// 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 nearly 2 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; further details can be found in the following section, <>. 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. WSPR and FST4W users with internet access can automatically upload 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.