It has been demonstrated that GPS slants can be used to reconstruct a three-dimensional water vapor distribution and have strong impact on water vapor forecasts if they are accurate measurements of integrated water vapor along GPS ray paths through a GPS tomography technique based upon Observation System Simulation Experiments (OSSE). For real time GPS data, there have been some 3DVAR and 4DVAR case studies showing the GPS data impact associated with other observations, backgrounds and/or numerical models. A real time GPS tomography analysis is a direct and independent verification of the GPS impact on water vapor analysis.

The International H2O Project (IHOP) 2002 is a field experiment over the Southern Great Plains (SGP) of the United States aiming at improvement of four-dimensional water vapor distribution analysis and applications for prediction of convection. In addition to other observation instruments, there is a relatively dense ground-based GPS network covering the right center of IHOP region. This GPS network makes a real time analysis of GPS slant observations possible. For the June 12-13, 2002 storm case over this IHOP area, a GPS tomography technique is applied to GPS slants processed using the Bernese 4.2 GPS Software. By adding proper constraints, the tomography analyses resemble some major water vapor patterns of satellite water vapor images over this region. After comparison to sounding data, we believe that these real time GPS slants contain important information regarding the water vapor distribution.