The strength of ground-based GPS is certainly not its absolute accuracy. Because of its sensitivity to signal multipath effects, varying the elevation angle cut-off limits - or using different schemes for down-weighting low elevation angle observations - will typically have a significant impact on the estimated ZTD value. A constant bias over decades is in principle not a problem but if there are variations at the time scales of years it will influence both NWP models and long term climate monitoring.
We will use long time series (> 5 years) of independent radiosonde and microwave radiometer data to study these effects and believe that a correct assessment can be made at the 5-10 mm level in ZTD. Very-Long-Baseline Interferometry (VLBI) is another method which will be used. Several European VLBI sites, e.g., Wettzeell, Matera, and Onsala, are co-located with important GPS sites in the IGS network, where data are publicly available. The VLBI estimates of ZTD are obtained from the same type of estimation technique as in GPS but due to the large directional antennas used the multipath effect is in practise eliminated. VLBI observations are, however, not continuous, but 24-hour observing sessions bi-weekly or monthly for more than five years provide a sufficient database. .