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Session B3: Lunar Navigation and Time

Lunar Navigation Satellite System and Positioning Accuracy Evaluation
Masaya Murata, Isao Kawano, and Satoshi Kogure, Japan Aerospace Exploration Agency, Japan
Location: Beacon B

Introduction:
The new era of the lunar exploration such as the US Artemis is approaching and the base camp establishment will be expected at the lunar South Pole region. Providing a stable and high-accurate lunar PNT service around that region is becoming essential and the LunaNet by the NASA and the Moonlight by the ESA are the candidate satellite constellation systems to address this issue. The JAXA is also investigating the PNT system called the Lunar Navigation Satellite System (LNSS) and we are currently paying attention to the deployment of the eight navigation satellites in the MLO (medium lunar orbit) called the elliptical lunar frozen orbit (ELFO). By providing the GPS-like signals for a user located at the South Pole region, our constellation was particularly designed to provide the high-accurate horizontal positioning accuracy at the South Pole region. We are also scrutinizing the feasibility on making the orbit and clock estimation of the LNSS satellites fully autonomous by employing the GNSS navigation technology for the ELFO. In this talk, our LNSS architecture is first shown and the expected GNSS navigation performance for the ELFO is discussed. Then, the evaluation results for the positioning accuracy at the South Pole region are provided. Our simulation results indicate that achieving the horizontal positioning accuracy lower than 40m is feasible.

Related works and our contributions:
In [1], a four-satellite constellation flying in the ELFOs was investigated and using the GPS navigation accuracy of 16.7m (1-sigma) and the cesium clock propagation error of about 1ns after 12 hours (1-sigma), the 3D positioning accuracy of between 100m and 200m around the South Pole region was reported when the satellite navigation system was available. In [2], the GPS navigation accuracy for the ELFO was evaluated and using the rubidium clock and inter-satellite ranging, the accuracy of about 20m was reported.
Different from [1], our constellation was designed to provide more than four visible satellites anytime at the South Pole region and to guarantee the stable and high-accurate horizontal positioning accuracy: our target accuracy is less than 40m.

Evaluation results and conclusions:
In our simulation, the GPS navigation accuracy for the LNSS satellite flying in the ELFO was evaluated and the orbit and clock estimation accuracy of 30m, 7.85m (clock bias), and 4.46cm/s (clock drift) was confirmed after the filter convergence. Using these preliminary results, the singe point positioning accuracy around the South Pole region was evaluated by varying the ephemerides prediction terms (EPR) from 1 minute to 10 minutes. According to the results obtained, the probabilities of achieving the horizontal positioning accuracy less than 40m were larger than 80 percent at the South Pole region when the EPR was 5 minutes and larger than 70 percent when the EPR was 10 minutes, respectively. The shorter EPR led to the higher positioning accuracy because the clock prediction errors became smaller. If the ephemeris generation is also performed onboard the LNSS satellites based on the onboard navigation results, the whole satellite navigation system becomes fully autonomous and liberated from the limitation to be caused by the operation of the earth monitoring stations.

References:
[1] M. Leonardi, et al., “Autonomous Lunar Satellite Navigation System: Preliminary Performance Assessment on South Pole,” ION ITM 2021, 2021.
[2] K. Iiyama, et al., “Autonomous and Decentralized Orbit Determination and Clock Offset Estimation of Lunar Navigation Satellites Using GPS Signals and Inter-satellite Ranging,” ION GNSS+2021, 2021.



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