In Eq. (1), a priori ZHD is evaluated from available surface
pressure using the modified Saastamoinen model
(Davis et al., 1985) and mfh and mfw are known mapping
functions relating the zenith delays with those in the slant
direction. For each observation, a combined ZTD value
is obtained, as the sum of the a priori ZHD and the estimated
ZWD. In principle, an error in the a priori ZHD will