In order to obtain a real-time atmospheric water vapor field of high temporal and spatial resolution, based on the principle of remote sensing of atmospheric precipitable water vapor(PWV)using a ground-based GPS, the observed data at certain stations in the Hong Kong CORS network were processed. According to the different time spans in calculating PWV with ultra-rapid ephemeris, three schemes were designed. In each scheme, PWV data calculated with final precise ephemeris, ultra-rapid ephemeris, and radiosonde data were compared. The results show that the predicted PWV data of the ultra-rapid ephemeris agree with those of the final precise ephemeris, and its accuracy satisfies the requirements of real-time service, such as weather forecasting.