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Session B2: Trends in GNSS Augmentation Systems

Enabling Satellite-Based Nationwide High Precision GNSS Positioning Through Homogeneous Network RTK and Comprehensive Residual Error Modeling
Byungwoon Park, Yongrae Jo, Sungik Kim, Yebin Lee, Sejong University; Yunho Cha, Sejong University, Hanwha Aerospace ; Cheolsoon Lim, Sejong University, LIGNex1
Alternate Number 2

Introduction
The evolution of global navigation satellite system (GNSS) technology has been instrumental in enhancing various high-precision positioning applications, including geodesy, surveying, autonomous vehicle navigation, and environmental monitoring. These advancements have mostly been anchored by real-time kinematic (RTK) and network RTK (N-RTK) systems, which provide centimeter-level positioning accuracy. However, these traditional systems encounter limitations due to spatial decorrelation errors over long distances and a limited number of service-available users due to the need for high-speed two-way communication channels. To address these challenges, satellite-based precision GNSS services have largely adopted state space representation (SSR)-based corrections. SSR corrections are ideally suited for one-way, low-bandwidth satellite broadcasts, as they are capable of generating a uniform message throughout the service area, making them indispensable for satellite-based services. Nonetheless, SSR-based corrections also have their limitations, including longer initialization times, poor performance in determining cycle ambiguity leading to frequent wrong determination, and considerable residual error bounds, restricting their use in mobile applications. If observation space representation (OSR) could be utilized in place of SSR for satellite-based services, it would facilitate rapid and reliable precise positioning for all users within the service area, regardless of user volume. This study introduces a novel approach known as homogeneous network RTK (HN-RTK), which is enhanced with advanced residual error modeling and designed to expand high-precision GNSS positioning services on a nationwide scale. By employing satellite-based correction broadcasts, HN-RTK aims to ensure wide accessibility and reliability, thereby effectively overcoming the inherent weakness of SSR-based corrections.
Homogeneous Network RTK (HN-RTK)
HN-RTK represents a paradigm shift in high-precision GNSS positioning technology. By synthesizing the strengths of OSR and SSR methodologies, HN-RTK provides a unified solution that addresses the spatial decorrelation error inherent in GNSS measurements. Traditional N-RTK systems rely on corrections from multiple reference stations, which are inherently constrained by the geographical coverage of the network and the user’s proximity to these stations. In these systems, the network that provides corrections is contingent upon the user’s approximate location within the conventional range of a 50 to 70 km radius, necessitating the user to transmit the coarse position to the server through a bidirectional communication link. This requirement inevitably limits the number of users that can be serviced simultaneously. HN-RTK, on the other hand, employs a single master station to broadcast corrections across an entire service area as shown in Figure 1, effectively creating multiple continuous correction planes that extend beyond 500 km [1]. This approach not only simplifies the correction delivery system but also ensures that all users, irrespective of their location within the service area, receive consistent and accurate corrections. Since HN-RTK is mathematically equivalent to N-RTK, the innovative design of HN-RTK significantly reduces initialization times while enhancing positional accuracy compared to conventional SSR-based methods, making it an ideal solution for applications requiring rapid and precise location information.
Advanced Residual Error Modeling
While HN-RTK substantially improves positioning accuracy, the issue of residual errors remains a concern. A basic first-order plane may not sufficiently capture the complex nature of spatial decorrelation errors, potentially leading to inaccurate results. Even with HN-RTK correction applied, residual errors still exist in user measurements, resulting in unmodeled factors within the network. To overcome this, we propose an advanced second-order convex model for estimating error uncertainty as shown in Figure 2 [2]. This model leverages additional data from monitoring stations strategically placed within the network. The Implementation of this model facilitates the prediction of the uncertainty levels of residual errors that remain after compensating for spatial decorrelation errors using traditional first-order correction planes. The uncertainty estimation can then be used to compute weight matrices for positioning and protection levels for the users.
Satellite-Enabled Nationwide Service
A cornerstone of the proposed HN-RTK system is its integration with satellite-based correction broadcast services. This integration marks a significant advancement in making high-precision GNSS positioning universally accessible, overcoming the limitations posed by terrestrial communication infrastructures. By broadcasting corrections through satellites, HN-RTK ensures that high-precision positioning information is available across the entire nation, including remote, urban, and maritime regions. This satellite-enabled approach not only expands the reach of GNSS positioning services but also enhances the robustness and reliability of the system against environmental and infrastructural challenges.
To see the feasibility of integrating the HN-RTK with satellite-based correction broadcast, this study implemented an HN-RTK correction generating module with single master station observables and multi-constellation FKP corrections. We conducted real-time experiments using GPS/Galileo/BeiDou observation data transmitted by CORS (Continuous Operating Reference Station) stations operated by the NGII (National Geographic Information Institute) in South Korea. As shown in Figure 3, 19 reference stations distributed in South Korea were selected, and the Delaunay triangulation method was used to form optimal triangle networks. Also, in consideration of the HN-RTK correction broadcast via satellites, the broadcast interval of the corrections was scheduled to a bandwidth limit of 2,000bps or less, and the performance sensitivity analysis was performed to analyze the effect of the correction scheduling on the positioning accuracy. To evaluate the performance of the HN-RTK according to baseline length, total 26 user stations. The HN-RTK performance was assessed in terms of position accuracy, carrier-phase ambiguity fix rate for the 26 static users using 3-hour GNSS observation data.
Figure 4 shows that the results of positioning performance sensitivity analysis with various broadcast intervals. To simulate the message bandwidth limitation of 2,000bps, the correction broadcast interval was set from 1 to 10 second for the main station observation and 1 to 30 second for the FKP corrections. In spite of the long correction message interval due to the bandwidth limitation, the positioning accuracy maintained to the cm-level and the accuracy degradation was only several mm. Therefore, it is expected that satellite-based nationwide precise positioning service using the HN-RTK method will be possible to provide the same correction messages throughout the entire area.
Impact and Future Directions
The development and implementation of HN-RTK, complemented by advanced residual error modeling and satellite-based correction broadcasts, promise to revolutionize the field of high-precision GNSS positioning. This technology opens new horizons for a multitude of applications, including precision agriculture, disaster management, and autonomous navigation, where the accuracy and reliability of positioning data are paramount. Looking forward, the continued refinement of HN-RTK and its integration with emerging satellite constellations and communication technologies will further enhance its performance and applicability. As we stand on the brink of this new era in GNSS positioning, the potential for innovation and the impact on society at large are boundless.
Conclusion
In conclusion, the integration of Homogeneous Network RTK with advanced residual error compensation and satellite-based correction broadcasts represents a transformative development in the field of GNSS positioning. By addressing the limitations of existing systems and extending high-precision positioning capabilities nationwide, this approach sets a new standard for accuracy, reliability, and accessibility in GNSS services. As we move forward, the implications of this technology for enhancing and enabling a wide range of applications are profound, marking a significant milestone in the ongoing evolution of GNSS positioning technology.
Reference
[1] Cheolsoon Lim, Byungwoon Park, Homogeneous Network RTK for Satellite Based Nationwide High Precision GNSS Positioning Service, ION GNSS+ 2022.
[2] Cha, Yunho, Lim, Cheolsoon, Lee, Yebin, Jo, Yongrae, Park, Byungwoon, Song, Junesol, "Homogeneous Network-RTK Correction Residual Modeling Techniques for Improving the User Positioning Performance," Proceedings of the 36th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2023), Denver, Colorado, September 2023, pp. 2539-2549.



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