Index: /trunk/doc/pslib/psLibADD.tex =================================================================== --- /trunk/doc/pslib/psLibADD.tex (revision 1749) +++ /trunk/doc/pslib/psLibADD.tex (revision 1750) @@ -1,3 +1,3 @@ -%%% $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} @@ -789,5 +789,5 @@ 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} @@ -879,50 +879,7 @@ 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} @@ -1017,6 +974,59 @@ 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. + %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%