GPS meteorology can be successfully used to model the tropospheric refraction effect on radiowave signals. We describe an approach to estimate and model the spatial distribution of the tropospheric water vapor. We refine the tomographic software package AWATOS which is based on the assimilation of GPS double difference observations. The observations are allocated to a voxel model, which is defined according to the distribution of the GPS stations. Performing a least-squares adjustment, the refractivity of each voxel is determined. Tests of the software are performed, based on simulated and real data. The latter were obtained during a dedicated field campaign on the Big Island of Hawaii. The tomographic profiles are compared with radiosonde data. The results obtained for continuous atmospheric conditions fit well, whereas larger differences occur in discontinuous atmospheric weather conditions. Overall, an accuracy of 5--20 ppm (refractivity units) has been achieved.