FIG Working Week, Helsinki, Finland, 29 May - 02 June 2017
Water vapor plays an important role in modelling atmosphere and climate studies. Moreover, long-term water vapor changes can be an independent source for detecting climate changes. Since Global Navigation Satellite Systems (GNSS) use microwaves passing through the atmosphere, the atmospheric effects should be modelled with high accuracy. Tropospheric effects on GNSS signals are estimated with the total zenith delay parameter (ZTD) which is the sum of the hydrostatic (ZHD) and wet zenith delay (ZWD). The first component can accurately be obtained from meteorological observations; the latter, however, can be computed by subtracting ZHD from ZTD (i.e. ZWD=ZTD-ZHD). Then, the weighted mean temperature (Tm) or the conversion factor (Q) is used for the conversion between the precipitable water vapor (PWV) and ZWD.
The parameters Tm and Q are derived from the analysis of radiosonde stations' profile observations. A number of Q and Tm models have been developed for each radiosonde station, radiosonde station group, countries and global fields such as Bevis Tm model and Emardson and Derks' Q models. Previously, an algorithm was developed using MatlabTM to compute Tm, Q, ZWD, and the PWV from the parameters of radiosonde profile data such as height (h), temperature (T), dew point temperature (Td), pressure (p) and relative humidity (H). By applying the least squares method to the results procured from the devised algorithm, the PWV models (Tm and Q models) utilized for Turkey have been derived using a year of radiosonde data (2011) from 4,103 radiosonde profile observations of Istanbul, Ankara, Samsun, Erzurum, Diyarbakir, Adana, Isparta and Izmir radiosonde stations. These models depend on different combinations of parameters such as the station temperature, the station latitude, the station height and day of year. In this study, the models developed are tested by comparing PWV_GNSS computed applying Tm and Q models to the ZTD estimates derived by Bernese and GAMIT/GLOBK software at the GNSS stations established in the close vicinity of Istanbul and Ankara radiosonde stations (PWV_RS) from October 2013 to December 2014. The GNSS and meteorological data are obtained from a project (no 112Y350) supported by the Scientific and Technological Research Council of Turkey (TUBITAK). The comparison results show that PWV_GNSS and PWV_RS are in high correlation (86 % for Ankara and 90% for Istanbul). Thus, the most applicable model for Turkey and the accuracy of GNSS meteorology are investigated.