id	summary	reporter	owner	description	type	status	priority	milestone	component	version	severity	resolution	keywords	cc
190	psTime: UT1-UTC and x_p,y_p table lookups	Paul Price	Paul Price	"Sorry for not specifying this in more detail before, but I've now rewritten the
appropriate SDRS sections to make this more clear (pasted below).  Please let me
know if there are any further questions.

\paragraph{Universal Time (UT1)}
\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 (\S\ref{sec:tables}).

[....]

\paragraph{Polar Coordinates}

The polar coordinates, $x_p$ and $y_p$, required for \code{SLA_AOPPA}
(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."	defect	closed	high		PSLib ADD	unspecified	normal	fixed		calvin.harman@…