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<H2><A NAME="SECTION00048000000000000000">SLA_AOPPA - Appt-to-Obs Parameters</A>
<A NAME="xref_SLA_AOPPA"> </A><A NAME="SLA_AOPPA"> </A>
</H2>
<DL>
<DT><STRONG>ACTION:</STRONG>
<DD>Pre-compute the set of apparent to observed place parameters
required by the ``quick'' routines sla_AOPQK and sla_OAPQK.
<P> <DT><STRONG>CALL:</STRONG>
<DD><TT>CALL sla_AOPPA (
DATE, DUT, ELONGM, PHIM, HM, XP, YP,
TDK, PMB, RH, WL, TLR, AOPRMS)</TT>
<P> </DL>
<P> <DL>
<DT><STRONG>GIVEN:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>DATE</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TD ALIGN="LEFT" NOWRAP>UTC date/time (Modified Julian Date, JD-2400000.5)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>DUT</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP><IMG WIDTH="16" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img19.gif"
ALT="$\Delta$">UT: UT1-UTC (UTC seconds)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>ELONGM</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>observer's mean longitude (radians, east +ve)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>PHIM</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>observer's mean geodetic latitude (radians)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>HM</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>observer's height above sea level (metres)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>XP,YP</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>polar motion <IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img20.gif"
ALT="$[\,x,y\,]$"> coordinates (radians)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>TDK</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>local ambient temperature (degrees K; std=273.155D0)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>PMB</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>local atmospheric pressure (mB; std=1013.25D0)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>RH</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>local relative humidity (in the range 0D0-1D0)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>WL</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>effective wavelength (<IMG WIDTH="26" HEIGHT="25" ALIGN="MIDDLE" BORDER="0"
SRC="img21.gif"
ALT="$\mu{\rm m}$">, <I>e.g.</I> 0.55D0)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>TLR</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>tropospheric lapse rate (degrees K per metre,
<I>e.g.</I> 0.0065D0)</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>RETURNED:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>AOPRMS</EM></TD>
<TH ALIGN="LEFT"><B>D(14)</B></TH>
<TD ALIGN="LEFT" NOWRAP>star-independent apparent-to-observed parameters:</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP COLSPAN=1>(1)</TD>
<TD></TD>
<TD ALIGN="LEFT" NOWRAP>geodetic latitude (radians)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP COLSPAN=1>(2,3)</TD>
<TD></TD>
<TD ALIGN="LEFT" NOWRAP>sine and cosine of geodetic latitude</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP COLSPAN=1>(4)</TD>
<TD></TD>
<TD ALIGN="LEFT" NOWRAP>magnitude of diurnal aberration vector</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP COLSPAN=1>(5)</TD>
<TD></TD>
<TD ALIGN="LEFT" NOWRAP>height (HM)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP COLSPAN=1>(6)</TD>
<TD></TD>
<TD ALIGN="LEFT" NOWRAP>ambient temperature (TDK)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP COLSPAN=1>(7)</TD>
<TD></TD>
<TD ALIGN="LEFT" NOWRAP>pressure (PMB)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP COLSPAN=1>(8)</TD>
<TD></TD>
<TD ALIGN="LEFT" NOWRAP>relative humidity (RH)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP COLSPAN=1>(9)</TD>
<TD></TD>
<TD ALIGN="LEFT" NOWRAP>wavelength (WL)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP COLSPAN=1>(10)</TD>
<TD></TD>
<TD ALIGN="LEFT" NOWRAP>lapse rate (TLR)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP COLSPAN=1>(11,12)</TD>
<TD></TD>
<TD ALIGN="LEFT" NOWRAP>refraction constants A and B (radians)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP COLSPAN=1>(13)</TD>
<TD></TD>
<TD ALIGN="LEFT" NOWRAP>longitude + eqn of equinoxes +
``sidereal <IMG WIDTH="16" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img19.gif"
ALT="$\Delta$">UT'' (radians)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP COLSPAN=1>(14)</TD>
<TD></TD>
<TD ALIGN="LEFT" NOWRAP>local apparent sidereal time (radians)</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>NOTES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>It is advisable to take great care with units, as even
unlikely values of the input parameters are accepted and
processed in accordance with the models used.
<DT>2.
<DD>The DATE argument is UTC expressed as an MJD. This is,
strictly speaking, wrong, because of leap seconds. However,
as long as the <IMG WIDTH="16" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img19.gif"
ALT="$\Delta$">UT and the UTC are consistent there
are no difficulties, except during a leap second. In this
case, the start of the 61st second of the final minute should
begin a new MJD day and the old pre-leap <IMG WIDTH="16" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img19.gif"
ALT="$\Delta$">UT should
continue to be used. As the 61st second completes, the MJD
should revert to the start of the day as, simultaneously,
the <IMG WIDTH="16" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img19.gif"
ALT="$\Delta$">UT changes by one second to its post-leap new value.
<DT>3.
<DD>The <IMG WIDTH="16" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img19.gif"
ALT="$\Delta$">UT (UT1-UTC) is tabulated in IERS circulars and
elsewhere. It increases by exactly one second at the end of
each UTC leap second, introduced in order to keep <IMG WIDTH="16" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img19.gif"
ALT="$\Delta$">UT
within <IMG WIDTH="15" HEIGHT="25" ALIGN="MIDDLE" BORDER="0"
SRC="img30.gif"
ALT="$\pm$"><IMG WIDTH="24" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img31.gif"
ALT="$0^{\rm s}\hspace{-0.3em}.9$">. The ``sidereal <IMG WIDTH="16" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img19.gif"
ALT="$\Delta$">UT'' which forms
part of AOPRMS(13) is the same quantity, but converted from solar
to sidereal seconds and expressed in radians.
<DT>4.
<DD>IMPORTANT - TAKE CARE WITH THE LONGITUDE SIGN CONVENTION. The
longitude required by the present routine is <B>east-positive</B>,
in accordance with geographical convention (and right-handed).
In particular, note that the longitudes returned by the
sla_OBS routine are west-positive (as in the <I>Astronomical
Almanac</I> before 1984) and must be reversed in sign before use in
the present routine.
<DT>5.
<DD>The polar coordinates XP,YP can be obtained from IERS
circulars and equivalent publications. The
maximum amplitude is about
<IMG WIDTH="23" HEIGHT="18" ALIGN="BOTTOM" BORDER="0"
SRC="img32.gif"
ALT="$0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.3$"> . If XP,YP values
are unavailable, use XP=YP=0D0. See page B60 of the 1988
<I>Astronomical Almanac</I> for a definition of the two angles.
<DT>6.
<DD>The height above sea level of the observing station, HM,
can be obtained from the <I>Astronomical Almanac</I> (Section J
in the 1988 edition), or via the routine sla_OBS. If P,
the pressure in mB, is available, an adequate
estimate of HM can be obtained from the following expression:
<BLOCKQUOTE><TT>HM=-29.3D0*TSL*LOG(P/1013.25D0)</TT>
</BLOCKQUOTE>
where TSL is the approximate sea-level air temperature in degrees K
(see <I>Astrophysical Quantities</I>, C.W.Allen, 3rd edition,
§52). Similarly, if the pressure P is not known,
it can be estimated from the height of the observing
station, HM as follows:
<BLOCKQUOTE><TT>P=1013.25D0*EXP(-HM/(29.3D0*TSL))</TT>
</BLOCKQUOTE>
Note, however, that the refraction is proportional to the
pressure and that an accurate P value is important for
precise work.
</DL></DL>
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<ADDRESS>
<I>SLALIB --- Positional Astronomy Library<BR>Starlink User Note 67<BR>P. T. Wallace<BR>12 October 1999<BR>E-mail:ptw@star.rl.ac.uk</I>
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