Determination and interpolation of parameters for precise conversion of GNSS wet zenith delay to precipitable water vapor in Turkey

Studies of estimating precipitable water vapor (PWV) from continuous Global Navigation Satellite System (GNSS) stations with high temporal and spatial resolution continuously have become popular in recent years. In this estimation process, the weighted mean temperature (T-m) and the conversion parameter (Q) are the most important parameters to convert tropospheric zenith delay (ZTD) to PWV. In this study, T-m and Q time series are derived by assessing 4103 profile observations of eight Turkish radiosonde stations (RS) for approximately one year. The T-m-T-s linear regression model is developed. In analogy to T-m-T-s model, the Q values are modelled based on different combinations of surface temperature (T-s), station latitude (), station height (H) and day of the year (DOY). To test the validity of these models, the GNSS derived PWV (PWVGNSS) values are computed from the GNSS data of just over a year for the Istanbul and Ankara RS-GNSS stations using the most precise T-m and Q models, and later they are compared with the PWVRS values. The mean of the differences obtained for the Ankara and Istanbul stations are found to be 1.4-1.6mm with a standard deviation of 1.7-1.8mm, respectively. Moreover, modelling and interpolating meteorological parameters such as temperature, pressure, as well as PWV and ZTD are tested using the spherical harmonic functions (SHF). The results indicate that SHF can be safely and accurately used for modelling and interpolating meteorological parameters and ZTD.