Abstract
A regional zenith tropospheric delay (ZTD) empirical model, referred to as SHAtropE (SHanghai Astronomical observatory tropospheric delay model—Extended), is developed and provides tropospheric propagation delay corrections for users in China and the surrounding areas with improved accuracy. The SHAtropE model was developed based on the ZTD time series of the continuous GNSS sites from the Crustal Movement Observation Network of China (CMONOC) and GNSS sites of surrounding areas. It combines the exponential and periodical functions and is provided as regional grids with a resolution of 2.5° × 2.0° in longitude and latitude. At each grid point, the exponential function converts the ZTD from the site height to the ellipsoid, and the periodical terms, including both annual and semi-annual periods, describe ZTD’s temporal variation. Moreover, SHAtropE also provides the predicted ZTD uncertainty, which is valuable in Precise Point Positioning (PPP) with ZTD being constrained for faster convergence. The data of 310 GNSS sites over 7 years were used to validate the new model. Results show that the SHAtropE ZTD has an accuracy of 3.5 cm in root mean square (RMS) quantity, which has a mean improvement of 35.2% and 5.4% over the UNB3m (5.4 cm) and GPT3 (3.7 cm) models, respectively. The predicted uncertainty of SHAtropE ZTD shows seasonal variations, where the values are larger in summer than in winter. By applying the SHAtropE model in the static PPP, the convergence time of GPS-only and BDS-only solutions are reduced by 8.1% and 14.5% respectively compared to the UNB3m model, and the reductions are 6.9% and 11.2% respectively for the GPT3 model. As no meteorological data are required for the implementation of the model, the SHAtropE could thus be a refined tropospheric model for GNSS users in mainland China and the surrounding areas. The method of modeling the ZTD uncertainty can also be used in further global tropospheric delay modeling.
Highlights
Through the path that radio signals traverse the troposphere, the signals are delayed to a magnitude of meters with respect to free-space propagation [1]
Tropospheric delay is usually modeled as the sum of a hydrostatic and a non-hydrostatic part, where the hydrostatic delay accounts for 90% of the zenith total delay (ZTD), while the wet delays contributes to the precipitable water vapor are hard to model since the water vapor varies a lot [5]
This study presents a regional ZTD empirical model, SHAtropE, for the regions of [70°E–135°E, 18°N–54°N]
Summary
Through the path that radio signals traverse the troposphere, the signals are delayed to a magnitude of meters with respect to free-space propagation [1]. The discrete tropospheric delays can be derived from radiosonde observations and Numerical Weather Model (NWM), e.g., European Centre for Medium-Range Weather Forecasts (ECMWF). The latter one has a global coverage and a ZTD accuracy of 1–2 cm can be achieved [12,13]. The empirical models could be determined using discrete ZTD data from radiosonde, NWM, and GNSS products. The improved version of GPT, i.e., GPT2 [21], GPT2w [22], and GPT3 [23], all use a global grid with better accuracy and more tropospheric information All these models determine the global meteorological parameters, e.g., pressure, temperature, water vapor, and they are used as inputs of other models, e.g., the Saasamoinen and the Askne and Nordius [24] models to calculate tropospheric delays.
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