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



next up previous
Next: SLA_REFCOQ - Refraction Constants (fast)
Up: SUBPROGRAM SPECIFICATIONS
Previous: SLA_RDPLAN - Apparent of Planet

SLALIB --- Positional Astronomy Library
Starlink User Note 67
P. T. Wallace
12 October 1999
E-mail:ptw@star.rl.ac.uk