The Zenith Total Delay uncertainty is a key component of the total uncertainty in GNSS-IPW measurements (Ning et al., 2016). If it is not handled in a proper way, it may drastically affect the GNSS-IPW uncertainty estimate. Fixing it equal to 4mm is just a compromise, excluding GAIA-CLIM outliers from longer time series.

When discussing GRUAN GNSS-IPW uncertainties, we only discuss data analysis using Precise Point Positioning (PPP) in the EPOS software package. While suggesting GRUAN GNSS-IPW uncertainties should be implemented by other data analysis centres, we talk about implementing the GNSS-IPW uncertainty analysis method as described by T. Ning et al. (AMT, 2016) in different software (i.e. not EPOS, solely used by GFZ and GRUAN data analysis). This task is not trivial; for example, the orbital error components described by J. Dousa (GPS Solutions, 2010) and used by T. Ning et al in AMT 2015 are not delivered for end users like ZTDs from IGS (or simply obtainable from standard software for GNSS-data analysis).

Preliminary analysis has been made (and is still in progress) on documentation and related articles published by the developers of Bernese and GAMIT/GLOBK software. ZTD uncertainty is known to be substantial if not the main contributor to the GNSS-IPW uncertainty budget. Therefore, it is essential to understand and to find recommendations when using uncertainty estimates obtained by different data processing software packages for undertaking GRUAN-type uncertainty analysis. The goal is to investigate at least two geodetic software packages using the same GNSS-data processing method, comparing the uncertainty definition and uncertainty handling, leading to (often remarkably) different numeric values of uncertainty estimates.