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SUBROUTINE sla_REFCO (HM, TDK, PMB, RH, WL, PHI, TLR, EPS,
: REFA, REFB)
*+
* - - - - - -
* R E F C O
* - - - - - -
*
* Determine the constants A and B in the atmospheric refraction
* model dZ = A tan Z + B tan**3 Z.
*
* Z is the "observed" zenith distance (i.e. affected by refraction)
* and dZ is what to add to Z to give the "topocentric" (i.e. in vacuo)
* zenith distance.
*
* Given:
* HM d height of the observer above sea level (metre)
* TDK d ambient temperature at the observer (deg K)
* PMB d pressure at the observer (millibar)
* RH d relative humidity at the observer (range 0-1)
* WL d effective wavelength of the source (micrometre)
* PHI d latitude of the observer (radian, astronomical)
* TLR d temperature lapse rate in the troposphere (degK/metre)
* EPS d precision required to terminate iteration (radian)
*
* Returned:
* REFA d tan Z coefficient (radian)
* REFB d tan**3 Z coefficient (radian)
*
* Called: sla_REFRO
*
* Notes:
*
* 1 Typical values for the TLR and EPS arguments might be 0.0065D0 and
* 1D-10 respectively.
*
* 2 The radio refraction is chosen by specifying WL > 100 micrometres.
*
* 3 The routine is a slower but more accurate alternative to the
* sla_REFCOQ routine. The constants it produces give perfect
* agreement with sla_REFRO at zenith distances arctan(1) (45 deg)
* and arctan(4) (about 76 deg). It achieves 0.5 arcsec accuracy
* for ZD < 80 deg, 0.01 arcsec accuracy for ZD < 60 deg, and
* 0.001 arcsec accuracy for ZD < 45 deg.
*
* P.T.Wallace Starlink 3 June 1997
*
* Copyright (C) 1997 Rutherford Appleton Laboratory
*-
IMPLICIT NONE
DOUBLE PRECISION HM,TDK,PMB,RH,WL,PHI,TLR,EPS,REFA,REFB
DOUBLE PRECISION ATN1,ATN4,R1,R2
* Sample zenith distances: arctan(1) and arctan(4)
PARAMETER (ATN1=0.7853981633974483D0,
: ATN4=1.325817663668033D0)
* Determine refraction for the two sample zenith distances
CALL sla_REFRO(ATN1,HM,TDK,PMB,RH,WL,PHI,TLR,EPS,R1)
CALL sla_REFRO(ATN4,HM,TDK,PMB,RH,WL,PHI,TLR,EPS,R2)
* Solve for refraction constants
REFA = (64D0*R1-R2)/60D0
REFB = (R2-4D0*R1)/60D0
END
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