Analysis of seasonal changes of zenith tropospheric delay components determined by the radio sounding and GNSS measurements data

The aim of the work is to analyze the change of hydrostatic and wet component values of zenith tropospheric delay (ZTD), determined for all seasons of the year. For today, ZTD components are determined mainly as follows: hydrostatic component – by using one of existing analytical models, mostly Saastamoinen model, and wet component – from GNSS measurements using simulated value of hydrostatic component. Also, in this study we evaluated the accuracy of the obtained values of hydrostatic and wet ZTD components for similar components, determined by radio sounding. For this purpose, we selected a pair of relatively close to each other station – aerological station and GNSS reference one. To implement the research methodology described above, we choose the Praha-Libus aerological station and the GOPE GNSS reference station. For processing and analysis, we selected the data from radio soundings of neutral atmosphere from the first station and the total values of ZTD (hydrostatic plus wet components) from the second one. Such data were selected monthly from the 1st to the 10th day of 2012 at 12 o’clock Universal Time. According to the radio sounding data, we determined the hydrostatic and the wet components of ZTD (set as reference) and the same number of total values of ZTD, derived for the same hour from GNSS measurements at the GOPE reference station. Based on these data, we determined the values of wet component of ZTD and compared them with the corresponding data, obtained from radio soundings. We found that the error of the hydrostatic component has a clear seasonal change ranging from only positive values in the range of 2 – 7 mm in January with a change cross zero in April (October), reaching only negative values in the range of 3 – 5 mm in July. As for the error of the wet component of ZTD, it should be noted that it takes only negative values during the year without clear seasonal course. Note that maximum absolute value of this error is in July, which exceeds 30 mm, due to the maximum content of water vapor in the troposphere at this time. However, only negative values of the wet component error indicate a systematic shift of its values. This paper provides recommendations for further research to improve the accuracy of determination of both hydrostatic and wet components of ZTD, as well as the reasons for seasonal changes in the accuracy of determination, especially the hydrostatic component.