to make good use of the
shortest meteor-trail reflections or "`pings`".  ISCAT uses free-form
messages up to 28 characters long, while MSK144 uses the same
structured messages as the slow modes and optionally an abbreviated
format with hashed callsigns.  

Note that some of the modes classified as slow can have T/R sequence
lengths as short the fast modes.  "`Slow`" in this sense implies
message frames being sent only once per transmission.  The fast modes
in _WSJT-X_ send their message frames repeatedly, as many times as
will fit into the Tx sequence length.

*WSPR* (pronounced "`whisper`") stands for **W**eak **S**ignal
**P**ropagation **R**eporter.  The WSPR protocol was designed for
probing potential propagation paths using low-power transmissions.
WSPR messages normally carry the transmitting station’s callsign,
grid locator, and transmitter power in dBm, and with two-minute
sequences they can be decoded at signal-to-noise ratios as low
as -31 dB in a 2500 Hz bandwidth. *FST4W* is designed for
similar purposes, but especially for use on LF and MF bands.
It includes optional sequence lengths as long as 30 minutes and
reaches sensitivity tresholds as low as -45 dB.  Users
with internet access can automatically upload WSPR and FST4W
reception reports to a central database called {wsprnet} that
provides a mapping facility, archival storage, and many other
features.

*Echo* mode allows you to detect and measure your own station's echoes
from the moon, even if they are far below the audible threshold.

_WSJT-X_ provides spectral displays for receiver passbands as wide as
5 kHz, flexible rig control for nearly all modern radios used by
amateurs, and a wide variety of special aids such as automatic Doppler
tracking for EME QSOs and Echo testing.  The program runs equally well
on Windows, Macintosh, and Linux systems, and installation packages
are available for all three platforms.

*Version Numbers:* _WSJT-X_ release numbers have major, minor, and
patch numbers separated by periods: for example, _WSJT-X_ Version
2.1.0.  Temporary _beta release_ candidates are sometimes made in
advance of a new general-availability release, in order to obtain user
feedback.  For example, version 2.1.0-rc1, 2.1.0-rc2, etc., would
be beta releases leading up to the final release of v2.1.0.
Release candidates should be used _only_ during a short testing
period. They carry an implied obligation to provide feedback to the
program development group.  Candidate releases should not be used on
the air after a full release with the same number is made.
