**ACTION:**- Determine the constants
*a*and*b*in the atmospheric refraction model , where is the*observed*zenith distance (*i.e.*affected by refraction) and is what to add to 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 () *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**coefficient (radians) *REFB***D**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 .
- 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
() and ().
At other zenith distances, the model achieves:
accuracy for ,

accuracy for , and

accuracy for .

Starlink User Note 67

P. T. Wallace

12 October 1999

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