Session A1: Alternatives, Backups, Complements to GNSS

Exploring the Utility of Doppler Shift Measurements for Enhanced GNSS Positioning
Lennon Headlee, Sherman Lo, Todd Walter, Stanford University
Location: Beacon A

The Global Navigation Satellite System (GNSS) has evolved into a critical infrastructure for a multitude of positioning, navigation, and timing applications; however, limitations in the accuracy and availability of traditional GNSS solutions, particularly those based solely on pseudorange measurements, persist—especially in environments with poor satellite visibility, high dynamics, or signal obstruction. To address these challenges, recent research has focused on integrating additional measurement types, such as Doppler shifts, to enhance GNSS performance. Doppler measurements, traditionally used for velocity estimation, offer a promising opportunity to augment position estimates by providing information about the rate of change of the range between the satellite and receiver. This research seeks to refine and extend previous work by advancing a combined GNSS solution that incorporates both pseudorange and Doppler measurements from various satellite constellations, with particular attention to the role of Low Earth Orbit (LEO) satellites. The core of this research is the development and validation of a combined solution that leverages Doppler shift measurements alongside pseudorange data to improve the estimation of user position, velocity, clock bias, and clock bias rate. By integrating both types of data, this approach seeks to overcome the limitations of traditional pseudorange-only GNSS solutions. The simplified combined solution has been initially developed and demonstrated, showing promise in improving positioning accuracy under specific conditions, such as challenging satellite geometries and in regions where Medium Earth Orbit (MEO) GNSS constellations alone do not provide sufficient satellite coverage; however, several critical areas remain for further exploration and refinement.
This work advances that combined solution by focusing on two main areas of improvement. First, a more representative Doppler error model will be developed and incorporated into the MATLAB Algorithm Availability Simulation Tool (MAAST) to enable more accurate simulation results. This involves a comprehensive review and incorporation of error sources specific to Doppler measurements, such as tropospheric and ionospheric effects, multipath interference, and clock/ephemeris errors. The sensitivity of the solution to Doppler error will be further examined through a series of simulations to identify conditions under which Doppler measurements provide significant benefits. These simulations will seek to explore the impact that parameters such as the number of satellites, stability of Doppler and clock bias rate measurements, and altitude of LEO satellites have on accuracy, integrity level, and availability. Second, the scope of the research will be broadened to include the incorporation of Doppler measurements from a variety of LEO satellite constellations. LEO satellites, such as those from the OneWeb constellation used in the earlier work, are characterized by their much higher velocities relative to MEO GNSS satellites. This increased velocity offers the potential for significant improvements in the precision of Doppler-based measurements; however, these constellations typically lack the atomic clock accuracy and precise downlink signals required for pseudorange measurements, limiting their utility in traditional GNSS solutions. This research will further explore the potential of LEO constellations to serve as Doppler-only sources of augmentation for GNSS positioning. By integrating LEO Doppler measurements into the combined solution, we aim to evaluate the potential benefits of using these constellations to enhance both positioning accuracy and availability.
Preliminary results indicate that the combined pseudorange and Doppler solution offers meaningful improvements in both positioning accuracy and signal availability, especially in areas with poor satellite geometry, such as polar regions. Analysis shows that the accuracy of Doppler measurements plays a critical role in determining the extent of these improvements. Furthermore, LEO Doppler measurements provide a notable enhancement to signal geometry, which in turn provides more consistent availability. This is particularly evident in polar regions, where the high inclination of LEO constellations offers enhanced satellite visibility compared to MEO constellations. The next phase of this research will focus on several key objectives. First, the simplified pseudorange rate solution will be replaced with a more representative model that accounts for additional error sources and real-world conditions. This will enable a more accurate assessment of the potential benefits of Doppler measurements in GNSS positioning. Second, the Doppler error models for both GNSS and LEO satellites will be further refined based on a more detailed review of the literature and potential experimentation. These refinements will improve the robustness of the simulations and provide more realistic estimates of the potential performance gains from Doppler augmentation. Third, the combined solution will be compared against other approaches from the literature to assess its relative strengths and weaknesses. This comparison will help identify the scenarios where Doppler augmentation offers the most significant benefits, as well as those where alternative approaches may be more effective. In addition to these technical advancements, this research aims to explore the real-world implementation of the combined solution using actual GNSS and LEO satellite data. This will provide an opportunity to validate the simulation results and assess the practical feasibility of Doppler-enhanced GNSS positioning.