Index: /trunk/doc/pslib/psLibADD.tex
===================================================================
--- /trunk/doc/pslib/psLibADD.tex	(revision 1557)
+++ /trunk/doc/pslib/psLibADD.tex	(revision 1558)
@@ -1,3 +1,3 @@
-%%% $Id: psLibADD.tex,v 1.26 2004-08-17 03:09:16 price Exp $
+%%% $Id: psLibADD.tex,v 1.27 2004-08-17 03:18:19 eugene Exp $
 \documentclass[panstarrs]{panstarrs}
 
@@ -694,12 +694,17 @@
 
 Correct time representation is critical in astronomical software.
-PSLib uses the \code{psTime} structure to represent all time
-values.  This structure represents a time which is equivalent to TAI
-(International Atomic Time) and has the following properties:
-\begin{itemize}
-\item it represents both seconds and microseconds
-\item the seconds are continuous (no leap seconds)
-\item the zero reference point is \tbd{1970/01/01,00:00:10} UTC.
-\end{itemize}
+PSLib uses the \code{psTime} structure to represent all time values.
+This structure represents a time which is consists of seconds and
+fractions of seconds in a time system defined by the \code{psTimeType}
+element \code{type}.  Two possible time systems are currently
+available: TAI and UTC.  Both are defined in terms of the reference
+epoch 1970-01-01T00:00:00Z, but with minor modifications for
+leap-seconds as needed.  The first represenatation, TAI (International
+Atomic Time), has seconds of uniform length and no leap seconds.  The
+exact zero reference is 1970/01/01,00:00:10 UTC.  The second
+representations is UTC, which has seconds of uniform length and
+leap-seconds as needed to adjust it to remain within 0.9 seconds of
+the Earth's rotation.  It has a zero-point of exactly
+1970/01/01,00:00:00 UTC.
 
 Julian Day (JD) and Modified Julian Day (MJD) are both continuous time
@@ -758,4 +763,19 @@
 \end{verbatim}
 
+The conversion from a time and longitude to local mean sidereal time
+is performed using the SLA Lib function \code{sla_GMST}.  This
+function requires the value $\Delta$ UT1 = UTC - UT1.  The value of
+$\Delta$ UT1 may be determined from the following site in real time:
+
+\code{ftp://maia.usno.navy.mil/ser7/ser7.dat}
+
+In addition, the long-term values may be determined from the table
+found at: \code{ftp://maia.usno.navy.mil/ser7/finals.all}.  See also
+the web page \code{http://maia.usno.navy.mil/}.  The most significant
+accuracy requirements are for the current value when calculating the
+LST.  For this purpose, the table above (\code{ser7.dat}), which
+provides predictions over a 2 month period, must be made available
+locally to PSLib and updated regularly. 
+
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