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DOUBLE PRECISION FUNCTION sla_DPAV ( V1, V2 )
*+
* - - - - -
* D P A V
* - - - - -
*
* Position angle of one celestial direction with respect to another.
*
* (double precision)
*
* Given:
* V1 d(3) direction cosines of one point
* V2 d(3) direction cosines of the other point
*
* (The coordinate frames correspond to RA,Dec, Long,Lat etc.)
*
* The result is the bearing (position angle), in radians, of point
* V2 with respect to point V1. It is in the range +/- pi. The
* sense is such that if V2 is a small distance east of V1, the
* bearing is about +pi/2. Zero is returned if the two points
* are coincident.
*
* V1 and V2 need not be unit vectors.
*
* The routine sla_DBEAR performs an equivalent function except
* that the points are specified in the form of spherical
* coordinates.
*
* Patrick Wallace Starlink 13 July 1997
*
* Copyright (C) 1997 Rutherford Appleton Laboratory
*-
IMPLICIT NONE
DOUBLE PRECISION V1(3),V2(3)
DOUBLE PRECISION X1,Y1,Z1,W,R,XU1,YU1,ZU1,DX,DY,DZ,SQ,CQ
* Unit vector to point 1
X1=V1(1)
Y1=V1(2)
Z1=V1(3)
W=SQRT(X1*X1+Y1*Y1+Z1*Z1)
IF (W.NE.0D0) THEN
X1=X1/W
Y1=Y1/W
Z1=Z1/W
END IF
* Unit vector "north" from point 1
R=SQRT(X1*X1+Y1*Y1)
IF (R.EQ.0.0) R=1D-5
W=Z1/R
XU1=-X1*W
YU1=-Y1*W
ZU1=R
* Vector from point 1 to point 2
DX=V2(1)-X1
DY=V2(2)-Y1
DZ=V2(3)-Z1
* Position angle
SQ=DX*YU1*Z1+DY*ZU1*X1+DZ*XU1*Y1-DZ*YU1*X1-DY*XU1*Z1-DX*ZU1*Y1
CQ=DX*XU1+DY*YU1+DZ*ZU1
IF (SQ.EQ.0D0.AND.CQ.EQ.0D0) CQ=1D0
sla_DPAV=ATAN2(SQ,CQ)
END
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