### SYNOPSYS

void slaRefco(hm, tdk, pmb, rh, wl, phi, tlr, eps, refa, refb)

double hm
double tdk
double pmb
double rh
double wl
double phi
double tlr
double eps
double *refa
double *refb

### DESCRIPTION

```
Determine constants A and B in atmospheric refraction model
dz = A tan z + B tan3 z (for optical wavelengths).

z is the "observed" zenith distance (i.e. affected by
refraction) and dz is what to add to z to give the "topocentric"
(i.e. in vacuo) zenith distance.

The constants are such that the model agrees precisely with
the full integration performed by the slaRefro routine at
zenith distances arctan(1) and arctan(4).

```

#### Given

```    hm    double    height of the observer above sea level (metre)
tdk   double    ambient temperature at the observer (deg k)
pmb   double    pressure at the observer (millibar)
rh    double    relative humidity at the observer (range 0-1)
wl    double    effective wavelength of the source (micrometre)
phi   double    latitude of the observer (radian, astronomical)
tlr   double    temperature lapse rate in the troposphere (degk/metre)
eps   double    precision required to terminate iteration (radian)

```

#### Returned

```    *refa double    tan z coefficient (radian)
*refb double    tan3 coefficient (radian)

```

### Called

``` slaRefro

Typical values for the tlr and eps arguments might be 0.0065 and
1e-10 respectively.

Relative to the comprehensive refraction model used by this routine,
the simple A tan z + B tan3 z formula achieves 0.5 arcsec accuracy
for ZD < 80 deg, 0.01 arcsec accuracy for ZD < 60 deg, and
0.001 arcsec accuracy for ZD < 45 deg.

P.T.Wallace   Starlink   31 October 1993
```