**ACTION:**- Adjust an unrefracted zenith distance to include the effect of
atmospheric refraction, using the simple
model.
**CALL:**`CALL sla_REFZ (ZU, REFA, REFB, ZR)`

**GIVEN:**-

*ZU***D**unrefracted zenith distance of the source (radians) *REFA***D**coefficient (radians) *REFB***D**coefficient (radians)

**RETURNED:**-

*ZR***D**refracted zenith distance (radians)

**NOTES:**- 1.
- This routine applies the adjustment for refraction in the
opposite sense to the usual one - it takes an unrefracted
(
*in vacuo*) position and produces an observed (refracted) position, whereas the model strictly applies to the case where an observed position is to have the refraction removed. The unrefracted to refracted case is harder, and requires an inverted form of the text-book refraction models; the formula used here is based on the Newton-Raphson method. For the utmost numerical consistency with the refracted to unrefracted model, two iterations are carried out, achieving agreement at the 10^{-11}arcsecond level for . The inherent accuracy of the model is, of course, far worse than this - see the documentation for sla_REFCO for more information. - 2.
- At , the rapidly-worsening
model is abandoned and an empirical formula takes over:
*F*is a factor chosen to meet the formula at . Over a wide range of observer heights and corresponding temperatures and pressures, the following levels of accuracy are achieved, relative to numerical integration through a model atmosphere:*error*< high-altitude < sites only - 3.
- See also the routine sla_REFV, which performs the adjustment in , and with the emphasis on speed rather than numerical accuracy.

Starlink User Note 67

P. T. Wallace

12 October 1999

E-mail:ptw@star.rl.ac.uk