  Tomographic techniques are successfully applied to obtain 4D images of the
tropospheric refractivity in a local dense network. In the lower atmosphere
both the small height and time scales and the non-dispersive nature of
tropospheric delays require a more careful analysis of the data. We show how
GPS data is processed to obtain the tropospheric slant delays using the
GIPSY-OASIS II software and define the concept of pseudo-wet delays, which will
be the observables in the tomographic software. We then discuss the inverse
problem in the 3D stochastic tomography, using simulated refractivity fields to
test the system and the impact of noise. Finally, we use data from the Kilauea
network in Hawaii and a local 4x4x41-voxel grid on a region of 400 Km$^2$ and
15 Km in height to produce 4D refractivity fields. Results are compared with
ECMWF forecast.

Geophysics;Atmospheric and Oceanic Physics