Factors Affecting Height Accuracy in Global Navigation Satellite Systems (GNSS)

By Shahabuddin Amerudin

Abstract

Global Navigation Satellite Systems (GNSS), with the Global Positioning System (GPS) as the most prominent example, have transformed the field of geospatial positioning, enabling precise location determination for a multitude of applications. While GNSS provides remarkable accuracy in horizontal position measurements, the accuracy of height measurements remains subject to various influencing factors. This article conducts an in-depth and comprehensive review of the factors impacting height accuracy in GNSS measurements, along with exploring the diverse techniques and technologies deployed to enhance height precision. Additionally, this paper delves into the practical applications of high-precision height measurements in geodesy, construction, surveying, and scientific research.

Introduction

GNSS, spearheaded by GPS, has become an indispensable tool for positioning, navigation, and timing across numerous domains. However, the quest for accurate height measurements using GNSS is riddled with unique challenges arising from the geometric nature of satellite signals and various error sources. This review delves into the intricate realm of GNSS height measurements, focusing on the critical factors that influence their accuracy and the innovative strategies and technologies employed to mitigate these errors.

Factors Influencing Height Accuracy in GNSS

Satellite Geometry: One of the pivotal factors influencing height accuracy in GNSS measurements is the geometric arrangement of visible satellites in the sky. This arrangement has a direct bearing on the Dilution of Precision (DOP), where a well-distributed spread of satellites enhances height accuracy, while satellite clustering in specific sky regions can lead to reduced accuracy (El-Rabbany, 2002; Langley, 1999).

Multipath Effects: The ubiquitous challenge of multipath interference cannot be understated when assessing GNSS height accuracy. Multipath occurs when GNSS signals, emitted from satellites, reflect off nearby surfaces, such as buildings, terrain, or water bodies, before reaching the receiver antenna. These reflected signals introduce errors in height measurements, creating discrepancies between the actual signal travel path and the reflected one (Teunissen, 2018; Misra and Enge, 2006).

Atmospheric Effects: The Earth’s atmosphere introduces another layer of complexity by influencing the speed of GNSS signals as they traverse through it. Two primary atmospheric effects impact height accuracy:

Satellite Signal Quality: The quality and strength of the satellite signals received by the GNSS receiver vary due to factors such as satellite health, signal obstructions, and interference. Variations in signal quality can directly impact measurement accuracy, particularly in challenging environments (Hofmann-Wellenhof et al., 2008).

Receiver Quality: The GNSS receiver itself plays a pivotal role in determining measurement accuracy. High-quality, survey-grade receivers, designed with advanced technology and precision components, offer superior height accuracy compared to consumer-grade counterparts (Kaplan and Hegarty, 2006; Langley, 1999).

Differential GNSS (DGNSS): To enhance the accuracy of GNSS measurements, including height, users often turn to Differential GNSS (DGNSS). Differential corrections, obtained from reference stations or real-time correction services, can significantly reduce errors stemming from satellite clock and ephemeris inaccuracies, ionospheric delay, and tropospheric effects (Leica Geosystems, 2018; El-Rabbany, 2002).

Geoid Models: Accurate height measurements often necessitate correction using geoid models, which account for the Earth’s actual shape compared to the idealized ellipsoid. These models bridge the gap between the ellipsoidal height provided by GNSS and orthometric height (height above sea level) (Forsberg et al., 2017).

Techniques to Enhance Height Accuracy

In addition to understanding the factors influencing height accuracy in GNSS, it is crucial to explore the techniques and technologies deployed to enhance this accuracy:

Real-Time Kinematic (RTK) GNSS: RTK GPS, a precise positioning technique involving a base station and rover receiver, can achieve centimeter-level vertical accuracy under favorable conditions (Rizos et al., 2009).

Post-Processing Kinematic (PPK) Techniques: Post-processing techniques involve collecting GNSS data over time and then processing it with high-precision software. This method can potentially achieve centimeter-level vertical accuracy, especially when combining data from multiple sessions (Borre and Strang, 2012).

Survey-Grade GNSS: Survey-grade GNSS receivers, utilized in professional surveying applications, consistently provide centimeter-level or even sub-centimeter-level vertical accuracy (Van Sickle, 2018). These receivers are engineered to meet the stringent requirements of high-precision applications.

Practical Applications of High-Precision Height Measurements

Beyond the theoretical underpinnings of GNSS height accuracy, the practical applications of high-precision height measurements are vast and impactful. These applications include:

Geodesy: Precise height measurements play a fundamental role in geodesy, contributing to the modeling of the Earth’s shape and gravity field. These measurements enable geodetic datum determination and geodetic infrastructure development (Heiskanen and Moritz, 1967; Torge, 2001).

Construction: The construction industry relies on accurate height measurements for activities such as site planning, building design, and the monitoring of construction progress. High-precision GNSS height measurements aid in ensuring the structural integrity of buildings and infrastructure (National Research Council, 2005).

Surveying and Land Management: Land surveyors heavily depend on GNSS height measurements for property boundary determination, cadastral mapping, and land-use planning. Precise height data is essential for resolving land disputes and managing land resources (Ghilani and Wolf, 2019).

Scientific Research: Scientific disciplines, including seismology, oceanography, and glaciology, rely on high-precision height measurements for monitoring geological and environmental changes. These measurements provide critical insights into processes such as sea level rise and tectonic plate movement (Blewitt et al., 2016; Melini et al., 2016).

Future Directions and Challenges

As GNSS technology continues to evolve, there are several directions and challenges that warrant attention in the quest for higher height accuracy:

Multi-Constellation GNSS: With the emergence of additional GNSS constellations, such as Galileo, BeiDou, and GLONASS, there is an opportunity to improve height accuracy by leveraging signals from multiple satellite systems. Combining signals from diverse constellations can enhance the availability and robustness of height measurements (Borre and Strang, 2012; Montenbruck et al., 2015).

Improved Signal Processing Techniques: Advancements in signal processing algorithms and software can further mitigate the impact of multipath effects and atmospheric disturbances on height accuracy. Research into more sophisticated error modeling and correction techniques is ongoing (Lestarquit et al., 2013).

Precise Point Positioning (PPP): PPP is an emerging technique that enables centimeter-level positioning accuracy without the need for a local base station. Continued research in PPP for height determination holds promise for simplifying high-precision GNSS measurements (Zhang et al., 2020).

Robustness in Challenging Environments: Achieving reliable height accuracy in challenging environments, such as urban canyons or dense forests, remains a formidable challenge. Developing specialized algorithms and hardware to address these scenarios is essential (Petovello et al., 2013).

Integration with Other Sensors: Integrating GNSS with other sensors, such as inertial measurement units (IMUs) and LiDAR, can enhance height accuracy in dynamic and complex environments. Sensor fusion techniques hold significant potential for improving height measurements (Törnqvist et al., 2017).

Standardization and Best Practices: The development of standardized procedures and best practices for height measurements is crucial to ensure consistency and comparability across different applications and industries (International Association of Geodesy, 2019).

Conclusion

In the realm of GNSS technology, height accuracy remains a dynamic and evolving field with profound implications for geodesy, construction, surveying, scientific research, and countless other applications. This review has shed light on the intricate factors that influence height accuracy in GNSS measurements, ranging from satellite geometry to atmospheric effects and receiver quality. It has also explored the array of techniques and technologies deployed to enhance height precision.

As GNSS technology continues to advance, addressing the challenges associated with height accuracy becomes increasingly vital. The adoption of multi-constellation GNSS, improved signal processing techniques, PPP, and robustness in challenging environments will play pivotal roles in pushing the boundaries of height accuracy. Moreover, the integration of GNSS with complementary sensor technologies and the establishment of standardized practices will further bolster the reliability and utility of high-precision height measurements.

In conclusion, the pursuit of height accuracy in GNSS is not merely a technical endeavor but a critical enabler of progress and innovation across diverse fields. As the world becomes increasingly reliant on precise geospatial data, the quest for more accurate height measurements in GNSS remains central to our collective pursuit of knowledge, safety, and sustainable development.

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Suggestion for Citation:
Amerudin, S. (2023). Factors Affecting Height Accuracy in Global Navigation Satellite Systems (GNSS). [Online] Available at: https://people.utm.my/shahabuddin/?p=7099 (Accessed: 13 September 2023).