Index: trunk/doc/pslib/psLibADD.tex
===================================================================
--- trunk/doc/pslib/psLibADD.tex	(revision 3428)
+++ trunk/doc/pslib/psLibADD.tex	(revision 3429)
@@ -1,6 +1,7 @@
-%%% $Id: psLibADD.tex,v 1.64 2005-03-16 00:29:31 jhoblitt Exp $
+%%% $Id: psLibADD.tex,v 1.65 2005-03-16 01:49:30 jhoblitt Exp $
 \documentclass[panstarrs]{panstarrs}
 
 \usepackage{amsmath}
+\usepackage{verbatim}
 
 % basic document variables
@@ -961,9 +962,44 @@
 \label{sec:ut1}
 
-Univseral Time is a measure of the rotation angle of the Earth.  When
-corrected for polar motion it is referred to as UT1.  This is distict
-from UT0 which does not involve corrections for polar motion.  UT1 may
-be calculated from UTC through a table lookup of the appropriate value
-of UTC - UT1.
+UT1 is directly tied to the rotation of the Earth.  Historically, time has been
+measured with respect to the rising and setting of the Sun. However, in the
+modern era of atomic clocks, the rotation of the Earth makes for a highly
+unstable time standard. Tidal effects, changes in the angular momentum of the
+atmosphere, seasonal changes in the polar ice caps, movement within the Earth's
+core, and other effects all cause measurable changes in the Earth's rotation on
+a daily basis.  However, UT1 is still vitally important for determining the
+orientation of the Earth with respect to the sky.
+
+Since 2003-01-01, UT1 has been defined as directly proportional to the Earth
+Rotation Angle (see IERS Technical Note 32\footnote{IERS Technical Note 32 -
+http://maia.usno.navy.mil/conv2003.html}).  Previous to that date, a different
+definition was in effect (see IERS Technical Note 21\footnote{IERS Technical
+Note 21 - http://maia.usno.navy.mil/conventions.html}).  We will always use the
+post-2003 definition.
+
+UT1 is continuously measured by the International Earth Rotation
+Service\footnote{IERS - http://maia.usno.navy.mil/}, and tabulated values of the
+offset of UT1 from UTC are published at regular intervals, along with predicted
+future values.  IERS Bulletin A gives "rapid response" values necessary for
+real-time and near real-time data analysis (such as Pan-STARRS Otis and IPP
+subsystems). Bulletin B gives the results of a final, definitive data
+reduction.  An amalgam of Bulletin A and B values is published daily on the
+IERS website\footnote{IERS Bulletin A \& B -
+http://maia.usno.navy.mil/ser7/finals2000A.daily} along with a desciption of
+the format\footnote{IERS finals2000A.daily table format -
+http://maia.usno.navy.mil/ser7/readme.finals2000A}.
+
+The UT1 offsets should be interpolated using the prescription of IERS Gazette
+\#13\footnote{IERS Gazette \#13 - http://maia.usno.navy.mil/iers-gaz13}.  This
+entails using a third order polynomial to interpolate the table values, and
+then applying a model for diurnal and semi-diurnal fluctuations due to tidal
+effects.  An example implimentation\footnote{interp.f
+ftp://maia.usno.navy.mil/dist/interp.f} written in Fortran is available.
+
+\verbatiminput{interp.f}
+
+The Polar motion X, and Y coordinates are also important for determining the
+orientation of the Earth with respect to the sky. This is also given in the
+IERS publications references above, and should be interpolated in the same way.
 
 \paragraph{Julian Day and Modified Julian Day}