SLA_REFCO - Refraction Constants

ACTION:

Determine the constants $a$ and $b$ in the atmospheric refraction model $\Delta \zeta = a \tan \zeta + b \tan^{3} \zeta$, 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 ($\mu{\rm m}$)
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 $>100$ $\mu{\rm m}$.
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 $\tan^{-1} 1$ ($45^\circ$) and $\tan^{-1} 4$ ($\sim 76^\circ$). At other zenith distances, the model achieves: $0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.5$ accuracy for $\zeta<80^{\circ}$, $0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.01$ accuracy for $\zeta<60^{\circ}$, and $0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.001$ accuracy for $\zeta<45^{\circ}$.