Publications

You can also find my articles at my ResearchGate and Google Scholar.

Maganize Articles

• [M1] Iiyama, K., Pullen, S., Gao, G., “Can Satellite-based Radionavigation be Extended to the Moon and Other Extraterrestrial Bodies?”, Inside GNSS, Vol. 20, No. 5, pp. 18--27, 2025, [URL] [BibTeX]

   @article{iiyama2025insidegnss_overview,
    title = {Can Satellite-based Radionavigation be Extended to the Moon and Other Extraterrestrial Bodies?},
    journal = {Inside GNSS},
    volume = {20},
    number = {5},
    pages = {18--27},
    year = {2025},
    month = {Sep-Oct},
    publisher = {Inside GNSS Media & Research LLC},
    url = {https://insidegnss.com/can-satellite-based-radionavigation-be-extended-to-the-moon-and-other-extraterrestrial-bodies/},
    author = {Iiyama, Keidai and Pullen, Sam and Gao, Grace},
  }

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Journal Articles Under Review

• [J9] Dai, A., Vila, C.G., Wu, A., Iiyama, K., Coimbra, K., Deng, T., Gao, G., “Full Stack Navigation, Mapping, and Planning for the Lunar Autonomy Challenge”, Submitted to NAVIGATION: Journal of the Institute of Navigation, 2025, (Under Review) [BibTeX] [Abstract]

   @article{dai2026lac,
    title = {Full Stack Navigation, Mapping, and Planning for the Lunar Autonomy Challenge},
    year = {2025},
    month = {},
    journal = {Submitted to NAVIGATION: Journal of the Institute of Navigation},
    url = {},
    author = {Dai, Adam and Vila, Casadesus Guillem and Wu, Asta and Iiyama, Keidai and Coimbra, Kaila and Deng, Thomas and Gao, Grace},
  }

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  Abstract: We present a modular, full-stack autonomy system for lunar surface navigation and mapping developed for the Lunar Autonomy Challenge. Operating in a GNSS-denied, visually challenging environment, our pipeline integrates semantic segmentation, stereo visual odometry, pose graph SLAM with loop closures, and layered planning and control. We leverage lightweight learning-based perception models for real- time segmentation and feature tracking and use a factor-graph backend to maintain globally consistent localization. High-level waypoint planning is designed to promote mapping coverage while encouraging frequent loop closures, and local motion planning uses arc sampling with geometric obstacle checks for efficient, reactive control. We evaluate our approach in the competition's high-fidelity lunar simulator, demonstrating centimeter-level localization accuracy, high-fidelity map generation, and strong repeatability across random seeds and rock distributions. Our solution achieved first place in the final competition evaluation.

• [J8] Iiyama, K., Gao, G., “Trade-off Analysis for Lunar Augmented Navigation Service (LANS) Constellation Design”, Submitted to NAVIGATION: Journal of the Institute of Navigation, 2025, (Minor Revision) [URL] [Paper PDF] [BibTeX] [Abstract]

   @article{iiyama2026constellationtradeoff,
    title = {Trade-off Analysis for Lunar Augmented Navigation Service (LANS) Constellation Design},
    year = {2025},
    month = {},
    journal = {Submitted to NAVIGATION: Journal of the Institute of Navigation},
    url = {https://arxiv.org/abs/2510.16030},
    author = {Iiyama, Keidai and Gao, Grace},
  }

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  Abstract: The establishment of a sustainable human presence on the Moon demands robust positioning, navigation, and timing (PNT) services capable of supporting both surface and orbital operations. This paper presents a comprehensive trade-off analysis of lunar frozen-orbit constellations for the Lunar Augmented Navigation Service (LANS), focusing on how the number of satellites and orbital parameters influence coverage, position dilution of precision (PDOP), orbit determination accuracy, receiver noise, and orbit insertion cost. Three Walker-constellation families based on frozen elliptical and circular orbits are examined to characterize their relative advantages across different semi-major axes and inclinations. Results show that larger semi-major axes enhance both polar and global coverage, though the optimal inclination depends on the constellation type and target service region. The south elliptical lunar frozen orbit (ELFO) Walker constellation provides superior performance for polar coverage and PDOP, whereas the circular lunar frozen orbit (CLFO) Walker configuration achieves the best global uniformity. Orbit determination errors and receiver noise both increase with larger semi-major axes and higher inclinations, reflecting weaker geometric observability and reduced received signal power at apolune for eccentric orbits. Orbit insertion analysis reveals clear trade-offs among transfer duration, characteristic energy (C3) at trans-lunar injection, and insertion Delta-V: shorter transfers require higher insertion Delta-V, while low-energy transfers achieve smaller Delta-V at the cost of months-long durations and higher C3. These findings provide a systematic framework for designing LANS constellations for both regional and global coverage.

• [J7] Iiyama, K., Gao, G., “GNSS-based Lunar Orbit and Clock Estimation With Stochastic Cloning UD Filter”, Journal of Guidance, Control, and Dynamics, 2026, (Accepted) [URL] [Paper PDF] [BibTeX] [Abstract]

   @article{iiyama2026udutdcp,
    title = {GNSS-based Lunar Orbit and Clock Estimation With Stochastic Cloning UD Filter},
    year = {2026},
    month = {},
    journal = {Journal of Guidance, Control, and Dynamics},
    url = {https://doi.org/10.48550/arXiv.2601.16393},
    author = {Iiyama, Keidai and Gao, Grace},
  }

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  Abstract: This paper presents a terrestrial GNSS-based orbit and clock estimation framework for lunar navigation satellites. To enable high-precision estimation under the low-observability conditions encountered at lunar distances, we develop a stochastic-cloning UD-factorized filter and delayed-state smoother that provide enhanced numerical stability when processing precise time-differenced carrier phase (TDCP) measurements. A comprehensive dynamics and measurement model is formulated, explicitly accounting for relativistic coupling between orbital and clock states, lunar time-scale transformations, and signal propagation delays including ionospheric, plasmaspheric, and Shapiro effects. The proposed approach is evaluated using high-fidelity Monte-Carlo simulations incorporating realistic multi-constellation GNSS geometry, broadcast ephemeris errors, lunar satellite dynamics, and ionospheric and plasmaspheric delay computed from empirical electron density models. Simulation results demonstrate that combining ionosphere-free pseudorange and TDCP measurements achieves meter-level orbit accuracy and sub-millimeter-per-second velocity accuracy, satisfying the stringent signal-in-space error requirements of future Lunar Augmented Navigation Services (LANS).

Journal Publications

• [J6] Iiyama, K., Gao, G., “Ionospheric and Plasmaspheric Delay Characterization for Lunar Terrestrial GNSS Receivers with Global Core Plasma Model”, NAVIGATION: Journal of the Institute of Navigation, Vol. 73, No. 1, 2026, [URL] [BibTeX] [Abstract]

   @article{iiyama2026plasmaspheresimulation,
    title = {Ionospheric and Plasmaspheric Delay Characterization for Lunar Terrestrial GNSS Receivers with Global Core Plasma Model},
    year = {2026},
    volume = {73},
    number = {1},
    month = {},
    doi = {10.33012/navi.785},
    keywords = {GNSS, ionosphere, lunar positioning, navigation, timing (PNT), plasmasphere, ray tracing},
    journal = {NAVIGATION: Journal of the Institute of Navigation},
    url = {https://navi.ion.org/content/73/1/navi.785},
    issn = {0028-1522},
    author = {Iiyama, Keidai and Gao, Grace},
  }

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  Abstract: Recent advancements in lunar positioning, navigation, and timing have demonstrated that global navigation satellite system (GNSS) signals, including weak side-lobe transmissions, can be exploited for lunar spacecraft positioning and timing. Although GNSS-based navigation on the Moon has been recently validated, unmodeled ionospheric and plasmaspheric delays remain a significant source of error, particularly given the unique signal geometry and extended propagation paths. This paper characterizes these delays using the Global Core Plasma Model and a custom low-cost ray-tracing algorithm that iteratively solves for bent signal paths. We simulate first-, second-, and third-order group delays, as well as excess path length from ray bending, for GNSS signals received at both lunar orbit and the lunar south pole under varying solar and geomagnetic conditions. Results show that mean group delays are typically on the order of meters, but can exceed 100 m for rays passing through high-electron-density regions (altitudes below 1000 km with respect to Earth) during high solar activity. We also show that bending delays are generally smaller but non-negligible when the ray passes through high-electron-density regions in the ionosphere. In addition, we quantify the influence of signal frequency, geomagnetic K p index, and solar R12 index. These results provide quantitative insight into the magnitude and variability of ionospheric and plasmaspheric delays for Earth-Moon GNSS signal paths and help characterize measurement errors affecting GNSS-based navigation at lunar receivers.

• [J5] Iiyama, K., Gao, G., “Ephemeris and Almanac Design for Lunar Navigation Satellites”, IEEE Transactions on Aerospace and Electronic Systems, Vol. 62, pp. 15842-15854, 2026, [URL] [BibTeX] [Abstract]

   @article{iiyama2026ephemeris,
    journal = {IEEE Transactions on Aerospace and Electronic Systems},
    title = {Ephemeris and Almanac Design for Lunar Navigation Satellites},
    year = {2026},
    volume = {62},
    number = {},
    pages = {15842-15854},
    keywords = {Moon;Navigation;Satellites;Feed in tariff;Fitting;Orbits;Orbits (stellar);Distance measurement;Timing;Polynomials;Almanac;ephemeris;Global Navigation Satellite Systems (GNSS);lunar positioning;navigation;and timing (PNT)},
    doi = {10.1109/TAES.2026.3721050},
    url = {https://doi.org/10.1109/TAES.2026.3721050},
    ISSN = {1557-9603},
    month = {},
    author = {Iiyama, Keidai and Gao, Grace},
  }

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  Abstract: This article presents almanac and ephemeris message representation for lunar navigation satellites supporting the Lunar Augmented Navigation System. The proposed method combines osculating orbital elements, Chebyshev polynomials, and Fourier series to efficiently represent lunar satellite trajectories subject to complex perturbations from lunar gravity and third-body effects. For the ephemeris, a hybrid Chebyshev–Fourier formulation improves fitting accuracy over long arcs while maintaining message compactness under the datasize constraint of the LunaNet Interoperability Specification. For the almanac, a compact low-order polynomial and Fourier model is introduced to capture mid-term orbital variations over a 15-day fitting arc. The approach is validated for multiple orbit regimes, including 30-, 24-, and 13.2-h elliptical Lunar frozen orbits and a 6-h polar orbit. Results show that the proposed framework achieves submeter position and sub-millimeter-per-second velocity fitting errors within the 900-bit limit for 6-h ephemeris arcs and almanac fitting accuracy sufficient for reliable satellite visibility identification in warm-start operations.

• [J4] Iiyama, K., Neamati, D., Gao, G., “Satellite Autonomous Clock Fault Monitoring with InterSatellite Ranges Using Euclidean Distance Matrices”, NAVIGATION: Journal of the Institute of Navigation, Vol. 73, No. 1, 2026, [URL] [BibTeX] [Abstract]

   @article{iiyama2026edmnavigation,
    title = {Satellite Autonomous Clock Fault Monitoring with InterSatellite Ranges Using Euclidean Distance Matrices},
    volume = {73},
    number = {1},
    year = {2026},
    doi = {10.33012/navi.764},
    publisher = {Institute of Navigation},
    issn = {0028-1522},
    url = {https://navi.ion.org/content/73/1/navi.764},
    keywords = {fault detection, inter-satellite links, lunar positioning, navigation, timing (PNT), satellite constellation},
    journal = {NAVIGATION: Journal of the Institute of Navigation},
    author = {Iiyama, Keidai and Neamati, Daniel and Gao, Grace},
  }

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  Abstract: To support reliable positioning, navigation, and timing services for lunar satellite constellations, this paper proposes a framework for detecting satellite clock phase jumps using dual one-way inter-satellite range measurements. The method models the constellation as a weighted graph and exploits rigidity properties to detect faults. Specifically, satellite clock faults are identified by monitoring the realizability of vertex-redundantly rigid subgraphs through the singular values of geometric-centered Euclidean distance matrices (GCEDMs). We establish the graph-topological conditions required for fault detection and derive the statistical distribution of the GCEDM test statistic under both nominal and faulty conditions. To address sparse link connectivity, we also introduce an ephemeris-augmented formulation that combines measured and estimated ranges and performs fault detection on fault-detectable subgraphs. Simulation results using a terrestrial Global Positioning System constellation and a notional lunar constellation demonstrate that the proposed approach provides robust fault detection under both dense and sparse measurement conditions.

• [J3] Cortinovis, M., Iiyama, K., Gao, G., “Satellite Ephemeris Parameterization Methods to Support Lunar Positioning, Navigation, and Timing Services”, NAVIGATION: Journal of the Institute of Navigation, Vol. 71, No. 4, 2024, [URL] [BibTeX] [Abstract]

   @article{cortinovis2024ephemerisnavigation,
    title = {Satellite Ephemeris Parameterization Methods to Support Lunar Positioning, Navigation, and Timing Services},
    volume = {71},
    number = {4},
    year = {2024},
    month = {12},
    publisher = {Institute of Navigation},
    issn = {0028-1522},
    url = {https://navi.ion.org/content/71/4/navi.664},
    doi = {10.33012/navi.664},
    journal = {NAVIGATION: Journal of the Institute of Navigation},
    author = {Cortinovis, Marta and Iiyama, Keidai and Gao, Grace},
  }

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  Abstract: Plans to establish a satellite network around the Moon to support communication, position, navigation, and timing services are rapidly evolving. Satellites that are part of this system broadcast their ephemeris as finite parameters to lunar users for user state estimation. In this work, we investigate lunar satellite ephemeris design to identify the optimal parameterization to broadcast to a lunar user. The proposed framework directly approximates the lunar satellite position and velocity in the inertial frame and obtains the conversion parameters necessary for state representation in the lunar fixed frame. The framework leverages signal-in-space-error requirements as constraints in the parameterization process to guide the search for the best ephemeris parameter set. We evaluate the performance of our proposed framework for satellites in a low lunar orbit and an elliptical lunar frozen orbit. The performance of different methods is assessed based on the precision of the ephemeris prediction, fit interval, and message size. We showcase the ability of the developed framework to approximate satellite ephemeris for both orbits to the desired precision by adjusting the fit interval and the number of parameters to broadcast. In particular, we demonstrate that formulations with a standard polynomial basis and a Chebyshev polynomial basis produce feasible solutions for ephemeris approximation at varying epochs in orbits, abiding by signal-in-space-error requirements.

• [J2] Iiyama, K., Bhamidipati, S., Gao, G., “Precise Positioning and Timekeeping in a Lunar Orbit via Terrestrial GPS Time-Differenced Carrier-Phase Measurements”, NAVIGATION: Journal of the Institute of Navigation, Vol. 71, No. 1, 2024, [URL] [BibTeX] [Abstract]

   @article{iiyama2024precisenavigation,
    title = {Precise Positioning and Timekeeping in a Lunar Orbit via Terrestrial GPS Time-Differenced Carrier-Phase Measurements},
    volume = {71},
    number = {1},
    year = {2024},
    month = {3},
    publisher = {Institute of Navigation},
    issn = {0028-1522},
    url = {https://doi.org/10.33012/navi.635},
    doi = {10.33012/navi.635},
    journal = {NAVIGATION: Journal of the Institute of Navigation},
    author = {Iiyama, Keidai and Bhamidipati, Sriramya and Gao, Grace},
  }

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  Abstract: There is a growing interest in the use of legacy terrestrial Global Positioning System (GPS) signals to determine the precise positioning and timing onboard a lunar satellite. Unlike prior works that utilize pseudoranges with meter-level accuracy, we propose a precise positioning and timekeeping technique that leverages carrier-phase measurements with millimeter-level accuracy (when integer ambiguities are correctly fixed). We design an extended Kalman filter framework that harnesses the intermittently available terrestrial GPS time-differenced carrier-phase (TDCP) values and gravitational accelerations predicted by the orbital filter. To estimate the process noise covariance, we implement an adaptive state noise compensation algorithm that adapts to the challenging lunar environment with weak gravity and strong third-body perturbations. Additionally, we perform measurement residual analysis to discard TDCP measurements corrupted by cycle slips and increased measurement noise. We present Monte-Carlo simulations of a lunar satellite in an elliptical lunar frozen orbit and quasi-frozen low lunar orbit, wherein we showcase higher positioning and timing accuracy as compared with the pseudorange-only navigation solution.

• [J1] Funase, R., Ikari, S., Miyoshi, K., Kawabata, Y., Nakajima, S., Nomura, S., Funabiki, N., Ishikawa, A., Kakihara, K., Matsushita, S., Takahashi, R., Yanagida, K., Mori, D., Murata, Y., Shibukawa, T., Suzumoto, R., Fujiwara, M., Tomita, K., Aohama, H., Iiyama, K., Ishiwata, S., Kondo, H., Mikuriya, W., Seki, H., Koizumi, H., Asakawa, J., Nishii, K., Hattori, A., Saito, Y., Kikuchi, K., Kobayashi, Y., Tomiki, A., Torii, W., Ito, T., Campagnola, S., Ozaki, N., Baresi, N., Yoshikawa, I., Yoshioka, K., Kuwabara, M., Hikida, R., Arao, S., Abe, S., Yanagisawa, M., Fuse, R., Masuda, Y., Yano, H., Hirai, T., Arai, K., Jitsukawa, R., Ishioka, E., Nakano, H., Ikenaga, T., Hashimoto, T., “Mission to earth--moon lagrange point by a 6u cubesat: Equuleus”, IEEE Aerospace and Electronic Systems Magazine, Vol. 35, No. 3, pp. 30--44, 2020, [URL] [BibTeX] [Abstract]

   @article{funase2020mission,
    title = {Mission to earth--moon lagrange point by a 6u cubesat: Equuleus},
    journal = {IEEE Aerospace and Electronic Systems Magazine},
    volume = {35},
    number = {3},
    pages = {30--44},
    year = {2020},
    publisher = {IEEE},
    url = {https://doi.org/10.1109/MAES.2019.2955577},
    author = {Funase, Ryu and Ikari, Satoshi and Miyoshi, Kota and Kawabata, Yosuke and Nakajima, Shintaro and Nomura, Shunichiro and Funabiki, Nobuhiro and Ishikawa, Akihiro and Kakihara, Kota and Matsushita, Shuhei and Takahashi, Ryohei and Yanagida, Kanta and Mori, Daiko and Murata, Yusuke and Shibukawa, Toshihiro and Suzumoto, Ryo and Fujiwara, Masahiro and Tomita, Kento and Aohama, Hiroki and Iiyama, Keidai and Ishiwata, Sho and Kondo, Hirotaka and Mikuriya, Wataru and Seki, Hiroto and Koizumi, Hiroyuki and Asakawa, Jun and Nishii, Keita and Hattori, Akihiro and Saito, Yuji and Kikuchi, Kosei and Kobayashi, Yuta and Tomiki, Atsushi and Torii, Wataru and Ito, Taichi and Campagnola, Stefano and Ozaki, Naoya and Baresi, Nicola and Yoshikawa, Ichiro and Yoshioka, Kazuo and Kuwabara, Masaki and Hikida, Reina and Arao, Shogo and Abe, Shinsuke and Yanagisawa, Masahisa and Fuse, Ryota and Masuda, Yosuke and Yano, Hajime and Hirai, Takayuki and Arai, Kazuyoshi and Jitsukawa, Ritsuko and Ishioka, Eigo and Nakano, Haruki and Ikenaga, Toshinori and Hashimoto, Tatsuaki},
  }

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  Abstract: EQUULEUS (EQUilibriUm Lunar-Earth point 6U Spacecraft) will be the world's smallest spacecraft to explore the Earth–Moon Lagrange point. It is being jointly developed by JAXA (Japan Aerospace Exploration Agency) and the University of Tokyo, and will be launched by NASA's Space Launch System Exploration Mission-1. The spacecraft will fly to a libration orbit around the Earth–Moon L2 point (EML2) and will demonstrate low-energy trajectory-control techniques within the Sun—Earth—Moon region for the first time by a nano-class spacecraft. EQUULEUS also carries three scientific observation missions: imaging of Earth's plasmasphere by extreme ultraviolet wavelength, lunar impact flash observation on the far side of the moon, and micrometeoroid flux measurements in the cis-lunar region. While all these missions have their own scientific objectives, they will also contribute to future human activity and/or infrastructure development in the cis-lunar region. Most parts of the spacecraft system use commercial off-the-shelf components, or are designed based on the experiences of various past space missions, with the exception of the newly developed water resistojet propulsion system. EQUULEUS uses X-band frequency for deep space telecommunication. Japanese deep space antennas (64-m and 34-m) will be nominally used for spacecraft operation, and support from the deep space network of JPL (Jet Propulsion Laboratory) is also being planned, especially for the initial phase of operation. The spacecraft will fly to EML2 in less than one year, and will remain there for scientific observations until shortly before the depletion of the onboard propellant, when the spacecraft will leave the orbit for space-debris compliance.

Conference Proceedings

2026

• [C25] Kura, M., Iiyama, K., Gao, G., “Sidelobe vs. Side of Main Lobe: LuGRE Experiment Data Analysis”, Proceedings of the Institute of Navigation GNSS+ conference (ION GNSS+ 2026), 2026, [Paper PDF] [Slides] [BibTeX] [Abstract]

   @article{kura2026sidelobe,
    title = {Sidelobe vs. Side of Main Lobe: LuGRE Experiment Data Analysis},
    pages = {},
    year = {2026},
    month = {September},
    journal = {Proceedings of the Institute of Navigation GNSS+ conference (ION GNSS+ 2026)},
    url = {},
    author = {Kura, Matthias and Iiyama, Keidai and Gao, Grace},
  }

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  Abstract: The Lunar GNSS Receiver Experiment (LuGRE) was the first mission to demonstrate Global Navigation Satellite System (GNSS) signal reception in lunar orbit and on the surface. This paper uses the LuGRE dataset to address two questions that prior work has not sufficiently examined: the contribution of antenna sidelobe reception to navigation availability, and the performance of Kalman-filter-based surface positioning. First, we locate the receiver directions within the transmit antenna gain pattern of each GPS and Galileo satellite to classify received signals into main lobe and sidelobe regions, and quantify their effect on Geometric Dilution of Precision (GDOP). Results show that the receiver consistently tracked signals from the side of the main lobe, while true sidelobe signals were received only occasionally. Nonetheless, across all measurement sets, 65% of least-squares solutions were enabled solely by sidelobe signals and a further 10% benefited from improved geometry, where GDOP was reduced by over an order of magnitude. The GDOP values remain far above the GDOP needed for adequate navigation performance. Second, we compare an Extended Kalman Filter (EKF), a Square-Root Extended Kalman Filter (SREKF), and a C/N0- Weighted Least Squares (WLS) baseline for surface positioning, under cold- and warm-start initialization and with an optional pseudo-altitude constraint. The pseudorange-only EKF achieved the best minimum errors of around 500 m, while adding the pseudo-altitude constraint reduced RMS error to roughly 8 km on larger sets. These results demonstrate that lunar surface positioning is achievable with GNSS pseudoranges alone, though residual errors remain insufficient for precise navigation due to the limited clock stability onboard LuGRE.

• [C24] Kura, M., Iiyama, K., Gao, G., “Optimal Reconfiguration of Lunar Navigation Constellations”, Proceedings of the Institute of Navigation GNSS+ conference (ION GNSS+ 2026), 2026, Best Presentation of the Session [Paper PDF] [Slides] [BibTeX] [Abstract]

   @article{kura2026reconfig,
    title = {Optimal Reconfiguration of Lunar Navigation Constellations},
    pages = {},
    year = {2026},
    month = {September},
    journal = {Proceedings of the Institute of Navigation GNSS+ conference (ION GNSS+ 2026)},
    url = {},
    author = {Kura, Matthias and Iiyama, Keidai and Gao, Grace},
  }

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  Abstract: Reliable positioning, navigation, and timing (PNT) services are essential to support the growing number of missions to the lunar surface and in lunar orbit. Lunar navigation constellations, such as NASA’s Lunar Communications Relay and Navigation Systems (LCRNS), are designed to provide continuous, high-accuracy positioning, navigation, and timing (PNT) coverage to the lunar South Pole region using navigation satellites in elliptical lunar frozen orbits (ELFOs). However, navigation satellite failures due to payload anomalies, aging, micrometeoroid impacts, or solar events can significantly degrade service availability, and the high cost and limited frequency of lunar launches make timely replacement difficult. This paper investigates the optimal reconfiguration of lunar navigation constellations to partially restore PNT availability following a single-satellite failure. The approach proceeds in two stages. First, a single-objective particle swarm optimization (PSO) framework is developed to optimize the true anomaly distribution of satellites within each orbital plane, maximizing the availability of Geometric Dilution of Precision (GDOP) below 6 for users in the lunar South Pole region. Using this PSO framework, we construct a database of optimal phasing for different numbers of planes and satellites. Second, a multi-objective reconfiguration problem is formulated to minimize the total ∆v and reconfiguration time required to transition the remaining satellites from their pre-failure- to their post-failure-optimal slots using in-plane Hohmann transfers, solved via an auction algorithm. Results from a case study on the LCRNS constellation demonstrate availability improvements of up to 13 percentage points following a single-satellite failure. Analysis across a design space of Southern ELFO constellations shows that reconfiguration yields the greatest benefit for constellations with up to 10 satellites, achieving improvements of up to 17 percentage points in GDOP availability. The associated ∆v and reconfiguration time are shown to be tunable via a weighting parameter, enabling operators to balance propellant use with time-to-recovery based on mission constraints. These results highlight constellation reconfiguration as a valuable operational tool, particularly for early-stage lunar navigation deployments.

2025

• [C23] Iiyama, K., Gao, G., “Ionospheric and Plasmaspheric Delay Characterization and Mitigation Methodologies for Lunar Terrestrial GNSS Receivers”, Proceedings of the Institute of Navigation GNSS+ conference (ION GNSS+ 2025), 2025, Best Presentation of the Session [URL] [Paper PDF] [Slides] [BibTeX] [Abstract]

   @article{iiyama2025plasma,
    title = {Ionospheric and Plasmaspheric Delay Characterization and Mitigation Methodologies for Lunar Terrestrial GNSS Receivers},
    pages = {},
    year = {2025},
    month = {September},
    journal = {Proceedings of the Institute of Navigation GNSS+ conference (ION GNSS+ 2025)},
    url = {https://doi.org/10.33012/2025.20343},
    doi = {10.33012/2025.20343},
    author = {Iiyama, Keidai and Gao, Grace},
  }

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  Abstract: Recent advancements in lunar positioning, navigation, and timing (PNT) have demonstrated that terrestrial GNSS signals, including weak sidelobe transmissions, can be exploited for lunar spacecraft positioning and timing. While GNSS-based navigation at the Moon has been validated recently, unmodeled ionospheric and plasmaspheric delays remain a significant error source, particularly given the unique signal geometry and extended propagation paths. This paper characterizes the plasmaspheric delay affecting GNSS signals observed at the lunar south pole and in lunar orbit using the Global Core Plasma Model (GCPM) and a custom ray-tracing algorithm to simulate both first-order delays, higher-order delays, and bending effects. Simulation results show that mean delays are on the order of 1 meter, but maximum delays can exceed 100 meters depending on the raypath, solar activity, and geomagnetic conditions. The study further compares mitigation strategies, including altitude masking, iono-free frequency combinations, and differential techniques. We found that a tangential altitude cutoff of 4000 km was effective in excluding signals traversing dense ionospheric regions. Single differencing is able to remove over 90\% of the ionospheric and plasmaspheric delays for two receivers with relative distances under 10,000 km. The iono-free combinations are effective to cancel 1st-order delays for signals that pass below 4000 km altitude, but for higher signals, the effect of inflated thermal noise due to frequency combination becomes more significant. These findings inform the design of robust lunar GNSS navigation systems that utilize terrestrial GNSS signals.

• [C22] Iiyama, K., Gao, G., “Constellation Design and Staged Development for the Lunar Navigation Satellite System”, Proceedings of the Institute of Navigation GNSS+ conference (ION GNSS+ 2025), 2025, [URL] [Paper PDF] [Slides] [BibTeX] [Abstract]

   @article{iiyama2025constellation,
    title = {Constellation Design and Staged Development for the Lunar Navigation Satellite System},
    pages = {},
    year = {2025},
    month = {September},
    journal = {Proceedings of the Institute of Navigation GNSS+ conference (ION GNSS+ 2025)},
    url = {https://doi.org/10.33012/2025.20257},
    doi = {10.33012/2025.20257},
    author = {Iiyama, Keidai and Gao, Grace},
  }

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  Abstract: The establishment of a sustainable human presence on the Moon demands robust positioning, navigation, and timing (PNT) services that can support both surface and orbital operations. Current lunar navigation system proposals focus on early deployment of Elliptical Lunar Frozen Orbits (ELFO) for South Pole coverage, but there remains no unified framework for designing constellations and staged deployment strategies that evolve from regional to global coverage. This paper presents a two-part methodology for lunar navigation constellation design. First, a grid search systematically characterizes how the number of satellites and frozen orbit parameters influence the south-pole region and global coverage, OD accuracy, and user range error, providing insights into orbit-performance trade-offs. Second, we introduce a staged-deployment optimization framework that jointly designs hybrid Walker constellations and deployment schedules across Initial, Extended, and Full Operational Capability phases. The framework employs a novel metric—discounted weighted dilution of precision satisfaction ratio—that integrates geometric effects, user range error, and satellite failure probabilities. Simulation results demonstrate the ability to generate Pareto-optimal constellations that minimize satellite count while ensuring performance targets at each stage. This staged approach enables efficient constellation growth from regional to global service, offering insights for future lunar navigation infrastructure development.

• [C21] Dai, A., Wu, A., Iiyama, K., Vila, G.C., Coimbra, K., Deng, T., Gao, G., “Full Stack Navigation, Mapping, and Planning for the Lunar Autonomy Challenge”, Proceedings of the Institute of Navigation GNSS+ conference (ION GNSS+ 2025), 2025, [URL] [Paper PDF] [Slides] [BibTeX] [Abstract]

   @article{dai2025lac,
    title = {Full Stack Navigation, Mapping, and Planning for the Lunar Autonomy Challenge},
    pages = {},
    year = {2025},
    month = {September},
    journal = {Proceedings of the Institute of Navigation GNSS+ conference (ION GNSS+ 2025)},
    url = {https://doi.org/10.33012/2025.20447},
    doi = {10.33012/2025.20447},
    author = {Dai, Adam and Wu, Asta and Iiyama, Keidai and Vila, Guillem Casadesus and Coimbra, Kaila and Deng, Thomas and Gao, Grace},
  }

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  Abstract: We present a modular, full-stack autonomy system for lunar surface navigation and mapping developed for the Lunar Autonomy Challenge. Operating in a GNSS-denied, visually challenging environment, our pipeline integrates semantic segmentation, stereo visual odometry, pose graph SLAM with loop closures, and layered planning and control. We leverage lightweight learning-based perception models for real-time segmentation and feature tracking and use a factor-graph backend to maintain globally consistent localization. High-level waypoint planning is designed to promote mapping coverage while encouraging frequent loop closures, and local motion planning uses arc sampling with geometric obstacle checks for efficient, reactive control. We evaluate our approach in the competition's high-fidelity lunar simulator, demonstrating centimeter-level localization accuracy, high-fidelity map generation, and strong repeatability across random seeds and rock distributions. Our solution achieved first place in the final competition evaluation.

• [C20] Vila, G.C., Iiyama, K., Gao, G., “LuPNT: An Open-Source Simulator for Lunar Communications, Positioning, Navigation, and Timing”, 2025 IEEE Aerospace Conference, 2025, [URL] [Paper PDF] [BibTeX] [Abstract]

   @article{guillem2025simulator,
    title = {LuPNT: An Open-Source Simulator for Lunar Communications, Positioning, Navigation, and Timing},
    pages = {1-18},
    year = {2025},
    month = {March},
    journal = {2025 IEEE Aerospace Conference},
    doi = {10.1109/AERO63441.2025.11068501},
    url = {https://doi.org/10.1109/AERO63441.2025.11068501},
    author = {Vila, Guillem Casadesus and Iiyama, Keidai and Gao, Grace},
  }

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  Abstract: The growing focus on missions to the Moon necessitates reliable communication and Positioning, Navigation, and Timing (PNT) services in cis-lunar space. To address the growing research needs in this field, we have been developing LuPNT, a comprehensive, open-source simulation framework that integrates astrodynamics, communication, and PNT functionalities into a single platform. The simulator is developed primarily in C++ for computational efficiency while also featuring Python bindings to facilitate rapid algorithmic development and user interaction. This paper introduces several new features and applications, including new core functionalities, the generation of realistic images for optical navigation, new measurement models including inter-satellite links, a modular state estimation framework, and contact plan design. We demonstrate the capability of the simulator through two different simulation scenarios: Elliptical Lunar Frozen Orbit (ELFO) navigation using side-lobe signals from multiple GNSS constellations, and ephemeris fitting for ELFO. Overall, LuPNT provides a unified, comprehensive tool for lunar PNT and communication research and advances the potential for lunar exploration by enabling the development, integration, and comparison of diverse PNT algorithms within a modular, extensible framework.

• [C19] Iiyama, K., Jun, W.W., Bhamidipati, S., Gao, G., Cheung, K., “Orbit Determination and Time Synchronization for the Future Mars Relay and Navigation Constellation”, 2025 IEEE Aerospace Conference, 2025, [URL] [Paper PDF] [Slides] [BibTeX] [Abstract]

   @article{iiyama2025odtsmars,
    title = {Orbit Determination and Time Synchronization for the Future Mars Relay and Navigation Constellation},
    pages = {1-14},
    year = {2025},
    month = {March},
    journal = {2025 IEEE Aerospace Conference},
    doi = {10.1109/AERO63441.2025.11068793},
    url = {https://doi.org/10.1109/AERO63441.2025.11068793},
    author = {Iiyama, Keidai and Jun, W William and Bhamidipati, Sriramya and Gao, Grace and Cheung, Kar-Ming},
  }

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  Abstract: To enable future human exploration on Mars, it is necessary to develop a dedicated network of satellites that provide both communication relay and positioning, navigation, and timing (PNT) service in Mars' orbit and on its surface. Additionally, it is desirable to operate the constellation semi-autonomously and reduce reliance on the oversubscribed Deep Space Network. In this work, we first address the design of the next-generation Mars constellation with a focus on the following three objectives: coverage between ±30° latitude, communication data volume, and surface user PNT performance. By analyzing the trade-off space, we propose a notional constellation comprising five satellites: two orbiters at areostationary orbit and three orbiters in an inclined ½ repeating ground track orbit, which balances the three objectives effectively. Second, we propose a semi-autonomous orbit determination and time synchronization (ODTS) framework for the MRN orbiters. Our proposed approach fuses range, range-rate, and time difference measurements from inter-satellite links (ISLs) and links with a Mars surface station (MSS), in a centralized batch filter. While eliminating the need for frequent DSN tracking, we demonstrate an improved orbit determination accuracy of up to meter level and timing accuracy of sub-nanosecond level by using a single MSS as an anchor node.

2024

• [C18] Iiyama, K., Neamati, D., Gao, G., “Autonomous Constellation Fault Monitoring with Inter-satellite Links: A Rigidity-Based Approach”, Proceedings of the Institute of Navigation GNSS+ conference (ION GNSS+ 2024), 2024, Best Presentation of the Session [URL] [Paper PDF] [Slides] [BibTeX] [Abstract]

   @article{iiyama2024fault,
    title = {Autonomous Constellation Fault Monitoring with Inter-satellite Links: A Rigidity-Based Approach},
    year = {2024},
    month = {9},
    journal = {Proceedings of the Institute of Navigation GNSS+ conference (ION GNSS+ 2024)},
    url = {https://doi.org/10.33012/2024.19935},
    doi = {10.33012/2024.19935},
    author = {Iiyama, Keidai and Neamati, Daniel and Gao, Grace},
  }

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  Abstract: To address the need for robust positioning, navigation, and timing services in lunar environments, this paper proposes a novel fault detection framework for satellite constellations using inter-satellite ranging (ISR). Traditionally, navigation satellites can depend on a robust network of ground-based stations for fault monitoring. However, due to cost constraints, a comprehensive ground segment on the lunar surface is impractical for lunar constellations. Our approach leverages vertex redundantly rigid graphs to detect faults without relying on precise ephemeris. We model satellite constellations as graphs where satellites are vertices and inter-satellite links are edges. We identify faults through the singular values of the geometric-centered Euclidean distance matrix (GCEDM) of 2-vertex redundantly rigid sub-graphs. The proposed method is validated through simulations of constellations around the Moon, demonstrating its effectiveness in various configurations. This research contributes to the reliable operation of satellite constellations for future lunar exploration missions.

• [C17] Iiyama*, K., Vila*, G.C., Gao, G., “Contact Plan Optimization and Distributed State Estimation for Delay Tolerant Satellite Networks”, 2024 IEEE Aerospace Conference, 2024, [URL] [Paper PDF] [Slides] [BibTeX] [Abstract]

   @article{iiyama2024contact,
    title = {Contact Plan Optimization and Distributed State Estimation for Delay Tolerant Satellite Networks},
    journal = {2024 IEEE Aerospace Conference},
    pages = {1--13},
    year = {2024},
    month = {3},
    doi = {10.1109/AERO58975.2024.10521114},
    url = {https://doi.org/10.1109/AERO58975.2024.10521114},
    author = {Iiyama*, Keidai and Vila*, Guillem Casadesus and Gao, Grace},
  }

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  Abstract: Future deep space exploration stands to benefit significantly from networks of small satellites, given their cost-effective communication and monitoring capabilities. However, challenges like frequent network partitions, high communication delays, and the demand for onboard orbit determination (OD) and time synchronization present obstacles for these satellite networks. One of the key problems in this context is how to schedule satellite communications and generate a contact plan. This paper presents an integrative solution that combines navigation and communications performance for contact plan optimization. Additionally, we propose a novel two-way ISL scheme to acquire range and differential clock bias estimates that take into account rapid inter-satellite range shifts and light time delays. Building on these, a distributed orbit and clock bias estimation algorithm is introduced, which emphasizes the use of batch processing to improve orbit observability. Performance metrics are validated through a simulation of a lunar navigation constellation, assessing OD error, time synchronization error, and inter-satellite communication delay. The combined insights and proposals presented, especially in terms of navigation and communications performance tradeoffs, offer promising avenues to enhance the operability and efficiency of small satellite networks on the Moon.

2023

• [C16] Iiyama*, K., Vila*, G.C., Gao, G., “LuPNT: Open-Source Simulator for Lunar Positioning, Navigation, and Timing”, Proceedings of the Institute of Navigation GNSS+ conference (ION GNSS+ 2023), 2023, [URL] [Paper PDF] [Slides] [Code] [BibTeX] [Abstract]

   @article{iiyama2023lupnt,
    title = {LuPNT: Open-Source Simulator for Lunar Positioning, Navigation, and Timing},
    journal = {Proceedings of the Institute of Navigation GNSS+ conference (ION GNSS+ 2023)},
    pages = {1499--1510},
    year = {2023},
    month = {9},
    url = {https://doi.org/10.33012/2023.19373},
    doi = {10.33012/2023.19373},
    author = {Iiyama*, Keidai and Vila*, Guillem Casadesus and Gao, Grace},
  }

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  Abstract: The growing focus on missions to the Moon necessitates reliable Positioning, Navigation, and Timing (PNT) services in cis-lunar space. This paper introduces a comprehensive, open-source simulation framework developed to address the growing research need in this field. Implemented primarily in C++ for computational efficiency, the framework also includes Python bindings to facilitate rapid algorithmic development and user interaction. The simulation core employs an event-based architecture to model asynchronous onboard applications and inter-satellite communication. High-fidelity lunar dynamics modeling, incorporating planetary ephemerides, high-order gravitational effects, third-body perturbations, and solar radiation pressure, is also integrated. This framework’s modular and extensible architecture allows for broad applicability across various research applications, from the performance analysis of Lunar Navigation Satellite System (LNSS) constellations to the assessment of orbit determination algorithms and lunar navigation payloads. In addition to describing the architectural and algorithmic facets of the framework, the paper presents use cases demonstrating its efficacy in onboard orbit determination and timing. Although the framework is initially targeted at lunar applications, its architectural design allows for straightforward adaptation to model PNT systems in other celestial domains, including Martian or Earth orbital environments.

• [C15] Iiyama, K., Gao, G., “Positioning and Timing of Distributed Lunar Satellites via Terrestrial GPS Differential Carrier Phase Measurements”, Proceedings of the Institute of Navigation GNSS+ conference (ION GNSS+ 2023), 2023, [URL] [Paper PDF] [Slides] [BibTeX] [Abstract]

   @article{iiyama2023differential,
    title = {Positioning and Timing of Distributed Lunar Satellites via Terrestrial GPS Differential Carrier Phase Measurements},
    journal = {Proceedings of the Institute of Navigation GNSS+ conference (ION GNSS+ 2023)},
    pages = {1511--1529},
    year = {2023},
    month = {9},
    url = {https://doi.org/10.33012/2023.19385},
    doi = {10.33012/2023.19385},
    author = {Iiyama, Keidai and Gao, Grace},
  }

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  Abstract: Relative positioning, navigation, and timing (PNT) are crucial to support proximity operations of multiple spacecraft in lunar orbit, which is expected to play a key role in upcoming lunar missions. We propose a relative positioning and timekeeping technique in lunar orbit that leverages differential carrier phase measurements of the terrestrial GPS signals. However, using GPS signals in the lunar orbit is challenging due to 1) the clustered GPS satellite direction leading to a low-observable system, 2) the nonexistence of ionosphere delay models for lunar orbit, and 3) the possibility of cycle slips in the low C/N0 signals. We designed a PNT framework that tackles the three challenges above. First, to robustly make the filter converge in a low-observable system, the proposed PNT framework estimates the absolute and relative states in two separate filters, where filter settings (e.g., process noise) can be tuned separately. Second, to remove the signal-in-space errors, the proposed filter utilizes three different differential measurements. The absolute filter estimates time-differenced carrier phase (TDCP) measurements in combination with the pseudorange and pseudorange rate measurements, avoiding the need for estimating the integer ambiguity terms that are low observable. The relative filter estimates the relative orbit and clock offsets by processing the single difference carrier phase (SDCP) measurements, where signal-in-space errors are removed thanks to the short inter-satellite distance compared to the Earth-Moon distance. Using the obtained single difference float ambiguity estimate, the relative filter also fixes the integer ambiguities in double difference carrier phase (DDCP) measurements to improve the relative orbit estimates. Finally, cycleslip corrupted carrier-phase measurements are removed by observing the residuals in the TDCP measurements. We demonstrate the filter’s performance through simulations of two closely operating lunar satellites with different clock grades in the elliptical lunar frozen orbit (ELFO), wherein we showcase higher positioning and timing accuracy compared to code phase-only PNT methods.

• [C14] Cortinovis, M., Iiyama, K., Gao, G., “Satellite ephemeris approximation methods to support lunar positioning, navigation, and timing services”, Proceedings of the Institute of Navigation GNSS+ conference (ION GNSS+ 2023), 2023, Best Presentation of the Session [URL] [Slides] [BibTeX] [Abstract]

   @article{cortinovis2023satellite,
    title = {Satellite ephemeris approximation methods to support lunar positioning, navigation, and timing services},
    journal = {Proceedings of the Institute of Navigation GNSS+ conference (ION GNSS+ 2023)},
    pages = {3647--3663},
    year = {2023},
    month = {9},
    paper = {https://drive.google.com/file/d/1z-j70jBJPtMAQ6cpA_ig5yjbLJkotSEX/view},
    url = {https://doi.org/10.33012/2023.19282},
    doi = {10.33012/2023.19282},
    author = {Cortinovis, Marta and Iiyama, Keidai and Gao, Grace},
  }

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  Abstract: Plans to establish a satellite network around the Moon to support communication, position, navigation, and timing services are rapidly evolving. Satellites that are part of this system broadcast their ephemeris as finite parameters to lunar users for user state estimation. We investigate lunar satellite ephemeris parameterization to identify the optimal parameterization to broadcast to a lunar user. The proposed framework directly approximates the lunar satellite position and velocity in the inertial frame, along with approximating conversion parameters necessary for representation in the lunar fixed frame. The framework leverages signal-in-space-error requirements as constraints in the approximation process to guide the search for the best parameterization. We evaluate the performance of our proposed framework for satellites in a Low Lunar Orbit and an Elliptical Lunar Frozen Orbit. The performance of different methods is assessed based on the precision of the ephemeris prediction, approximation interval, and message size. We showcase the ability of the developed framework to approximate satellite ephemeris for both orbits to the desired precision by adjusting the approximation period and the number of broadcasted parameters. Particularly, we demonstrate that model formulations with a standard polynomial basis and a Chebyshev polynomial basis produce feasible solutions for ephemeris approximation at varying epochs in orbits, abiding by stringent signal-in-space-error requirements.

• [C13] Shimane, Y., Iiyama, K., “Methods for Dual-Objective High Energy Tour Design”, AAS/AIAA Astrodynamics Specialist Conference, 2023, [Paper PDF] [BibTeX] [Abstract]

   @article{shimane2023dual,
    title = {Methods for Dual-Objective High Energy Tour Design},
    journal = {AAS/AIAA Astrodynamics Specialist Conference},
    location = {Big sky, MT},
    year = {2023},
    month = {8},
    author = {Shimane, Yuri and Iiyama, Keidai},
  }

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  Abstract: High-energy, multi-fly-by tours are a complex design task that typically arises in planetary moon tours, such as those of the Jupiterian or Saturnian systems. In- spired by this challenge, the 2022 edition of the Space Optimisation Competition (SpOC) organized by the Advanced Concepts Team included a problem involving a Delta V and time of flight optimization of a tour visiting the seven planets of the Trappist-1 star system. This work introduces the preliminary analyses and heuristics developed to tackle the multiobjective optimization problem, along with the results found for the Trappist-1 tour problem

• [C12] Iiyama, K., Bhamidipati, S., Gao, G., “Terrestrial GPS time-differenced carrier-phase positioning of lunar surface users”, 2023 IEEE Aerospace Conference, 2023, [URL] [Paper PDF] [Slides] [BibTeX]

   @article{iiyama2023terrestrial,
    title = {Terrestrial GPS time-differenced carrier-phase positioning of lunar surface users},
    journal = {2023 IEEE Aerospace Conference},
    pages = {1--9},
    year = {2023},
    month = {3},
    organization = {IEEE},
    url = {https://doi.org/10.1109/AERO55745.2023.10115673},
    doi = {10.1109/AERO55745.2023.10115673},
    author = {Iiyama, Keidai and Bhamidipati, Sriramya and Gao, Grace},
  }

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• [C11] Iiyama, K., Sriramya, , Gao, G., “Precise Positioning and Timekeeping in a Lunar Orbit via Terrestrial GPS Time-Differenced Carrier-Phase Measurements”, Proceedings of the 2023 International Technical Meeting of The Institute of Navigation, 2023, [URL] [Paper PDF] [Slides] [BibTeX] [Abstract]

   @article{iiyama2024precise,
    title = {Precise Positioning and Timekeeping in a Lunar Orbit via Terrestrial GPS Time-Differenced Carrier-Phase Measurements},
    location = {Long Beach, CA},
    year = {2023},
    journal = {Proceedings of the 2023 International Technical Meeting of The Institute of Navigation},
    url = {https://doi.org/10.33012/2023.18597},
    doi = {10.33012/2023.18597},
    author = {Iiyama, Keidai and Sriramya,  and Gao, Grace},
  }

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  Abstract: There is a growing interest in the use of legacy terrestrial Global Positioning System (GPS) signals to determine the precise positioning and timing onboard a lunar satellite. Unlike prior works that utilize pseudoranges with meter-level accuracy, we propose a precise positioning and timekeeping technique that leverages carrier-phase measurements with millimeter-level accuracy (when integer ambiguities are correctly fixed). We design an extended Kalman filter framework that harnesses the intermittently available terrestrial GPS time-differenced carrier-phase (TDCP) values and gravitational accelerations predicted by the orbital filter. To estimate the process noise covariance, we implement an adaptive state noise compensation algorithm that adapts to the challenging lunar environment with weak gravity and strong third-body perturbations. Additionally, we perform measurement residual analysis to discard TDCP measurements corrupted by cycle slips and increased measurement noise. We present Monte-Carlo simulations of a lunar satellite in an elliptical lunar frozen orbit and quasi-frozen low lunar orbit, wherein we showcase higher positioning and timing accuracy as compared with the pseudorange-only navigation solution.

2022

• [C10] Murata, M., Koga, M., Nakajima, Y., Yasumitsu, R., Araki, T., Makino, K., Akiyama, K., Yamamoto, T., Tanabe, K., Kogure, S., Sato, N., Toyama, D., Kitamura, K., Miyasaka, K., Kawaguchi, T., Sato, Y., Kakihara, K., Shibukawa, T., Iiyama, K., Tanaka, T., “Lunar navigation satellite system: Mission, system overview, and demonstration”, 39th International Communications Satellite Systems Conference (ICSSC 2022), 2022, [URL] [Paper PDF] [BibTeX] [Abstract]

   @article{murata2022lunar,
    title = {Lunar navigation satellite system: Mission, system overview, and demonstration},
    journal = {39th International Communications Satellite Systems Conference (ICSSC 2022)},
    volume = {2022},
    pages = {12--15},
    year = {2022},
    month = {10},
    url = {https://doi.org/10.1049/icp.2023.1355},
    doi = {10.1049/icp.2023.1355},
    author = {Murata, M. and Koga, M. and Nakajima, Y. and Yasumitsu, R. and Araki, T. and Makino, K. and Akiyama, K. and Yamamoto, T. and Tanabe, K. and Kogure, S. and Sato, N. and Toyama, D. and Kitamura, K. and Miyasaka, K. and Kawaguchi, T. and Sato, Y. and Kakihara, K. and Shibukawa, T. and Iiyama, K. and Tanaka, T.},
  }

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  Abstract: In this paper, our lunar communication and navigation system called the lunar navigation satellite system (LNSS) is presented. The mission of the LNSS is to provide the communication, positioning, navigation, and timing (CPNT) service at the lunar South Pole region. Our primary lunar user is a pressurized rover that will explore the South Pole region and the LNSS has been designed to help the rover locate its own position on moon surface satellite images. The LNSS satellites will be also utilized as communication link nodes between the earth and the lunar surface user. The expected positioning accuracy at the South Pole region by the LNSS will be evaluated by our demonstration mission currently scheduled in 2028. The LNSS is not only for our lunar users, but also to contribute to the system of lunar communication and navigation satellite systems that will be completed by the international collaboration for the benefit of all humankind.

• [C9] Bhamidipati, S., Iiyama*, K., Mina*, T., Gao, G., “Time-transfer from terrestrial gps for distributed lunar surface communication networks”, 2022 ieee aerospace conference (aero), 2022, [URL] [Paper PDF] [Slides] [BibTeX] [Abstract]

   @article{bhamidipati2022time,
    title = {Time-transfer from terrestrial gps for distributed lunar surface communication networks},
    journal = {2022 ieee aerospace conference (aero)},
    pages = {1--15},
    year = {2022},
    organization = {IEEE},
    month = {3},
    url = {https://doi.org/10.1109/AERO53065.2022.9843716},
    doi = {10.1109/AERO53065.2022.9843716},
    author = {Bhamidipati, Sriramya and Iiyama*, Keidai and Mina*, Tara and Gao, Grace},
  }

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  Abstract: Reliable lunar communication infrastructure will be instrumental in building a sustainable human presence on the Moon. Recently, NASA has awarded a $14.1 million contract to Nokia for developing a 4G Long-Term Evolution (LTE) network on the Moon. In particular, there has been a great emphasis on designing a low-power and an ultra-compact LTE solution for the future Lunar Surface Communication Network (LSCN) due to its ease of transportation and potential for scalable deployment on the lunar surface. Ensuring a reliable LSCN design requires the base stations to be time-synchronized, and to maintain a precise globally referenced timing standard. We propose an LSCN design with distributed time-transfer from Earth-GPS to maintain precise timing across the network. We design a diffusion Kalman filter framework that utilizes the intermittently available Earth-GPS signals and the crosslink signals exchanged among the LSCN base stations to facilitate global time synchronization, while alleviating the timing stability and SWaP requirements for the on-site clocks. We validate our proposed technique by simulating the Earth-GPS satellites and multiple base stations near the Moon's Haworth Crater using the Systems Tool Kit (STK) software. We simulate the visibility and carrier-to-noise-density ratio of the Earth-GPS satellites at each LSCN base station, while examining the timing accuracy performance when each base station is equipped with a low-SWaP Chip Scale Atomic Clock (CSAC). We demonstrate the robustness of our distributed time-transfer technique as the error covariance of the transmission delay is varied.

• [C8] Iiyama, K., Kruger, J., D’Amico, S., “Autonomous Distributed Angles-Only Navigation and Timekeeping in Lunar Orbit”, Proceedings of the 2022 International Technical Meeting of the Institute of Navigation, 2022, [URL] [Paper PDF] [Slides] [BibTeX] [Abstract]

   @article{iiyama2022anglesonly,
    title = {Autonomous Distributed Angles-Only Navigation and Timekeeping in Lunar Orbit},
    journal = {Proceedings of the 2022 International Technical Meeting of the Institute of Navigation},
    location = {Long Beach, CA},
    pages = {453--470},
    year = {2022},
    month = {1},
    url = {https://doi.org/10.33012/2022.18207},
    doi = {10.33012/2022.18207},
    author = {Iiyama, Keidai and Kruger, Jusitn and D’Amico, Simone},
  }

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  Abstract: This paper demonstrates an algorithmic framework for autonomous, distributed navigation and timekeeping for spacecraft swarms and constellations using angles-only measurements from onboard cameras. Angles-only methods are compelling as they reduce reliance on external measurement sources. However, prior flight demonstrations have faced limitations, including 1) inability to treat multi-agent space systems including multiple observers and targets in an accurate and timely manner, 2) lack of autonomy and reliance on external state information, and 3) treatment of primarily Earth-orbiting scenarios. The Absolute and Relative Trajectory Measurement System (ARTMS) discussed in this paper overcomes these challenges to enable future lunar missions. It consists of three novel algorithms: 1) Image Processing, which tracks and identifies targets in images and computes their bearing angles; 2) Batch Orbit Determination, which computes a swarm state initialization from angles-only measurements; and 3) Sequential Orbit Determination, which uses an unscented Kalman filter to refine the swarm state, seamlessly fusing measurements from multiple observers to achieve the necessary robustness and autonomy. This paper augments ARTMS for lunar navigation and its theoretical performance is investigated through a quantitative observability analysis. High-fidelity simulations with a star tracker in the loop demonstrate successful navigation of swarms and constellations in low lunar orbits, near-rectilinear halo orbits, and elliptic frozen orbits. ARTMS achieves absolute orbit estimation for all swarm members using only inter-satellite angles with simultaneous estimation of differential clock offsets and ballistic coefficients. It therefore presents an important capability for the support of future lunar and planetary exploration.

2021

• [C7] Iiyama, K., Kawabata, Y., Funase, R., “Autonomous and decentralized orbit determination and clock offset estimation of lunar navigation satellites using GPS signals and inter-satellite ranging”, Proceedings of the Institute of Navigation GNSS+ conference (ION GNSS+ 2021), 2021, [URL] [Paper PDF] [Slides] [BibTeX] [Abstract]

   @article{iiyama2021autonomousdecentralized,
    title = {Autonomous and decentralized orbit determination and clock offset estimation of lunar navigation satellites using GPS signals and inter-satellite ranging},
    journal = {Proceedings of the Institute of Navigation GNSS+ conference (ION GNSS+ 2021)},
    pages = {936--949},
    year = {2021},
    month = {9},
    url = {https://doi.org/10.33012/2021.18019},
    doi = {10.33012/2021.18019},
    author = {Iiyama, Keidai and Kawabata, Yosuke and Funase, Ryu},
  }

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  Abstract: The development of a robust navigation infrastructure in cis-lunar space is crucial for the coming new era of advanced lunar exploration. NASA has set a goal to develop an extensible and scalable lunar communication and navigation architecture called LunaNet. For the flexibility and robustness of the system, the architecture is desired to have an autonomous and decentralized operation capability. This paper presents and demonstrates a decentralized and autonomous state estimation algorithm for SmallSats that provides positioning, navigation, and timing service, each equipped with a GNSS receiver, chip-scale atomic clock, and inter-satellite communication module. In the framework, each satellite individually estimates its own state and clock offset with a decentralized Schmidt Extended Kalman Filter by processing weak GNSS signal and inter-satellite range measurements. The algorithm is validated with Monte-Carlo state estimate simulations of five satellites at the hybrid constellation of lunar frozen orbit and the Near-Rectilinear Halo Orbit (NRHO). The proposed filter converged in all Monte-Carlo cases with clock offset errors below 0.15 microseconds and 1.0 microseconds in the frozen orbit and the NRHO, respectively.

2020

• [C6] Tomita, K., Skinner, K., Iiyama, K., Jagatia, B., Nakagawa, T., Ho, K., “Hazard detection algorithm for planetary landing using semantic segmentation”, ASCEND 2020, 2020, [URL] [BibTeX] [Abstract]

   @article{tomita2020hazard,
    title = {Hazard detection algorithm for planetary landing using semantic segmentation},
    journal = {ASCEND 2020},
    pages = {4150},
    year = {2020},
    month = {11},
    location = {Las Vegas, NV},
    url = {https://doi.org/10.2514/6.2020-4150},
    doi = {10.2514/6.2020-4150},
    author = {Tomita, Kento and Skinner, Katherine and Iiyama, Keidai and Jagatia, Bhavi and Nakagawa, Tatsuwaki and Ho, Koki},
  }

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  Abstract: This paper presents an application of a computer vision technique called Semantic Segmentation to a Hazard Detection (HD) algorithm for planetary landing. Three convolutional neural network (CNN) architectures are trained with binary (safe or unsafe) safety maps and multi-hazard-class safety maps. Their performance is compared together with a replicated state-of-the-art (SOA) HD algorithm from NASA's Autonomous Landing Hazard Avoidance Technology (ALHAT) project. To train and test each method, we prepared a dataset that is rich enough to reproduce the fine hazardous features by developing a realistic Digital Elevation Map (DEM) generator. Our DEM generator produces realistic terrains in arbitrary high resolution and the safeness of each pixel of DEMs is examined by computing the maximum possible slope and roughness for a given lander geometry and landing attitude. The results show that all three CNN architectures perform better than the replicated SOA HD algorithm for noised DEMs. The networks trained with multi-hazard-class safety maps result in better accuracy in terms of mean intersection over union (mIoU) but do not improve pixel accuracy.

• [C5] Iiyama, K., Tomita, K., Jagatia, B.A., Nakagawa, T., Ho, K., “Deep reinforcement learning for safe landing site selection with concurrent consideration of divert maneuvers”, AAS/AIAA Astrodynamics Specialist Conference, 2020, [URL] [Paper PDF] [Slides] [BibTeX] [Abstract]

   @article{iiyama2021deep,
    title = {Deep reinforcement learning for safe landing site selection with concurrent consideration of divert maneuvers},
    journal = {AAS/AIAA Astrodynamics Specialist Conference},
    year = {2020},
    month = {8},
    url = {https://arxiv.org/abs/2102.12432},
    doi = {10.48550/arXiv.2102.12432},
    location = {Lake Tahoe, CA},
    author = {Iiyama, Keidai and Tomita, Kento and Jagatia, Bhavi A and Nakagawa, Tatsuwaki and Ho, Koki},
  }

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  Abstract: This research proposes a new integrated framework for identifying safe landing locations and planning in-flight divert maneuvers. The state-of-the-art algorithms for landing zone selection utilize local terrain features such as slopes and roughness to judge the safety and priority of the landing point. However, when there are additional chances of observation and diverting in the future, these algorithms are not able to evaluate the safety of the decision itself to target the selected landing point considering the overall descent trajectory. In response to this challenge, we propose a reinforcement learning framework that optimizes a landing site selection strategy concurrently with a guidance and control strategy to the target landing site. The trained agent could evaluate and select landing sites with explicit consideration of the terrain features, quality of future observations, and control to achieve a safe and efficient landing trajectory at a system-level. The proposed framework was able to achieve 94.8 % of successful landing in highly challenging landing sites where over 80% of the area around the initial target lading point is hazardous, by effectively updating the target landing site and feedback control gain during descent.

• [C4] Shibukawa, T., Matsushita, S., Iiyama, K., Ishikawa, A., Nishii, K., Funase, R., “Flight Model Thermal Design and Verification for the 6U Deep Space CubeSat EQUULEUS”, 2020 International Conference on Environmental Systems, 2020, [URL] [Paper PDF] [BibTeX] [Abstract]

   @article{shibukawa2020flight,
    title = {Flight Model Thermal Design and Verification for the 6U Deep Space CubeSat EQUULEUS},
    year = {2020},
    journal = {2020 International Conference on Environmental Systems},
    month = {7},
    url = {https://hdl.handle.net/2346/86455},
    location = {Lisbon, Portugal (postponed)},
    author = {Shibukawa, Toshihiro and Matsushita, Shuhei and Iiyama, Keidai and Ishikawa, Akihiro and Nishii, Keita and Funase, Ryu},
  }

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  Abstract: EQUULEUS is a 6U CubeSat developed by the University of Tokyo and JAXA, which will fly to a libration orbit around the second Earth-Moon Lagrange point (EML2) as a secondary payload of the Space Launch System (SLS). At the destination and on the way there, EQUULEUS will conduct three scientific missions such as the detection of lunar impact flashes. To realize these missions, the thermal design of EQUULEUS faces constraints coupled with other subsystems. The Engineering Model (EM) was designed to clear these constraints, but after verification by a thermal vacuum test, one mission component did not meet the temperature requirements. To clear the problems in the EM, changes in operation strategies, temperature range management, and thermal interface materials were made. These design changes were verified by another thermal vacuum test, and we proved that EQUULEUS can meet the constraints to reach EML2 and conduct its scientific missions.

2019

• [C3] Matsushita, S., Shibukawa, T., Iiyama, K., Funase, R., “Thermal Design and Validation for a 6U Deep Space CubeSat EQUULEUS under Constraints Tightly Coupled with Orbital Design and Water Propulsion System”, 49th International Conference on Environmental Systems, 2019, [URL] [Paper PDF] [BibTeX] [Abstract]

   @article{matsushita2019thermal,
    title = {Thermal Design and Validation for a 6U Deep Space CubeSat EQUULEUS under Constraints Tightly Coupled with Orbital Design and Water Propulsion System},
    year = {2019},
    month = {7},
    journal = {49th International Conference on Environmental Systems},
    url = {https://hdl.handle.net/2346/84425},
    author = {Matsushita, Shuhei and Shibukawa, Toshihiro and Iiyama, Keidai and Funase, Ryu},
  }

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  Abstract: EQUULEUS, a 6U CubeSat co-developed by the University of Tokyo and JAXA, will fly to a libration orbit around the second Earth-Moon Lagrange point (EML2), and conduct scientific observations such as detection of the lunar impact flashes. To realize these observations, thermal design of EQUULEUS faces three constraints coupled with the design of other subsystems. Firstly, because there is an uncertainty of the launch date, EQUULEUS will have to consider the variation of the sun direction at thrusting phases, which changes the heater wattage to achieve the required thrust value. Secondly, at the observation phase around EML2, EQUULEUS will also experience the sunlight from various direction. Lastly, the water resistojet propulsion system adopted for trajectory control and angular momentum unloading of EQUULEUS makes the thermal design more difficult in terms of freezing and the latent heat for vaporization. Under the constraints, we proposed the thermal design of EQUULEUS Engineering Model (EM), validated the design by numerical simulations and the thermal vacuum test.

• [C2] Iiyama, K., “Optimization of the Navigation satellite constellation and Lunar Monitoring Station for Lunar Global Navigation Satellite System”, 32nd International Symposium on Space Technology and Science, 2019, [Paper PDF] [BibTeX] [Abstract]

   @article{iiyama2019optimization,
    title = {Optimization of the Navigation satellite constellation and Lunar Monitoring Station for Lunar Global Navigation Satellite System},
    year = {2019},
    month = {6},
    journal = {32nd International Symposium on Space Technology and Science},
    location = {Fukui, Japan},
    author = {Iiyama, Keidai},
  }

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  Abstract: The Lunar Global Navigation Satellite System (LGNSS) could significantly improve lunar missions' operational capability and flexibility by providing real-time, continuous, and highly accurate positioning services to lunar users. To achieve accurate user positioning, the orbit and clock bias of the navigation satellites have to be estimated accurately. An effective way to accurately estimate the orbit of navigation satellites in the lunar fixed frame is to process the psudorange data generated from the received navigation signals at lunar monitoring stations (LMSs) on the lunar surface. This paper presents a method to simulate the orbit determination error of the navigation satellites and the positioning error of the lunar user, assuming the usage of LMSs. We also propose a method to optimize the satellite constellation and LMS configuration to minimize the user positioning error. When optimizing, we reduced the number of design parameters by assuming the geometric symmetry of the navigation satellite constellation and LMS arrangement. Considering the sensitivity of the LMS arrangement and satellite constellation on the user positioning error, step by step optimization method is proposed. In the proposed method, the candidate constellation is first narrowed down by satellite visibility and PDOP analysis, followed by the LMS arrangement optimization for the selected constellations. Finally, the user positioning accuracy performance of the optimized configuration for 20 navigation satellites and 8 LMSs is analyzed. The results showed that the obtained constellation could achieve positioning errors below 10m in 3$\sigma$ for the global average.

• [C1] Shibukawa, T., Matsushita, S., Iiyama, K., Funase, R., “Reflection and Verification of Thermal Design under Tightly-Coupled Constraints to the 6U Deep Space CubeSat EQUULEUS”, 32nd International Symposium on Space Technology and Science, 2019, [BibTeX]

   @article{shibukawa2019reflection,
    title = {Reflection and Verification of Thermal Design under Tightly-Coupled Constraints to the 6U Deep Space CubeSat EQUULEUS},
    year = {2019},
    month = {6},
    journal = {32nd International Symposium on Space Technology and Science},
    location = {Fukui, Japan},
    author = {Shibukawa, Toshihiro and Matsushita, Shuhei and Iiyama, Keidai and Funase, Ryu},
  }

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