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 and
experimental branch of the program WSJT.
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WSJT-X Version 2.0  offers twelve different protocols  or modes: FST4,
FST4W, FT4, FT8, JT4, JT9, JT65, QRA64, ISCAT, MSK144, WSPR, and Echo.
The first  eight are designed  for making reliable QSOs  under extreme
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
a number of advantages over  JT65, including better performance on the
very weakest signals.   We imagine that over time it  may replace JT65
for EME use.  JT9 was originally designed for the LF, MF, and lower HF
bands.  Its submode JT9A is 2  dB more sensitive than JT65 while using
less than 10%  of the bandwidth. FST4, and  its quasi-beacon companion
mode  FST4W  (see  below),  are  designed for  LF  and  MF  and  offer
significant advantages over JT9 and WSPR  on those bands. 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.  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.
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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.
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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 they can be  decoded at signal-to-noise
ratios as  low as  -28 dB  in a  2500 Hz  bandwidth.  WSPR  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. WSPR has a  new companion
FST4W which has  a similar message content but offers  up to 30 minute
transmission periods for greater sensitivity on LF and MF.
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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.
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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.
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Be sure to read the online WSJT-X User's Guide.
