Apply Refraction to ZD
CALL sla_REFZ (ZU, REFA, REFB, ZR)
ZU  D  unrefracted zenith distance of the source (radians) 
 
REFA  D  $tan\zeta $ coefficient (radians) 


REFB  D  ${tan}^{3}\zeta $ coefficient (radians) 
ZR  D  refracted zenith distance (radians) 
where $E=9{0}^{\circ}{\zeta}_{true}$ and $F$ is a factor chosen to meet the $\Delta \zeta =atan\zeta +b{tan}^{3}\zeta $ formula at $\zeta =8{3}^{\circ}$.
For optical/IR wavelengths, over a wide range of observer heights and corresponding temperatures and pressures, the following levels of accuracy (worst case) are achieved, relative to numerical integration through a model atmosphere:
${\zeta}_{obs}$  error  
$8{0}^{\circ}$  ′′07  
$8{1}^{\circ}$  ′′13  
$8{2}^{\circ}$  ′′24  
$8{3}^{\circ}$  ′′47  
$8{4}^{\circ}$  ′′62  
$8{5}^{\circ}$  ′′64  
$8{6}^{\circ}$  $8\phantom{\rule{0.54753pt}{0ex}}$${}^{\prime}{\phantom{\rule{1.09506pt}{0ex}}}^{\prime}$  
$8{7}^{\circ}$  $10\phantom{\rule{0.54753pt}{0ex}}$${}^{\prime}{\phantom{\rule{1.09506pt}{0ex}}}^{\prime}$  
$8{8}^{\circ}$  $15\phantom{\rule{0.54753pt}{0ex}}$${}^{\prime}{\phantom{\rule{1.09506pt}{0ex}}}^{\prime}$  
$8{9}^{\circ}$  $30\phantom{\rule{0.54753pt}{0ex}}$${}^{\prime}{\phantom{\rule{1.09506pt}{0ex}}}^{\prime}$  
$9{0}^{\circ}$  $60\phantom{\rule{0.54753pt}{0ex}}$${}^{\prime}{\phantom{\rule{1.09506pt}{0ex}}}^{\prime}$  
$9{1}^{\circ}$  $150\phantom{\rule{0.54753pt}{0ex}}$${}^{\prime}{\phantom{\rule{1.09506pt}{0ex}}}^{\prime}$  $<$ highaltitude 
$9{2}^{\circ}$  $400\phantom{\rule{0.54753pt}{0ex}}$${}^{\prime}{\phantom{\rule{1.09506pt}{0ex}}}^{\prime}$  $<$ sites only 
The highZD model is scaled to match the normal model at the transition point; there is no glitch.