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slalib/polmo.f

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+      SUBROUTINE sla_POLMO ( ELONGM, PHIM, XP, YP, ELONG, PHI, DAZ )
+*+
+*     - - - - - -
+*      P O L M O
+*     - - - - - -
+*
+*  Polar motion:  correct site longitude and latitude for polar
+*  motion and calculate azimuth difference between celestial and
+*  terrestrial poles.
+*
+*  Given:
+*     ELONGM   d      mean longitude of the observer (radians, east +ve)
+*     PHIM     d      mean geodetic latitude of the observer (radians)
+*     XP       d      polar motion x-coordinate (radians)
+*     YP       d      polar motion y-coordinate (radians)
+*
+*  Returned:
+*     ELONG    d      true longitude of the observer (radians, east +ve)
+*     PHI      d      true geodetic latitude of the observer (radians)
+*     DAZ      d      azimuth correction (terrestrial-celestial, radians)
+*
+*  Notes:
+*
+*   1)  "Mean" longitude and latitude are the (fixed) values for the
+*       site's location with respect to the IERS terrestrial reference
+*       frame;  the latitude is geodetic.  TAKE CARE WITH THE LONGITUDE
+*       SIGN CONVENTION.  The longitudes used by the present routine
+*       are east-positive, in accordance with geographical convention
+*       (and right-handed).  In particular, note that the longitudes
+*       returned by the sla_OBS routine are west-positive, following
+*       astronomical usage, and must be reversed in sign before use in
+*       the present routine.
+*
+*   2)  XP and YP are the (changing) coordinates of the Celestial
+*       Ephemeris Pole with respect to the IERS Reference Pole.
+*       XP is positive along the meridian at longitude 0 degrees,
+*       and YP is positive along the meridian at longitude
+*       270 degrees (i.e. 90 degrees west).  Values for XP,YP can
+*       be obtained from IERS circulars and equivalent publications;
+*       the maximum amplitude observed so far is about 0.3 arcseconds.
+*
+*   3)  "True" longitude and latitude are the (moving) values for
+*       the site's location with respect to the celestial ephemeris
+*       pole and the meridian which corresponds to the Greenwich
+*       apparent sidereal time.  The true longitude and latitude
+*       link the terrestrial coordinates with the standard celestial
+*       models (for precession, nutation, sidereal time etc).
+*
+*   4)  The azimuths produced by sla_AOP and sla_AOPQK are with
+*       respect to due north as defined by the Celestial Ephemeris
+*       Pole, and can therefore be called "celestial azimuths".
+*       However, a telescope fixed to the Earth measures azimuth
+*       essentially with respect to due north as defined by the
+*       IERS Reference Pole, and can therefore be called "terrestrial
+*       azimuth".  Uncorrected, this would manifest itself as a
+*       changing "azimuth zero-point error".  The value DAZ is the
+*       correction to be added to a celestial azimuth to produce
+*       a terrestrial azimuth.
+*
+*   5)  The present routine is rigorous.  For most practical
+*       purposes, the following simplified formulae provide an
+*       adequate approximation:
+*
+*       ELONG = ELONGM+XP*COS(ELONGM)-YP*SIN(ELONGM)
+*       PHI   = PHIM+(XP*SIN(ELONGM)+YP*COS(ELONGM))*TAN(PHIM)
+*       DAZ   = -SQRT(XP*XP+YP*YP)*COS(ELONGM-ATAN2(XP,YP))/COS(PHIM)
+*
+*       An alternative formulation for DAZ is:
+*
+*       X = COS(ELONGM)*COS(PHIM)
+*       Y = SIN(ELONGM)*COS(PHIM)
+*       DAZ = ATAN2(-X*YP-Y*XP,X*X+Y*Y)
+*
+*   Reference:  Seidelmann, P.K. (ed), 1992.  "Explanatory Supplement
+*               to the Astronomical Almanac", ISBN 0-935702-68-7,
+*               sections 3.27, 4.25, 4.52.
+*
+*  P.T.Wallace   Starlink   30 November 2000
+*
+*  Copyright (C) 2000 Rutherford Appleton Laboratory
+*
+*  License:
+*    This program is free software; you can redistribute it and/or modify
+*    it under the terms of the GNU General Public License as published by
+*    the Free Software Foundation; either version 2 of the License, or
+*    (at your option) any later version.
+*
+*    This program is distributed in the hope that it will be useful,
+*    but WITHOUT ANY WARRANTY; without even the implied warranty of
+*    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+*    GNU General Public License for more details.
+*
+*    You should have received a copy of the GNU General Public License
+*    along with this program (see SLA_CONDITIONS); if not, write to the 
+*    Free Software Foundation, Inc., 59 Temple Place, Suite 330, 
+*    Boston, MA  02111-1307  USA
+*
+*-
+
+      IMPLICIT NONE
+
+      DOUBLE PRECISION ELONGM,PHIM,XP,YP,ELONG,PHI,DAZ
+
+      DOUBLE PRECISION SEL,CEL,SPH,CPH,XM,YM,ZM,XNM,YNM,ZNM,
+     :                 SXP,CXP,SYP,CYP,ZW,XT,YT,ZT,XNT,YNT
+
+
+
+*  Site mean longitude and mean geodetic latitude as a Cartesian vector
+      SEL=SIN(ELONGM)
+      CEL=COS(ELONGM)
+      SPH=SIN(PHIM)
+      CPH=COS(PHIM)
+
+      XM=CEL*CPH
+      YM=SEL*CPH
+      ZM=SPH
+
+*  Rotate site vector by polar motion, Y-component then X-component
+      SXP=SIN(XP)
+      CXP=COS(XP)
+      SYP=SIN(YP)
+      CYP=COS(YP)
+
+      ZW=(-YM*SYP+ZM*CYP)
+
+      XT=XM*CXP-ZW*SXP
+      YT=YM*CYP+ZM*SYP
+      ZT=XM*SXP+ZW*CXP
+
+*  Rotate also the geocentric direction of the terrestrial pole (0,0,1)
+      XNM=-SXP*CYP
+      YNM=SYP
+      ZNM=CXP*CYP
+
+      CPH=SQRT(XT*XT+YT*YT)
+      IF (CPH.EQ.0D0) XT=1D0
+      SEL=YT/CPH
+      CEL=XT/CPH
+
+*  Return true longitude and true geodetic latitude of site
+      IF (XT.NE.0D0.OR.YT.NE.0D0) THEN
+         ELONG=ATAN2(YT,XT)
+      ELSE
+         ELONG=0D0
+      END IF
+      PHI=ATAN2(ZT,CPH)
+
+*  Return current azimuth of terrestrial pole seen from site position
+      XNT=(XNM*CEL+YNM*SEL)*ZT-ZNM*CPH
+      YNT=-XNM*SEL+YNM*CEL
+      IF (XNT.NE.0D0.OR.YNT.NE.0D0) THEN
+         DAZ=ATAN2(-YNT,-XNT)
+      ELSE
+         DAZ=0D0
+      END IF
+
+      END