Changeset 3428

Timestamp:

Mar 15, 2005, 2:29:31 PM (21 years ago)

Author:

jhoblitt

Message:

misc. Time cleanups

Location:

trunk/doc/pslib

Files:

• ChangeLogADD.tex (modified) (1 diff)
• psLibADD.tex (modified) (9 diffs)

trunk/doc/pslib/ChangeLogADD.tex

@@ -27 +27 @@
   \code{PS_RESAMPLE_LAGRANGE}.
 \item Added section on FITS WCS.
+\item Renamed TDT to TT plus misc. cleanups in the Time section.
 \end{itemize}

trunk/doc/pslib/psLibADD.tex

@@ -1 @@
-%%% $Id: psLibADD.tex,v 1.63 2005-02-14 21:06:55 eugene Exp $
+%%% $Id: psLibADD.tex,v 1.64 2005-03-16 00:29:31 jhoblitt Exp $
 \documentclass[panstarrs]{panstarrs}
 
@@ -783 +783 @@
 \subsubsection{Time}
 
-Correct time representation is critical in astronomical software.
-PSLib uses the \code{psTime} structure to represent 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 representation 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.
+Correct time representation is \emph{critical} in astronomical software.  PSLib
+uses the \code{psTime} structure to represent 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 (SI seconds) and no leap seconds.
+The exact zero reference is ``1970-01-01T00:00:10Z'' UTC.  The second
+representation 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 ``1970-01-01T00:00:00Z'' UTC.
 
 \paragraph{Coordinated Universal Time (UTC)}
@@ -803 +802 @@
 insertion of leap second whenever UTC-UT1 $\ge$ 0.9s.  By definition
 UTC-TAI is an integer number of seconds.  UTC went into effect on
-"1972-01-01T00:00:00" and is defined as being TAI-UTC = 10s on that
+``1972-01-01T00:00:00Z'' and is defined as being TAI-UTC = 10s on that
 date.  For dates prior to 1972-01-01 a fixed offset of 10s relative
 to TAI will be assumed.
@@ -811 +810 @@
 \end{equation}
 
-Leapseconds are declared by the International Earth Rotation and
-Reference Systems Service (IERS).  Leapseconds only occur in the UTC
-time system and cannot be accurately predicted due to variations in
-the Earth's rotational period.  To determine the number of leapsecond
-in a given UTC date a table of leapseconds as annouced by the IERS
-must be consulted.  This table will have to be updated each time a new
-leapsecond occurs.
-
-For ease of conversion, UTC should be represented as the number of
-seconds since the UNIX epoch of "1970-01-01T00:00:00".
+Leapseconds are declared by the International Earth Rotation and Reference
+Systems Service (IERS).  Leapseconds only occur in the UTC time system and
+cannot be accurately predicted due to variations in the Earth's rotational
+period.  To determine the number of leapsecond in a given UTC date a table of
+leapseconds as annouced by the IERS must be consulted.  This table will have to
+be updated each time a new leapsecond occurs.
+
+For ease of conversion, UTC should be represented as the number of seconds
+since the UNIX epoch of ``1970-01-01T00:00:00Z''.  \emph{Times will always be
+expressed in the 'UTC' timezone.  Use of the local timezone is forbidden.}
 
 \paragraph{International Atomic Time (TAI)}
@@ -982 +981 @@
 \end{verbatim}
 
-$2451545.0$ JD $= 51544.5$ MJD is equivalent to "2000-01-01T00:00:00".
+$2451545.0$ JD $= 51544.5$ MJD is equivalent to ``2000-01-01T00:00:00Z''.
 
 \begin{equation}
@@ -988 +987 @@
 \end{equation}
 
-\paragraph{Terrestrial Dynamical Time (TDT)}
-
-Terrestrial Dynamical Time (TDT) is defined as a fixed offset from
-TAI.  Its only purpose as far as we are concerned is for its utility
-in obtaining the GMST.
-
-\begin{equation}
-{\rm TDT} = {\rm TAI} + 32.184{\rm s}
-\end{equation}
-
-\paragraph{TDT as Julian Centuries since J2000.0}
-
-The algorithm for calulating GMST requires TDT formated in Julian centruies
+\paragraph{Terrestrial Time (TT)}
+
+Terrestrial Time (TT) is defined as a fixed offset from TAI.
+
+\begin{equation}
+{\rm TT} = {\rm TAI} + 32.184{\rm s}
+\end{equation}
+
+\paragraph{TT as Julian Centuries since J2000.0}
+
+The algorithm for calulating GMST requires TT formated in Julian centruies
 since the J2000.0 epoch.
 
-\begin{equation}
-t_u = \frac{{\rm JD}_{\rm TDT} - 2451545.0}{36525}
+\begin{equation} t_u = \frac{{\rm JD}_{\rm TT} - 2451545.0}{36525}
 \end{equation}
 
 \paragraph{UT1 as Julian Centuries since J2000.0}
 
-The algorithm for calulating GMST requires UT1 be formated in Julian
-centuries since the J2000.0 epoch.
+The algorithm for calulating GMST requires UT1 be formated in Julian centuries
+since the J2000.0 epoch.
 
 \begin{equation}
@@ -1018 +1014 @@
 \paragraph{Greenwich Mean Sidereal Time (GMST)}
 
-Greenwich Mean Sidereal Time (GMST) is caclulated from UT1 and TDT.
-This algorithm requires that both time inputs are expressed as Julian
-centuries since J2000.0.
+Greenwich Mean Sidereal Time (GMST) is caclulated from UT1 and TT.  This
+algorithm requires that both time inputs are expressed as Julian centuries
+since J2000.0 \footnote{Expressions to implement the IAU 2000 definition of UT1
+- http://www.edpsciences.org/articles//aa/abs/2003/30/aa3487/aa3487.html}.
 
 Here $t_u$ is UT1 expressed in Julian centuries since J2000.0, and $t$
-is TDT expressed in Julian centuries since J2000.0.
+is TT expressed in Julian centuries since J2000.0.
 
 \begin{eqnarray}
@@ -1034 +1031 @@
 Gives $GMST00$ in seconds.
 
+
 \paragraph{Longitude}
 
@@ -1045 +1043 @@
 \paragraph{Local Mean Sidereal Time (LMST)}
 
-Local Mean Sidereal Time (LMST) is Greenwich Mean Sideral Time (GMST)
-plus the observer's location in East longitude. Calculating LMST
-requires the input of Universal Time (UT1), Terrestrial Dynamical Time
-(TDT) and a longitude (measured East of Greenwich).
+Local Mean Sidereal Time (LMST) is Greenwich Mean Sideral Time (GMST) plus the
+observer's location in East longitude. Calculating LMST requires the input of
+Universal Time (UT1), Terrestrial Dynamical Time (TT) and a longitude (measured
+East of Greenwich).
 
 \begin{equation}