Trend estimation of zenith total delays at IGS stations by using nonparametric methods
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ORIGINAL PAPER
Trend estimation of zenith total delays at IGS stations by using nonparametric methods Cansu Beşel 1 & Emine Tanır Kayıkçı 1 Received: 4 April 2018 / Accepted: 10 July 2019 # Saudi Society for Geosciences 2019
Abstract Global Navigation Satellite System (GNSS) observations play an important role in climate studies because of the advantages they provide. An analysis of the GNSS observations can be used to obtain the zenith total delay (ZTD) parameter, which represents the effect of weather conditions. In this study, the trends of the time series of IGS (International GNSS Service) Repro1 ZTD data recorded between 1995 and 2010 at 19 GNSS stations throughout Turkey and Europe reprocessed in the framework of COST Action ES1206 were investigated. The trends of the time series were estimated by using the Mann–Kendall rank correlation test in addition to Spearman’s rho test and Sen’s slope method. In addition, the Run (Swed–Eisenhart) test was performed to test for homogeneity. The aim of this study was to evaluate the advantages and disadvantages of the applications of different methods to the estimation of trends in the ZTD time series. According to the Mann–Kendall rank correlation test, the ANKR, GRAS, HERS and MAS1 stations exhibited an increasing trend. Using the Spearman rho test, increasing trends were observed at the ANKR, EBRE and MAS1 stations, while decreasing trends were observed at the BRUS and GRAS stations. Finally, negative slope values were obtained at the BRUS, GRAS, GRAZ, HERS, JOZE, PENC and WTZR stations by using the Sen slope method. Keywords Global Navigation Satellite System . Zenith total delay . Trend analysis . Nonparametric methods
Introduction Currently, climate change is one of the most considered environmental issues (Demircan et al. 2016). Owing to their advantages, Global Navigation Satellite System (GNSS) observations play an important role in climate studies. GNSS signals change as they propagate through the atmosphere. Different layers of the atmosphere transform signals for various reasons. For example, different atmospheric layers have different refraction indexes; thus, when a GNSS signal moves through an atmospheric layer, it is exposed to a propagation delay due to the properties of that layer. This delay is divided into two categories: tropospheric and ionospheric. Editorial handling: Nilanchal Patel * Cansu Beşel [email protected] Emine Tanır Kayıkçı [email protected] 1
Department of Geomatics Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey
GNSS signals are exposed to tropospheric delays before reaching receivers located on the earth. This delay is transformed into the zenith direction and obtained as the zenith total delay (ZTD; Bevis et al. 1992; Baldysz et al. 2016). The size and seasonal variations of the ZTD depend on the latitude, longitude and distance from water masses. Therefore, the ZTD provides information on the regional weather conditions. All of these factors encourage scientists to observe the changes taking place throug
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