  We have analyzed Zenith Wet Delay (ZWD) time series from an experiment over
the Madrid (Spain) area obtained from 5 GPS receivers using two different
techniques. In the first case a delay correlation analysis of the ZWD
time-series has been carried out. We show that for this small network (with a
spatial scale of less than 100 km) the correlation between the time series is
very strong, and that using windowing techniques a reliable correlation delay
time series can be produced for each pair of sites (10 such pairs are
available). We use this delay time series together with a frozen flow model to
estimate the velocity of a passing front, and compare the results to
meteorological data and Numerical Weather Prediction output, showing good
agreement. In the second approach, the data is analyzed using Empirical
Orthogonal Functions. We demonstrate that the temporally demeaned and
normalized analysis yields information about the passing of fronts, while the
spatially demeaned data yields orographic information. A common second mode
highlights the underlying wave behavior.
