### SLA_REFCO

Refraction Constants

ACTION:
Determine the constants $a$ and $b$ in the atmospheric refraction model $\Delta \zeta =atan\zeta +b{tan}^{3}\zeta$, where $\zeta$ is the observed zenith distance (i.e. affected by refraction) and $\Delta \zeta$ is what to add to $\zeta$ to give the topocentric (i.e. in vacuo) zenith distance.
CALL:
CALL sla_REFCO (HM, TDK, PMB, RH, WL, PHI, TLR, EPS, REFA, REFB)
##### GIVEN:
 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)

##### RETURNED:
 REFA D $tan\zeta$ coefficient (radians) REFB D ${tan}^{3}\zeta$ coefficient (radians)

NOTES:
(1)
Suggested values for the TLR and EPS arguments are 0.0065D0 and 1D$-$8 respectively. The signs of both are immaterial.
(2)
The radio refraction is chosen by specifying WL $>100$ $\mu m$.
(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 ${tan}^{-1}1$ ($4{5}^{\circ }$) and ${tan}^{-1}4$ ($\sim 7{6}^{\circ }$). At other zenith distances, the model achieves: ′′05 accuracy for $\zeta <8{0}^{\circ }$, ′′001 accuracy for $\zeta <6{0}^{\circ }$, and ′′0001 accuracy for $\zeta <4{5}^{\circ }$.