Refraction Constants
CALL sla_REFCO (HM, TDK, PMB, RH, WL, PHI, TLR, EPS, REFA, REFB)
HM  D  height of the observer above sea level (metre) 
 
TDK  D  ambient temperature at the observer K) 
 
PMB  D  pressure at the observer (mb) 
 
RH  D  relative humidity at the observer (range 0 – 1) 
 
WL  D  effective wavelength of the source ($\mu m$) 


PHI  D  latitude of the observer (radian, astronomical) 


TLR  D  temperature lapse rate in the troposphere ( K per metre) 


EPS  D  precision required to terminate iteration (radian) 
REFA  D  $tan\zeta $ coefficient (radians) 


REFB  D  ${tan}^{3}\zeta $ coefficient (radians) 