SUBROUTINE iau_HD2AE ( HA, DEC, PHI, AZ, EL )
*+
* - - - - - - - - - -
* i a u _ H D 2 A E
* - - - - - - - - - -
*
* Equatorial to horizon coordinates: transform hour angle and
* declination to azimuth and altitude.
*
* This routine is part of the International Astronomical Union's
* SOFA (Standards of Fundamental Astronomy) software collection.
*
* Status: support routine.
*
* Given:
* HA d hour angle (local)
* DEC d declination
* PHI d site latitude
*
* Returned:
* AZ d azimuth
* EL d altitude (informally, elevation)
*
* Notes:
*
* 1) All the arguments are angles in radians.
*
* 2) Azimuth is returned in the range 0-2pi; north is zero, and east
* is +pi/2. Altitude is returned in the range +/- pi/2.
*
* 3) The latitude PHI is pi/2 minus the angle between the Earth's
* rotation axis and the adopted zenith. In many applications it
* will be sufficient to use the published geodetic latitude of the
* site. In very precise (sub-arcsecond) applications, PHI can be
* corrected for polar motion.
*
* 4) The returned azimuth AZ is with respect to the rotational north
* pole, as opposed to the ITRS pole, and for sub-arcsecond accuracy
* will need to be adjusted for polar motion if it is to be with
* respect to north on a map of the Earth's surface.
*
* 5) Should the user wish to work with respect to the astronomical
* zenith rather than the geodetic zenith, PHI will need to be
* adjusted for deflection of the vertical (often tens of
* arcseconds), and the zero point of HA will also be affected.
*
* 6) The transformation is the same as Vh = Rz(pi)*Ry(pi/2-phi)*Ve,
* where Vh and Ve are lefthanded unit vectors in the (az,el) and
* (ha,dec) systems respectively and Ry and Rz are rotations about
* first the y-axis and then the z-axis. (n.b. Rz(pi) simply
* reverses the signs of the x and y components.) For efficiency,
* the algorithm is written out rather than calling other utility
* functions. For applications that require even greater
* efficiency, additional savings are possible if constant terms
* such as functions of latitude are computed once and for all.
*
* 7) Again for efficiency, no range checking of arguments is carried
* out.
*
* Last revision: 2018 January 2
*
* SOFA release 2021-05-12
*
* Copyright (C) 2021 IAU SOFA Board. See notes at end.
*
*-----------------------------------------------------------------------
IMPLICIT NONE
DOUBLE PRECISION HA, DEC, PHI, AZ, EL
DOUBLE PRECISION D2PI
PARAMETER ( D2PI = 6.283185307179586476925287D0 )
DOUBLE PRECISION SH, CH, SD, CD, SP, CP, X, Y, Z, R, A
* Useful trig functions.
SH = SIN(HA)
CH = COS(HA)
SD = SIN(DEC)
CD = COS(DEC)
SP = SIN(PHI)
CP = COS(PHI)
* Az,Alt unit vector.
X = - CH*CD*SP + SD*CP
Y = - SH*CD
Z = CH*CD*CP + SD*SP
* To spherical.
R = SQRT(X*X + Y*Y)
IF ( R.EQ.0D0 ) THEN
A = 0D0
ELSE
A = ATAN2(Y,X)
END IF
IF ( A.LT.0D0 ) A = A+D2PI
AZ = A
EL = ATAN2(Z,R)
* Finished.
*+----------------------------------------------------------------------
*
* Copyright (C) 2021
* Standards Of Fundamental Astronomy Board
* of the International Astronomical Union.
*
* =====================
* SOFA Software License
* =====================
*
* NOTICE TO USER:
*
* BY USING THIS SOFTWARE YOU ACCEPT THE FOLLOWING SIX TERMS AND
* CONDITIONS WHICH APPLY TO ITS USE.
*
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*
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*
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*
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*
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*
*-----------------------------------------------------------------------
END