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

Navigation technology in the Cislunar region is essential for future activities. However, GNSS alone is insufficient due to the poor geometric distribution of satellites. This paper explores an innovative hybrid navigation system that combines GNSS and AOWR (Asynchronous One-Way Range) in the Cislunar region. The AOWR scheme synchronizes clocks between the ground station and the spacecraft by iteratively exchanging information via 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 to three. This reduction not only simplifies the navigation process but also significantly improves positioning accuracy.
The proposed hybrid system leverages synchronized clocks for GNSS-based navigation, reducing the number of estimations to three. Analyzing the DOP (Dilution of Precision) for this scheme provides quantitative insights into its potential positioning accuracy, which is significantly improved. The scheme requires communication with the ground station to conduct clock synchronization via AOWR. By utilizing a highly stable and compact clock, such as the CSAC (Chip Scale Atomic Clock), the synchronized time can be propagated for a certain period, allowing the navigation system to operate onboard without continuous communication with the ground station.
Several technical challenges remain in realizing the hybrid navigation system, including clock sharing between the GNSS receiver and the AOWR component, enhancing link margins, and refining the navigation filter. This paper focuses on the clock-sharing technique.
Clock sharing is achieved through pseudo-range-based clock offset measurements between the AOWR component and the GNSS receiver. The ground GNSS receiver calculates its own clock offset as a product of positioning and, using signals from the AOWR, determines the AOWR's clock offset. By combining both offsets, the AOWR's clock offset relative to GNSS time is derived. Since the AOWR synchronizes clocks between the spacecraft and ground, the spacecraft's AOWR clock offset can also be determined. Additionally, the AOWR and the spacecraft's GNSS receiver can measure their clock offsets without needing GNSS positioning.
A hardware-based simulation was conducted to demonstrate the hybrid navigation scheme. This demonstration utilized SDR (Software-Defined Radio) devices for the GPS simulator, the GNSS receiver, and the AOWR component. To prevent time drift between the GNSS receiver and the AOWR system on the ground or spacecraft, OCXOs (Oven-Controlled Crystal Oscillators) were used as external oscillators on both. The clock stability of the OCXOs ensures that the clock offset of the GNSS receiver and AOWR component on the spacecraft changes at a very slow rate (e.g., 1E-9). Therefore, strict synchronization is not required for sharing the clock offset information. Even with a 1-second delay in sharing information, the resulting positioning error is less than one meter, which is acceptable for cislunar navigation. When using CSAC, accuracy can be maintained below one meter for delays of more than 1 minute, which is sufficient even when accounting for communication delays caused by the distance between Earth and the spacecraft.
As a preliminary study, static positioning at the lunar distance was conducted with and without this hybrid scheme. In this simulation, only the GPS constellation was used, considering the GPS antenna patterns but not power attenuation due to distance. The experiment showed positioning accuracy within tens of meters, a significant improvement compared to the over one-kilometer error observed without the clock-sharing scheme. Furthermore, substantial improvement was seen in the positioning error along the line of sight (LOS) from the Earth to the Moon, a result attributable to the hybrid scheme. This improvement is due to the enhanced accuracy of receiver clock offset estimation achieved through the AOWR system, which operates independently of GNSS.
In the presentation, the author will provide more details on these simulations, as well as results from other practical user-positioning scenarios. For future studies, enhancing the link margin is critical, as this will not only allow for the reception of GNSS signals but also enable communication with small ground stations for AOWR. The manuscript will cover potential solutions to this challenge. Additionally, the expandability of this scheme beyond cislunar space will be discussed in future studies.