image: The radial axis represents the elevation angle, the azimuth angle is represented by the angular axis, and the color represents the values of non-isotropic (∆), which is the difference between the SPD at different azimuth angles and the average SPD at the same elevation angle. Based on Fig. 11, considering the elevation angle range of 10°–20°, it can be observed that the tropospheric delay values exhibit some variation within four azimuth angle intervals: 45°–135°, 225°–315°, 315°–45°, and 135°–225°. More specifically, the values within the 45°–135° and 225°–315° intervals are similar, while the 315°–45° and 135°–225° intervals display noticeable disparities compared to the other intervals.
Credit: Satellite Navigation
A new study unveils a critical aspect of tropospheric delays affecting Global Navigation Satellite Systems (GNSS) - their non-isotropic nature. By analyzing Slant Path Delays (SPD) across different azimuth angles, researchers have discovered significant variations that challenge the conventional isotropic and anisotropic assumptions in tropospheric modeling.
GNSS provide invaluable positioning data for countless applications, from everyday navigation to scientific research. Tropospheric delays, caused by the refractive properties of the atmosphere, significantly impact the accuracy of GNSS positioning. The standard practice of multiplying Zenith Tropospheric Delay (ZTD) by a Mapping Function (MF) to derive SPD operates under an assumption of atmospheric isotropy, limiting precision in GNSS applications.
On a recently published study (doi: 10.1186/s43020-023-00122-5) in the journal Satellite Navigation, researchers from Shandong University of Science and Technology introduces a novel concept that SPDs are non-isotropic with respect to azimuth angles, departing from traditional isotropic and anisotropic assumptions. They utilized three different mapping functions and conducted evaluations at five International GNSS Service (IGS) stations, employing the ray-tracing method as a benchmark. The study compared SPD accuracy using Vienna Mapping Function 3 (VMF3) and found the smallest residual between VMF3-derived SPDs and ray-traced SPDs. Surprisingly, introducing a horizontal gradient correction for azimuth-dependent SPD variations showed no significant improvement in accuracy.
Dr. Ying Xu, the leading researcher of this study, emphasizes the importance of this research: "This revelation of non-isotropic tropospheric delays is a game-changer for high-precision GNSS applications. By acknowledging and understanding these variations across azimuth angles, we can develop more accurate models, significantly enhancing the reliability of GNSS positioning systems."
The discovery of non-isotropic behavior in SPD across different azimuth angles highlights a pivotal aspect previously overlooked in tropospheric delay modeling. This insight challenges existing methodologies and suggests the need for new models that accurately represent the complex dynamics of the troposphere. Such advancements are crucial for applications requiring high-precision GNSS positioning, such as geodesy, navigation, and atmospheric sciences.
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Reference
DOI
Original Source URL
https://doi.org/10.1186/s43020-023-00122-5
Funding information
This work is supported by the National Natural Science Foundation of China (No. 42174035).
About Satellite Navigation
Satellite Navigation (E-ISSN: 2662-1363; ISSN: 2662-9291) is the official journal of Aerospace Information Research Institute, Chinese Academy of Sciences. The aims is to report innovative ideas, new results or progress on the theoretical techniques and applications of satellite navigation. The journal welcomes original articles, reviews and commentaries.
Journal
Satellite Navigation
Subject of Research
Not applicable
Article Title
An initial investigation of the non-isotropic feature of GNSS tropospheric delay
Article Publication Date
15-Jan-2024
COI Statement
The authors declare that they have no competing interests