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 height of the observer above sea level (metre)

TDK D ambient temperature at the observer (degrees K)

PMB D pressure at the observer (mB)

RH D relative humidity at the observer (range 0-1)

$$\mu{\rm m}$$ WL D

PHI D latitude of the observer (radian, astronomical)

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

EPS D precision required to terminate iteration (radian)

RETURNED:

$$\tan \zeta$$ REFA

$$\tan^{3} \zeta$$ REFB D

NOTES:

1.

Suggested values for the TLR and EPS arguments are 0.0065D0 and 1D-8 respectively.

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 $\zeta<45^{\circ}$.