Introduction Delay of the arrival time of GPS microwave against arrival time in vacuum due to refraction of (neutral or no-ionizing) atmosphere is primary error source in GPS positioning. However, the delay information can be used for meteorology as measurement of integrated refraction of atmosphere if we can estimate it accurately. The most common use of atmospheric delay estimate in GNSS processing is to convert it to moisture information. Contribution from dry atmosphere, called zenith hydrostatic delay, is subtracted from zenith tropospheric delay (ZTD) based on surface pressure observation. The resultant product is called zenith wet delay (ZWD). Then mapping factor based on mean temperature of atmosphere weighted with moisture (surface temperature can be used based on strong correlation between the mean temperature and surface temperature) is used to convert ZWD to precipitable water vapor (PWV). GNSS delivered PWV is used as input for numerical weather prediction model (NWP) operated by weather center to improve moisture field in the initial condition in NWP. The positive impacts of GNSS PWV on NWP have been observed in short-term precipitation forecast and moisture forecast. The illustration above shows conceptual figure of PWV retrieved in GNSS (GPS, GLONASS, GALILEO, and QZSS). Definition of PWV in meteorology is integrated water vapor in the pillar of 1m2 in the bottom area into zenith angle, while PWV retrieval in GNSS is "averaged value" in the inverse cone above GNSS antenna with its observation area depending on cut-off angle of GNSS observation, where mapping function is used to map slant-pass tropospheric delay to ZTD.