Computer Sciences Corporation (CSC) is monitoring the quality of the wet tropospheric path delay (PD) retrieval generated from the GFO Water Vapor Radiometer (WVR) data by making inter-comparisons of the GFO path delay with measurements from a global network of radiosonde observations (RaObs) of the atmosphere. Routine twice-daily RaOb launches are provided by the national weather service of a large number of participating countries around the world. Many of these launch sites are on small isolated islands, far from any significant land masses, and a number of these islands lie on or very near a GFO ground track. Overpasses by GFO within 300 km of such a site, and which occur within 360 min of a RaOb launch, are used as an inter-comparison point. The atmospheric profiles of temperature, pressure, and water vapor which are measured by the RaOb are used to perform a numerical calculation of integrated refractivity through the troposphere. The result is an independent, direct measurement for comparison with the WVR Path Delay retrieval.