Minor tweaks to User Guide.

git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@7384 ab8295b8-cf94-4d9e-aec4-7959e3be5d79

doc/user_guide/en/vhf-features.adoc

@@ -154,32 +154,38 @@ image::JT65B.png[align="center",alt="JT65B"]
 
 === QRA64
 
-QRA64 is an experimental mode in the Version 1.7 alpha release of
+QRA64 is an experimental mode in Version 1.7 of _WSJT-X_.  The mode is
-_WSJT-X_.  The mode is designed especially for EME on VHF and higher
+designed especially for EME on VHF and higher bands; its operation is
-bands; operation is generally similar to JT65.  The following screen
+generally similar to JT65.  The following screen shot shows an example
-shot shows an example of a QRA64C transmission from DL7YC recorded at
+of a QRA64C transmission from DL7YC recorded at G3WDG over the EME
-G3WDG over the EME path at 24 GHz.  Doppler spread on the path was 78
+path at 24 GHz.  Doppler spread on the path was 78 Hz, so although the
-Hz, so although the signal is reasonably strong its tones are
+signal is reasonably strong its tones are broadened enough to make
-broadened enough to make them hard to see on the waterfall.  The red
+them hard to see on the waterfall.  The red curve shows that the
-curve shows that the decoder has achieved synchronization with a
+decoder has achieved synchronization with a signal at approximately
-signal at approximately 967 Hz.
+967 Hz.
 
 image::QRA64.png[align="center",alt="QRA64"]
 
 The QRA64 decoder makes no use of a callsign database.  Instead, it
-takes advantage of _a priori_ (already known) information such as the
+takes advantage of _a priori_ (AP) information such as the one's own
-one's own callsign and the encoded form of message word `CQ`.  In
+callsign and the encoded form of message word `CQ`.  In normal usage,
-normal usage, as a QSO progresses the available _a priori_ (AP)
+as a QSO progresses the available AP information increases to include
-information increases to include the callsign of the station being
+the callsign of the station being worked and perhaps also his/her
-worked and perhaps also his/her 4-digit grid locator.  The decoder
+4-digit grid locator.  The decoder always begins by attempting to
-always begins by attempting to decode the full message using no AP
+decode the full message using no AP information.  If this attempt
-information.  If this attempt fails, additional attempts are made
+fails, additional attempts are made using available AP information to
-using available AP information to provide initial hypotheses about the
+provide initial hypotheses about the message content.  At the end of
-message content.  At the end of each iteration the decoder computes
+each iteration the decoder computes the extrinsic probability of the
-the extrinsic probability of the most likely value for each of the
+most likely value for each of the message's 12 six-bit information
-message's 12 six-bit information symbols.  A decode is declared only
+symbols.  A decode is declared only when the total probability for all
-when the total probability for all 12 symbols has converged to an
+12 symbols has converged to an unambiguous value very close to 1.
-unambiguous value very close to 1.
+
+TIP: In _WSJT-X_ Version 1.7 QRA64 is different from JT65 in that the
+decoder attempts to find and decode only a single signal in the
+receiver passband.  If many signals are present you may be able to
+decode them by double-clicking on the lowest tone of each one in the
+waterfall.  A multi-decoder like those for JT65 and JT9 has not
+yet been written.
 
 === ISCAT