GNSS, which encompasses systems like GPS, GLONASS, and Galileo, is fundamental to a myriad of applications ranging from everyday navigation to intricate scientific research. The accuracy of these systems is paramount, and one significant factor affecting this accuracy is the tropospheric delay.

The Earth's troposphere, the lowest atmospheric layer, contains water vapor and other elements that refract satellite signals, leading to delays. These delays are traditionally accounted for using Zenith Tropospheric Delay (ZTD) and a mapping function that adjusts ZTD based on the satellite's elevation angle. However, these models often assume that the troposphere's impact on signals is uniform (isotropic) or, at best, predictably variable (anisotropic).

The study from Shandong University introduces a different approach by asserting that SPDs are non-isotropic with respect to azimuth angles. This insight challenges the longstanding isotropy and anisotropy models in tropospheric delay calculation.

The researchers employed three distinct mapping functions and conducted evaluations at five International GNSS Service (IGS) stations. These evaluations involved comparing the accuracy of SPDs derived from the Vienna Mapping Function 3 (VMF3) against those obtained through ray-tracing, a method considered a benchmark in this context.

One of the key findings of the study was the smallest residual between VMF3-derived SPDs and ray-traced SPDs, highlighting the potential of VMF3 in enhancing GNSS accuracy. Interestingly, the study also found that introducing a horizontal gradient correction to account for azimuth-dependent SPD variations did not significantly improve accuracy. This suggests that the non-isotropic nature of tropospheric delays is more complex than previously understood.

Dr. Ying Xu, the lead researcher, highlighted the significance of these findings, stating, "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 is not just a theoretical exercise; it has practical implications for a range of applications. High-precision GNSS positioning is vital in fields like geodesy, where accurate measurement of the Earth's shape, orientation in space, and gravity field is critical. It also plays a significant role in atmospheric sciences, where precise data is crucial for weather forecasting and climate studies.

This study's findings challenge existing methodologies and call for the development of new models that can accurately represent the tropospheric delays' complex dynamics. Such advancements are crucial for improving the reliability and accuracy of GNSS applications, impacting a wide array of sectors, from navigation and transportation to scientific research and defense.

Research Report:An initial investigation of the non-isotropic feature of GNSS tropospheric delay

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