JAXA Supercomputer System Annual Report February 2025-January 2026

Report Number: R25EDU20199

Subject Category: Space and Astronautical Science

PDF (to be added)

• Responsible Representative: Ryuichi Fujimoto, Professor, Institute of Space and Astronautical Science, Department of Space Astronomy and Astrophysics
• Contact Information: Ryo Nagata(nagata.ryo@jaxa.jp)
• Members: Yusuke Itai, Ryo Nagata, Shugo Oguri, Rion Takahashi, Karen Tateoka, Hayato Takakura

Abstract

LiteBIRD is a strategic large-class mission of the Institute of Space and Astronautical Science (ISAS), JAXA. The satellite will be equipped with a wide-field reflective telescope incorporating several thousand detectors and will observe over the broad frequency range of 34–448 GHz. Its scientific goal is to probe the physics of the early Universe by measuring the polarization of the cosmic microwave background (CMB) with high precision in search of signatures of primordial inflation. To detect the weak CMB polarization, contamination from strong radiation in the Galactic plane could lead to large systematic errors. Accurate characterization of the beam properties (antenna patterns) is therefore of critical importance. In this project, we compute antenna patterns using POji, a physical optics simulation code being developed for LiteBIRD to improve understanding of the mission's observational performance and contribute to the reduction of systematic errors.

Reference URL

Please refer to https://www.isas.jaxa.jp/missions/spacecraft/future/litebird.html .

Reasons and benefits of using JAXA Supercomputer System

The mirrors of the LiteBIRD telescope are about 1 m, while the shortest observation wavelength (at 448 GHz) is 0.669 mm. Treating higher-frequency bands requires a finer mesh size to discretize the mirror surfaces, resulting in a rapid increase in computational cost. Halving the mesh size along each side results in a quadratic increase in the number of meshes on the mirror surface. Furthermore, because LiteBIRD employs a Crossed-Dragone optical system consisting of two reflectors, the computation time increases according to a fourth-power law. Estimating the simulations required for LiteBIRD based on this fourth-power rule shows that even a standard high-performance workstation PC would require approximately 720.5 days of computation time. Moreover, since LiteBIRD has several thousand detectors, this further expands to millions of days, making it unfeasible within a practical timeframe in a standalone environment. Our strategy for coping with this increasing simulation scale is to make use of the parallel processing capability of JAXA's supercomputer.

Achievements of the Year

During this fiscal year, we validated the computational accuracy of the physical optics simulation code POji and evaluated the validity of the far-field antenna patterns by comparison with near-field measurement results. For the mesh size, in which the surface is represented by discretized points, we confirmed, with all other conditions held fixed, that the discrepancy decreases as the mesh is refined. This demonstrates that the simulation can achieve sufficient numerical accuracy to meet the requirements imposed by LiteBIRD (Fig. 1). In addition, for the quarter-scale model of the LiteBIRD telescope at 180 GHz and for the central focal-plane position, we compared the far-field antenna patterns obtained with POji against those derived from near-field measurements (H. Takakura et al., 2022, Proc. SPIE, 1218052). The comparison showed that the ring-like sidelobe structure is qualitatively reproduced in the two-dimensional maps (Fig. 2). Moreover, in the +45degrees diagonal cut, which was adopted to avoid contamination from scan noise, the main-lobe width and the positions and amplitudes of the nearby sidelobe sequence were found to agree well down to roughly -50 dB relative to the peak (Fig. 3).

Fig.1: Dependence of numerical accuracy on simulation mesh size.

Fig.2: Far-field antenna patterns at 180GHz, Poji simulation (left) and real measurement (right).

Fig.3: One-dimensional cut of the far-field antenna patterns for the central focal-plane position along its +45 degrees.

Publications

- Oral Presentations

2026 Spring Meeting of the Astronomical Society of Japan: Observation Equipment (Radio) V143a

Usage of JSS

Computational Information

• Process Parallelization Methods: MPI
• Thread Parallelization Methods: N/A
• Number of Processes: 48 - 48000
• Elapsed Time per Case: 4 Hour(s)

JSS3 Resources Used

Fraction of Usage in Total Resources*1(%): 0.02

Details

Please refer to System Configuration of JSS3 for the system configuration and major specifications of JSS3.

*1: Fraction of Usage in Total Resources: Weighted average of three resource types (Computing, File System, and Archiver).

*2: Fraction of Usage：Percentage of usage relative to each resource used in one year.

ISV Software Licenses Used

*2: Fraction of Usage：Percentage of usage relative to each resource used in one year.

JAXA Supercomputer System Annual Report February 2025-January 2026