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  2.5 offers eleven  different protocols or  modes: FT4,
FT8, JT4,  JT9, JT65, Q65,  FST4, 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  Q65 were designed for EME  (“moonbounce”), but not
limited to just  that propagation path, on the VHF/UHF  bands and JT65
has also proven very effective  for worldwide QRP communication on the
HF bands. Q65  has a number of advantages over  JT65, including better
performance on  the very weakest  signals and variants  with different
T/R period lengths. We imagine that  over time it may replace JT65 for
EME use,  it has  also proved  to be  very effective  for iono-scatter
paths on 6m. 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.  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. 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.  QSOs are possible at signal
levels 10 to 15 dB below  those required for CW. FST4 has similarities
in  use to  JT9 but  offers more  flexibility as  it offers  different
period lengths allowing  QSO completion time to be  traded off against
sensitivity. In  its base form  of FST4-60A it has  better sensitivity
than JT9A  and should be considered  as an upgrade where  JT9 has been
the preferred slow QSO mode.
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Note that even  though their T/R sequences are short,  FT4 and FT8 are
classified as  slow modes because  their message frames are  sent only
once per  transmission. All  fast modes in  WSJT-X send  their message
frames repeatedly,  as many  times as  will fit  into the  Tx sequence
length.
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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`.  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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FST4W, like WSPR,  is a quasi-beacon mode, it targets  LF and MF bands
and offers a number of T/R periods form 2 minutes up to 30 minutes for
the most  challenging weak signal  paths. Similarly to  WSPR reception
reports can be automatically uploaded to the WSPRnet.org web service.
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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.
