TUBITAK Project, 2013 - 2016
GNSS, which provides spatial information with high accuracy and reliability, has been an indispensable tool for disciplines such as engineering, navigation, surveying, environmental, geology, etc. As a result, the creation of network of continuous GNSS stations has become imminent for many countries. Thus, in our country, 146 reference stations covering the entire country (with 80-100km interranges) Turkey National Permanent GPS Network - Active (TUSAGA-Active) supported by TUBITAK, came into use in 2009.
In the satellite-to-receiver communications of GNSS, the carrier microwaves are affected from the troposphere and these tropospheric effects are obtained at [mm] accuracy. A portion of total tropospheric zenith delay obtained from zenith directions of stations is due to water vapour. Thus, precipitable water vapour is estimated accurately and reliably from the total tropospheric zenith delay of a station and the meteorological observations of the station. There is a possibility for monitoring water vapour from the amount of precipitable water vapour of the TUSAGA-active stations in Turkey that will be obtained.
Water vapour plays an important role as a basic climate variable in the thermodynamics and dynamics of the storm systems at the atmosphere and in hydrological cycles of local, regional and global scales. Moreover, the distribution of atmospheric water vapor is difficult to solve because of its rapid change in spatial and temporal scales. Therefore, development of monitoring the atmospheric water vapour will provide with an accurate prediction of condensation in advance and with a better understanding of the global climate.
Estimation of atmospheric water vapour is performed by various methods and techniques such as water vapor gradiometers, solar spectrometers, radiosondes, lidars, etc. However, these techniques are expensive and their spatial and temporal solutions are weak, and also they are not equipped to work in all weather conditions. For these reasons, the methods and techniques referred to as the 'GPS Meteorology' (GPS MET) and the 'GPS Tomography' has been developed. The continuous GPS networks established for geodetic, engineering, navigation, etc. purposes are still working and can be converted into GPS MET networks with an extra small cost. Thus, for the entire country or region, the information of continuous precipitable water vapour at the accuracy of radiosonde will be accessible.
When water vapor is known, tropospheric water vapour can be computed by the equation given by Hogg et al (1981). When the zenith delay is known, the calculation of water vapor from the equation of the index of refraction can be obtained by Askne and Nodius (1987). Bevis et al (1992) have developed an equation which gives the weighted average temperature (Tm) depending on the surface temperature for the United States. Tm is the basic parameter of this equation. Emardson et al (2000) has developed a conversion factor Q between the zenith delay and water vapour for the Europe. The recent studies on this area are given in references.
In this project, the weighted average temperature Tm and Q parameters, which are described for the total tropospheric zenith delay and precipitable water vapor, respectively, will be determined by the analysis of Turkey's radiosonde profiles. Accuracy and reliability of these parameters will be checked by data from the continuous GPS stations which will be established within the scope of this project in Istanbul and Ankara radiosonde stations representing two different climate zones. Appropriate Tm and Q models will be investigated and developed for different regions and all over Turkey.
By using Tm and Q parameters developed for Turkey, meteorology files of TUSAGA-active stations (which do not carry out meteorological observations) for the selected test area will be created. Test area will cover latitudes between 38° - 42° and longitudes between 29° - 33°. Then the amounts of precipitable water vapour will be calculated from these results retrospectively.
In order to obtain meteorological data from TUSAGA-active stations from observations of meteorological stations around these stations, the numerical temperature, pressure and humidity models will be investigated and be created. The total tropospheric zenith delay values and precipitable water vapour will also be modelled from these numerical models.
Studies in producing the basic parameters of the estimation of precipitable water vapour by GPS for Turkey and in obtaining precipitable water vapour at the accuracy of radiosondes are qualifications for creating an infrastructure. They will be the first studies ever conducted in this area. This will lead to use TUSAGA-Active which is already running for different purposes for GPS meteorology purposes. The precipitable water vapour data obtained continuously at the accuracy of radiosonde for all of Turkey will be used as a basis data for research on the weather and hydrological disaster prediction, long-time hydrological cycle, atmospheric chemistry and global climate. The results will be the basis data for global research.
Tropospheric zenith delay and precipitable water vapour of GPS stations, etc data are modelled within a superficial network. Thus, the temporal solutions of the horizontal distiributions of the parameters in question are carried out. This information which is obtained for large areas in a short time is the basic data for a storm, flood, etc. prediction. For these reasons, modeling research will be one of the first studies for Turkey. These studies are essential for the development of a global numerical model.
This research will be a great contribution to the use of the most accurate atmospheric parameter estimation models of the high-precise studies which is required by satellite and space techniques, such as GPS, VLBI,etc.