The ZTD (mm) is the 0-level product of the GPS data processing. It should be used for assimilation purpose. The IWV (kg/m**2) is the 1-level product of the GPS data. The conversion from ZTD to IWV is
The zenith total delay ZTD (mm) is the 0-level product of the GPS data processing. It should be used for assimilation purpose. The integrated water vapour IWV (kg/m**2) is the 1-level product of the GPS