Adjust an unrefracted zenith distance to include the effect of atmospheric refraction, using the simple A tan z + B tan3 z model.
zu double unrefracted zenith distance of the source (radian) refa double A: tan z coefficient (radian) refb double B: tan3 z coefficient (radian)
*zr double refracted zenith distance (radian) Note that this routine applies the adjustment for refraction in the opposite sense to the usual one - it takes an unrefracted (in vacuo) position and produces an observed (refracted) position, whereas the basic A tan z + B tan3 z model strictly applies to the case where a refracted position is available and must be corrected for refraction. This requires an inverted form of the refraction expression; The formula used here is based on the Newton/Raphson method. For numerical consistency with the refracted to unrefracted model, two iterations are used; The error is less than 1e-11 arcseconds at 80 degrees ZD, and is still under 1 milliarcsecond at 88 degrees. For the results to be useful, zu should be in the range zero to 89 degrees (expressed in radians). For zu bigger than 89 degrees, the value for zu = 89 degrees is returned. No check is made for zu very large or negative. See also the routine slaRefv, which performs the adjustment in Cartesian az/el coordinates. P.T.Wallace Starlink 15 July 1993