Determine constants A and B in atmospheric refraction model dz = A tan z + B tan3 z (for optical wavelengths). 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. The constants are such that the model agrees precisely with the full integration performed by the slaRefro routine at zenith distances arctan(1) and arctan(4).
hm double height of the observer above sea level (metre) tdk double ambient temperature at the observer (deg k) pmb double pressure at the observer (millibar) rh double relative humidity at the observer (range 0-1) wl double effective wavelength of the source (micrometre) phi double latitude of the observer (radian, astronomical) tlr double temperature lapse rate in the troposphere (degk/metre) eps double precision required to terminate iteration (radian)
*refa double tan z coefficient (radian) *refb double tan3 coefficient (radian)
slaRefro Typical values for the tlr and eps arguments might be 0.0065 and 1e-10 respectively. Relative to the comprehensive refraction model used by this routine, the simple A tan z + B tan3 z formula 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 31 October 1993