Evaluation of Zenith Tropospheric Delay Derived from Ray-Traced VMF3 Product over the West African Region Using GNSS Observations

Stefania Bonafoni,Collins Fosu,Samuel Osah,Isaac Dadzie,Akwasi A. Acheampong

doi:10.1155/2021/8836806

Abstract

The growing demand for Global Navigation Satellite System (GNSS) technology has necessitated the establishment of a vast and ever-growing network of International GNSS Service (IGS) tracking stations worldwide. The IGS provides highly accurate and highly reliable daily time-series Zenith Tropospheric Delay (ZTD) products using data from the member sites towards the use of GNSS for precise geodetic, climatological, and meteorological applications. However, if for reasons like poor internet connectivity, equipment failure, and power outages, the IGS station is inaccessible or malfunctioning, and gaps are created in the data archive resulting in degrading the quality of the ZTD and precipitable water vapour (PWV) estimation. To address this challenge as a means of providing an alternative data source to improve the continuous availability of ZTD data and as a backup data in the event that the IGS site data are missing or unavailable in West Africa, this paper compares the sitewise operational Vienna Mapping Functions 3 (VMF3) ZTD product with the IGS final ZTD product over five IGS stations in West Africa. Eight different statistical evaluation metrics, such as the mean bias (MB), mean absolute error (MAE), root mean squared error (RMSE), Pearson correlation coefficient (r), coefficient of determination (r2), refined index of agreement (IAr), Nash–Sutcliffe coefficient of efficiency (NSE), and the fraction of prediction within a factor of two (FAC2), are employed to determine the degree of agreement between the VMF3 and IGS tropospheric products. The results show that the VMF3-ZTD product performed excellently and matches very well with the IGS final ZTD product with an average MB, MAE, RMSE, r, r2, NSE, IAr, and FAC2 of 0.38 cm, 0.87 cm, 1.11 cm, 0.988, 0.976, 0.967, 0.992, and 1.00 (100%), respectively. This result is an indication that the VMF3-ZTD product is accurate enough to be used as an alternative source of ZTD data to augment the IGS final ZTD product for positioning and meteorological applications in West Africa.

Highlights

Introduction e impact of theEarth’s neutral atmosphere on Global Navigation Satellite System (GNSS) signals has become a major concern in GNSS Positioning, Navigation, and Timing (PNT) applications
Vienna Mapping Functions 3 (VMF3) is the successor of VMF1 realized on both 1° × 1° and 5°× 5 ° global grids [17]. e sitewise tropospheric delay products provided for the International GNSS Service (IGS) stations include zenith hydrostatic delay (ZHD), zenith wet delay (ZWD), meteorological parameters like pressure (P), temperature (T), and water vapour partial pressure (e), and mapping function coefficients for both hydrostatic and wet components. e Zenith Tropospheric Delay (ZTD) is obtained by adding ZHD and ZWD. e VMF3-ZTD data are available at https://vmf.geo.tuwien.ac.at/trop_products/ GNSS/VMF3/
To evaluate the performance of the VMF3-ZTD model predictions, it is important to measure how well its predictions match the IGSZTD values. e mean bias (MB), mean absolute error (MAE), and root mean squared error (RMSE) help to quantify how close the VMF3-ZTD values are to the IGS-ZTD values. e smaller the MB, MAE, and RMSE values are, the closer the VMF3-ZTD predictions are to the IGS-ZTD values, and the better the predictions

Summary

Research Article

Evaluation of Zenith Tropospheric Delay Derived from Ray-Traced VMF3 Product over the West African Region Using GNSS Observations. Ssenyunzi et al [31] assessed the performance of ERA5 data in retrieving PWV over East African tropical region All of these studies suggest that GNSS-derived ZTD may be useful for improving the amount of water vapour retrieval from the atmosphere for meteorological applications in African and for the study and monitoring of the West African Monsoons. (ii) An additional ZTD data source for estimating PWV that can help improve continuous observations of water vapour in West Africa for meteorological applications and scientific investigations such as improving climate models to better understand the impact of climate change in the region, which will have a direct impact on agriculture, water resources, and energy. Tm can be obtained from global or regional Tm models such as follows [5]: Tm 0.72 · Ts + 70.2

Materials and Methods

Methodology

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