Session A4: PNT Solutions for Space Applications

High Precision Spacecraft Landing on the Moon Through Lunar Satellite Navigation Systems and Local Ultra-Wideband Positioning Network
Danim Jung and Euiho Kim, Hongik University
Location: Beacon A

Amid escalating competition in space technology development, interest in lunar exploration, particularly at the lunar South Pole, has considerably grown in recent years. Consequently, the demand for a globally applicable lunar navigation system is increasing to support various planned lunar missions. The Global Navigation Satellite System (GNSS) serves as a primary means of navigation on Earth for various applications, including car driving, machine control, and high-precision aircraft landing. Although some GNSS signals are receivable on the Moon, their positioning accuracy and availability are expected to be very poor for future lunar missions such as spacecraft landing, surface exploration, and human habitat construction. Therefore, a satellite-based positioning system, similar to GNSS, is highly desirable on the Moon.
In response to this need, the National Aeronautics and Space Agency (NASA) and the European Space Agency (ESA) have recently begun developing the Lunar Navigation Satellite System (LNSS) through NASA’s LunaNet and ESA’s Moonlight programs, in collaboration with other international partners. LunaNet is introduced as a crucial technology enabling NASA's Artemis program to stay connected. Serving as a conceptual and technical blueprint for lunar communications and navigation, LunaNet establishes operational standards through the LunaNet Interoperability Specification (LNIS), with its draft released in August 2023. The development of LunaNet is a joint effort involving NASA, ESA, and the Japan Aerospace Exploration Agency (JAXA). NASA's Lunar Communication Relay and Navigation System (LCRNS) plans to initiate satellite launches in 2025, paralleled by ESA's Moonlight program, which aims to launch the Lunar Pathfinder and subsequently begin the Lunar Communication & Navigation Services (LCNS) by 2027. JAXA's Lunar Navigation Satellite System (LNSS) is scheduled to commence in 2028. The strategic objective is to deploy four LCRNS satellites, one LCNS satellite, and one LNSS satellite by 2028, expanding to four LCRNS satellites, four LCNS satellites, and two LNSS satellites by 2031.
These initiatives have led to recent advances in lunar exploration, prompting the proposal of various satellite orbits to create an effective lunar navigation system. Proposed orbits include the Elliptical Lunar Frozen Orbit (ELFO), Near-Rectilinear Halo Orbit (NRHO), Low Lunar Orbit (LLO), and Prograde Circular Orbit (PCO), with the ELFO receiving considerable attention due to its broad coverage and high availability over the lunar South Pole. Concurrently, the development of vehicles such as rovers and landers is progressing, along with advancements in payloads like receivers for missions planned in cis-lunar space and on lunar orbits/surfaces. Under NASA's Commercial Lunar Payload Services (CLPS) program, the Blue Ghost lander, equipped with the Lunar GNSS Receiver Experiment (LuGRE), is scheduled for a 2024 launch. Similarly, ESA's Lunar Pathfinder mission is set to launch NaviMoon, a receiver designed for space navigation, in 2025. In 2023, NASA also introduced the NavCube3-mini (NC3m) GNSS Receiver for lunar low-orbit and navigation.
On the international stage, significant progress in lunar exploration has been made by various space agencies. The China National Space Administration (CNSA) made notable strides in 2020 when the Chang’e 5 lander returned lunar samples to Earth for the first time in four decades. Upcoming missions include Chang’e 6, aimed at retrieving samples from the Moon's far side in 2024, and Chang’e 7, scheduled for 2026 to explore the lunar South Pole and deploy a resource-scouting rover. The Indian Space Research Organisation (ISRO) marked a significant achievement with its Chandrayaan-3 mission, successfully landing near the lunar South Pole in August 2023 and deploying the Pragyan rover. Looking ahead to the 2030s, ESA’s development of the Argonaut, or the European Large Logistic Lander (EL3), under its Terrae Novae 2030+ strategy, plans to complement NASA’s Artemis program by delivering cargo and scientific instruments, furthering sustainable lunar exploration and preparing for future human missions.
Despite these international efforts, lunar landing missions remain highly challenging. The landing mission consists of several phases, each requiring specific navigation accuracy as dictated by agencies such as NASA and ESA. The Representative Lunar Entry, Descent, and Landing (EDL) trajectory includes: 1) Power Descent Initiation (PDI), 2) Braking Phase (TRN), 3) Pitch-up Maneuver, 4) Approach Phase (HDA, HRN), 5) Terminal Descent Phase, and 6) Touchdown. The required navigation accuracy for each phase is set according to the mission's level of complexity, especially concerning landing operations. During lunar surface landings, landers encounter multiple challenges, including precision in landing, communication delays, data transfer issues, and constraints on operational lifespan and resources. These problems are particularly pronounced in regions such as the lunar South Pole and the Far Side of the Moon, where communication with Earth is not only delayed but also frequently disrupted. Moreover, these regions include vast areas where GNSS signals are not only ineffective but also completely unreceivable.
Given these considerations, the LNSS, which provides comprehensive communication and navigation services across the Moon, could serve as an indispensable resource. For missions necessitating a soft landing, the navigation accuracy afforded by the LNSS alone is sufficient to meet the specific accuracy requirements demanded of landers executing soft landing operations, underscoring its value as a critical resource for lunar landing operations. However, due to limited resources on the Moon, such as monitoring stations, the positioning performance of LNSS would be much poorer than that of GNSS. Therefore, a fusion of GNSS and LNSS is expected to result in 3D positioning accuracy that exceeds the requirements of NASA’s Autonomous Landing and Hazard Avoidance Technology (ALHAT) project for a safe spacecraft landing and ESA for safe spacecraft landings.
In this paper, we propose a local Ultra-Wideband (UWB) based positioning system to be integrated with GNSS and LNSS to enhance positioning accuracy during lunar landing missions. Our strategy for establishing the proposed local UWB positioning network involves deploying multiple mobile rovers from a mothership, which autonomously navigate to their destinations. The positioning accuracy of the UWB network primarily depends on the layout of the network and the fidelity of known UWB antenna coordinates, i.e., the rovers’ destinations.
To develop an efficient UWB network layout, we utilized our in-house genetic algorithms to optimize the network layout, considering RF signal propagation patterns and positioning accuracy requirements during the landing phases. For this study, we selected a candidate landing site by inspecting the lunar South Pole topography from NASA’s Digital Elevation Map (DEM). The spacecraft landing trajectory was based on previous Apollo missions and the proposed Argonaut mission.
Achieving high fidelity in rover antenna coordinates, which can be several tens of meters when using GNSS and LNSS only, required a cooperative positioning technique employing GNSS, LNSS, and the UWB network. This technique reduces the positioning uncertainty of the rover antenna to approximately 1 to 4 meters (1-sigma). To assess positioning performance during spacecraft landing phases, we conducted a simulation-based sensitivity analysis of the UWB network integrated with GNSS and LNSS, demonstrating positioning accuracy along the lander’s trajectory. Our results showed that the optimized UWB network with GNSS and LNSS could provide substantial support for spacecraft landing missions, notably improving positioning accuracy in the horizontal direction to a maximum of 0.7 meters (3-sigma).
Based on these results, the proposed UWB network can significantly aid both landing and exploration missions on the lunar surface by providing reliable positioning and communication support, making it a valuable resource crucial for mission success.