SLA_REFCO

Refraction Constants

ACTION:: Determine the constants $a$ and $b$ in the atmospheric refraction model $Δ ζ = a tan ζ + b {tan}^{3} ζ$ , where $ζ$ is the observed zenith distance (i.e. affected by refraction) and $Δ ζ$ is what to add to $ζ$ to give the topocentric (i.e. in vacuo) zenith distance.
CALL:: 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 ( $μ m$ )

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)

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 $> 100$ $μ m$ .
(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 $ζ < 8 0^{\circ}$ , ′′001 accuracy for $ζ < 6 0^{\circ}$ , and ′′0001 accuracy for $ζ < 4 5^{\circ}$ .