Abstract
Using the observations from local and regional GPS networks, the estimation of slant wet delays (SWDs) is possible for each line of sight between satellite and receiver. The observations of SWD are used to model horizontal and vertical variations of the wet refractivity in the atmosphere above the study area. This work is done using the tomography method. In tomography, the horizontal variations of tropospheric wet refractivity are modeled with the polynomial in degree and rank of 2 with latitude and longitude as variables. Also, altitude variations are modeled in the form of discrete layers with constant heights. The main innovation is to estimate the tropospheric parameters for each line of sight by the artificial neural networks (ANNs). The SWD obtained from GPS observations for the different signals at each station is compared with the SWD generated by the ANNs (SWDGPS–SWDANNs). The square of the difference between these two values is introduced as the cost function in the ANNs. To evaluate, we used observations from October 27 to 31, 2011. The availability of GPS and radiosonde data is the main reason for choosing this timeframe. The correlation coefficient, root mean square error (RMSE), and relative error allow for evaluation of the proposed model. The results were also compared with the results of the voxel-based troposphere tomography method. For a more detailed evaluation, four test stations are selected and ANN zenith wet delays (ZWDANN) are compared with the ZWDGPS. Observations of test stations are not used in the modeling step. The correlation coefficient in the testing step for TomoANN and Tomovoxel is 0.9006 and 0.8863, respectively. The mean RMSE at 5 days for TomoANN and Tomovoxel is calculated as 0.63 and 0.71 mm/km, respectively. Also, the average relative error at the four test stations for TomoANN is 15.37% and for Tomovoxel it is 19.69%. The results demonstrate the better capability of the proposed method in the modeling of the tropospheric wet refractivity in the region of Iran.
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Data Availability
Tabriz radiosonde data can be obtained from the Iranian Meteorological Organization. Also, observations of GPS stations in the northwest of Iran (Azarbayjan network) are available at the National Cartographic Center of Iran. All data and results in this paper can be provided to readers by contacting the corresponding author.
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Acknowledgement
The authors of this paper appreciate the National Cartographic Center (NCC) of Iran for providing GPS data from the IPGN. Also, the authors thank the reviewers and editor, which provided valuable insights to improve the results of the paper. This paper was supported by the Iran National Science Foundation and K. N. Toosi University of Technology (Project No. 96008789).
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Ghaffari Razin, MR., Voosoghi, B. Estimation of tropospheric wet refractivity using tomography method and artificial neural networks in Iranian case study. GPS Solut 24, 65 (2020). https://doi.org/10.1007/s10291-020-00979-y
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DOI: https://doi.org/10.1007/s10291-020-00979-y