Abstract: An atmospheric water vapor sensing system using Global Positioning Satellites (GPS) to determine the refractivity and Slant-path Water Vapor (SWV) between an Earth-based GPS receiver and a plurality of satellite-based GPS transmitters. Either the refractivity measure or the SWV measure can be input directly into weather forecasting models. SWV is the integrated atmospheric water vapor in a column of atmosphere at an acute angular line-of-sight path between the Earth-based GPS receiver and any one of the plurality of satellite-based GPS transmitters. SWV sensing facilitates atmospheric soundings over land and water to provide global scale high resolution atmospheric tomography for use in operational meteorology and meteorological forecasting.