A now standard alternative is to estimate the zenith delay from the GPS data as part of the geodetic solution, which has the virtue of absorbing residual errors in the dry delay model as well. Possibly the most effective approach is to model the zenith delay as a stochastic process and estimate a constrained adjustment every few minutes.
Further study is needed to identify preferred mapping functions for different sites and seasons.
The high cost of WVRs, however, may limit their use. The advent of dense regional arrays promises a further advance in atmospheric calibration. The flip side of error removal is error source observation, and the earth's atmosphere is a particularly rich source to observe. Objectives range from regional mapping of tropospheric water vapor for weather prediction to global recovery of stratospheric temperatures for studying long term climate change. Space based techniques can provide that knowledge by a different approach.