Another alternative is to assume that you know the water vapor content,
either from a previous measurement and assuming the weather is stable, or
from theoretical curves as a function of pressure, temperature and relative
humidity. With the knowledge of the atmospheric parameters, the model
predicts and . In this mode, instead of above, you
must supply the amount of precipitable water vapor in millimeter.
is then given by (from Eq. 7)
This is what happens in OBS when used in AUTO mode (specified by OBSSET CAL AUTO) and when you do a CAL command without the argument COLD.