Session A4: PNT Solutions for Space Applications

Clock Transfer and Positioning Method in Cislunar Space by Innovative Hybrid Device
Shingo Nishimoto and Junichiro Kawaguchi, Australian National University
Location: Beacon A
Date/Time: Thursday, Jan. 30, 11:48 a.m.

Navigation technology in the Cislunar region is essential for future activities. However, GNSS alone is insufficient to achieve practical positioning accuracy due to the poor geometric distribution of satellites in this region. This paper explores an innovative hybrid navigation system that combines GNSS with Asynchronous One-Way Range (AOWR) technology for navigation in Cislunar space. The AOWR scheme synchronizes clocks between the ground station and the spacecraft by iteratively exchanging information using only a pair of entities. While GNSS-based navigation typically requires determining four parameters, including the receiver clock offset, integrating the AOWR system reduces this requirement to three parameters. The analysis of the Dilution of Precision (DOP) for this scheme provides quantitative insights into its potential for significantly improved positioning accuracy. The AOWR scheme requires communication with a ground station to achieve clock synchronization. By utilizing a highly stable and compact clock, such as the Chip-Scale Atomic Clock (CSAC), the synchronized time can be propagated for a certain period, enabling the navigation system to function onboard without continuous communication with the ground station. There are several technical challenges in realizing this hybrid navigation system, including clock sharing between the GNSS receiver and the AOWR component, improving link margins, and refining the navigation filter. This paper focuses on the clock-sharing technique and the demonstration of Cislunar positioning. Clock sharing leverages the pseudo-satellite’s signal to measure the clock offset between the AOWR component and the GNSS receiver. The ground GNSS receiver determines its clock offset as part of its positioning process. Using signals from the AOWR system, the ground station determines the AOWR component’s clock offset. Since the AOWR synchronizes clocks between the spacecraft and the ground, the spacecraft’s GNSS receiver clock offset can also be determined without requiring GNSS-based positioning. A hardware-based simulation was conducted to evaluate the hybrid scheme and compare it with navigation using only GNSS, employing software-defined radio (SDR) devices. The simulation included both static and dynamic scenarios. In the static case, at a distance of 4 × 105 km, the positioning accuracy achieved was less than 50 m in most cases, whereas the position error was more than 1 km by GNSS alone. This result highlights the hybrid scheme’s ability to eliminate the line-of-sight (LOS) direction uncertainty. For the dynamic scenario, the Low Lunar Orbit (LLO) was simulated, and the positioning accuracy improved significantly, from more than 990.1 m to 19.6 m in 3D RMS.