Changeset 1750

Timestamp:

Sep 8, 2004, 4:37:42 PM (22 years ago)

Author:

Paul Price

Message:

Clarified the lookup tables for psTime.

File:

• trunk/doc/pslib/psLibADD.tex (modified) (4 diffs)

trunk/doc/pslib/psLibADD.tex

@@ -1 @@
-%%% $Id: psLibADD.tex,v 1.42 2004-09-08 19:33:10 price Exp $
+%%% $Id: psLibADD.tex,v 1.43 2004-09-09 02:37:42 price Exp $
 \documentclass[panstarrs]{panstarrs}
 
@@ -789 +789 @@
 
 This data is available from
-\code{http://hpiers.obspm.fr/eop-pc/earthor/utc/TAI-UTC_tab.html}
+\code{ftp://maia.usno.navy.mil/ser7/tai-utc.dat}
 
 \paragraph{Gregorian dates to seconds}
@@ -879 +879 @@
 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.
-
-The offset of UTC from UT1, $\Delta$ UT1 = UTC - UT1, may be
-determined from the following site in real time:
-
-\code{ftp://maia.usno.navy.mil/ser7/finals.all}
-
-\noindent with explanatory guide at
-
-\code{ftp://maia.usno.navy.mil/ser7/readme.finals}
-
-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.
-
-For the present time, it should be assumed that this table resides on
-local disk in a known location (i.e., there is no need that it is
-downloaded from the internet by PSLib).  Later, the location of this
-file will be made configurable.
-
-For dates within the range of the table, the value for the offset
-between UTC and UT1 shall be derived from linear interpolation between
-the nearest entries in the table.  For dates earlier the range of the
-above table, a warning shall be generated, and the values calculated
-from a different table (an estimate, instead of observations),
-obtained from:
-
-\code{http://hpiers.obspm.fr/eoppc/eop/eopc01/eopc01.1900-2004}
-
-Dates outside the ranges of the above tables shall generate a warning.
-Dates later than those covered by the table shall be extrapolated
-using the formula contained in the Bulletin A
-(\code{ftp://maia.usno.navy.mil/ser7/finals.all}); the formula shall
-be easily configurable without recompiling (as the formula may change
-with each Bulletin A).  Dates prior to the those covered by the table
-shall simply assume that TDT is identical to UTC (which, from Reingold
-\& Dershowitz Figure 12.2 appears accurate to $\sim 10$ sec to about
-1700).  Archeoastronomy is not a perceived PSLib application, so there
-is no need for high precision at such early dates.
-
-These tables shall be read in only when required by the user, and
-shall remain in memory until the termination of the program.  An
-additional function, \code{psTimeTableReset} should be provided in
-order to force the reloading of the time tables.
+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 (\S\ref{sec:tables}).
 
 \paragraph{Julian Day and Modified Julian Day}
@@ -1017 +974 @@
 
 The polar coordinates, $x_p$ and $y_p$, required for \code{SLA_AOPPA}
-(and hence the \code{psGrommit}s), may be calculated in a similar
-manner as for the offset of UT1 from UTC (\S\ref{sec:ut1}).
+(and hence the \code{psGrommit}s), may be calculated through table
+lookups (\S\ref{sec:tables}).
+
+\paragraph{Table Lookups}
+\label{sec:tables}
+
+The offset of UTC from UT1, $\Delta$ UT1 = UTC - UT1, as well as the
+pole coordinates, $x_p$ and $y_p$, may be determined from table
+lookups.  Tables are available covering different time periods and
+with different time resolution:
+
+\begin{itemize}
+\item \code{ftp://maia.usno.navy.mil/ser7/ser7.dat}
+\item \code{ftp://maia.usno.navy.mil/ser7/finals.all} with explanatory
+  guide at \code{ftp://maia.usno.navy.mil/ser7/readme.finals} . See also
+  the web page \code{http://maia.usno.navy.mil/}.
+\item \code{http://hpiers.obspm.fr/eoppc/eop/eopc01/eopc01.1900-2004}
+  (contains estimates prior to 1972).
+\end{itemize}
+
+For the present time, it should be assumed that these tables reside on
+local disk in known locations (i.e., there is no need that they are
+downloaded from the internet and parsed by PSLib), containing the
+minimum required information in a format that may be read quickly by
+PSLib as required.
+
+The location of these files, their priority order, and the ``from''
+and ``to'' dates of applicability will be known beforehand.
+\tbd{Later, the location of these files, as well as the order of their
+specification, will be made configurable.}
+
+When a value is required, the table with the highest priority shall be
+checked to see if the date is within the range of applicability for
+the table.  If not, the table with the next highest priority shall be
+checked.  If a table is found that is applicable, then the appropriate
+value shall be derived from linear interpolation between the nearest
+entries in the table.  If no table is found that is applicable, and
+the required date is later than those in the table, a warning shall be
+generated, and a formula shall be applied (specifically, the formula
+contained in the Bulletin A,
+\code{ftp://maia.usno.navy.mil/ser7/ser7.dat}); the formula must be
+easily configurable (i.e., not requiring recompilation, as the formula
+will likely change with each Bulletin A).  For dates prior to those
+covered in the tables, the function shall generate a warning and
+assume that TDT is identical to UTC (which, from Reingold \&
+Dershowitz Figure 12.2 appears accurate to $\sim 10$ sec to about
+1700), and that the polar motion ($x_p,y_p$) are zero.
+Archeoastronomy is not a perceived PSLib application, so there is no
+need for high precision at such early dates.
+
+The tables shall be read in only when required by the user, and shall
+remain in memory until the termination of the program.  An additional
+function, \code{psTimeTableReset} should be provided in order to force
+the reloading of the time tables.
+
 
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