Optical Clocks in GNSS Positioning
Creators: name: Boldbaatar, Enkhtuvshin
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401306
subject: Surveying (incl. hydrographic surveying)
classificationCode: 370603
subject: Geodesy
classificationCode: 370606
subject: Gravimetrics

description: Atomic clocks are highly precise timing devices crucial for Positioning, Navigation, and Timing (PNT) applications both on Earth and in space. In recent years, advancements in technology have led to the introduction of more precise timing solutions based on optical technology. State-of-the-art optical clocks, considered the next evolution of traditional atomic clocks, have achieved an unprecedented uncertainty of 1 × 10−18, surpassing the best atomic clock performance by two orders of magnitude. This breakthrough has garnered significant attention from academia and industry, opening up new opportunities for exploitation.

Despite this progress, limited research has been conducted to assess the impact of optical clocks on Global Navigation Satellite System (GNSS) precise positioning applications. Consequently, it remains uncertain whether more precise clocks would improve positioning solutions, from the perspectives of both GNSS community and clock manufacturers. Most research efforts have been directed towards clock development, further advancements, fundamental physics tests, gravity field determination, time and frequency transfer experiments, and metrology applications, with limited focus on evaluating the practical implications of optical clocks for GNSS positioning.

To address this gap, this thesis aims to investigate the advancements in optical clocks in comparison to atomic clocks and their potential to enhance GNSS positioning applications. The study addresses three primary research questions: exploring the development, characteristics, and significance of optical clocks in PNT; comparing their frequency stability with current GNSS satellites; and evaluating their impact on GNSS positioning performance.

To answer the research questions, the research conducts two key assessments along with an overview of optical clock development in existing literature. The first assessment involves a comprehensive analysis of clock stability, comparing the frequency stability of three state-of-the-art optical clocks with existing atomic clocks onboard GNSS satellites as of 2022. Using Allan Deviation (ADEV) metrics, this comparison reveals that optical clocks exhibit superior stability, indicating potential improvements in satellite-based positioning systems. Furthermore, the literature highlights challenges in optical clock maturity, including Size, Weight, and Power consumption (SWaP) parameters for practical deployments, and suggests future research directions to address these challenges.

The second assessment evaluates the impact of optical clocks on GNSS user positioning precision and the success rate in resolving carrier-phase ambiguities through a simulated GNSS Precise Point Positioning – Real Time Kinematic (PPP-RTK) application in MATLAB. The simulation assumes deployment of the three aforementioned optical clocks on GNSS satellites as the primary source of oscillation. Each clock undergoes an identical simulation scenario to isolate its impact from other error sources. The corresponding ADEV values driven by white noises are modelled as process noise in the Kalman Filter processing. The results demonstrate that optical clocks significantly enhance positioning precision and reliability, highlighting their potential to improve GNSS PPP-RTK applications.