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.
by Simon Mansfield
Sydney, Australia (SPX)
Jan 23, 2024
Researchers from Shandong University of Science and Technology have published a study that sheds new light on the intricate nature of tropospheric delays impacting Global Navigation Satellite Systems (GNSS). This study, featured in the journal Satellite Navigation, goes beyond the traditional isotropic and anisotropic assumptions in tropospheric modeling, introducing a novel perspective on the non-isotropic characteristics of Slant Path Delays (SPD).
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
Aerospace Information Research Institute
GPS Applications, Technology and Suppliers
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 inherent fluctuational instabilities 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 slant path delays (SPD) operates under an assumption of atmospheric isotropy, limiting precision in GNSS applications. Researchers from Shandong University of Science and Technology introduce 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 Global Navigation Satellite Systems (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. Acknowledging and understanding these variations across azimuth angles, a more accurate model can be developed, significantly enhancing the reliability of GNSS positioning systems.