Designing a penetrating radar system for lunar surveys as part of the HARLOCK project

Developing science-driven instrumentation and methodologies for the investigation of lunar subsurface materials, such as water ice, and surface to near-surface mineral resources is the main goal of HARLOCK (High-resolution Autonomous Resource Lunar Observation & Characterization Kit) project, which is a strategic Italian project coordinated by CNR and INAF, as part of the PRORIS initiative [1].

Among the HARLOCK technologies, the penetrating radar is one of the few ones having a high Technology Readiness Level since it has been an effective payload for rover and lander adopted in Moon observation missions, for instance the China missions Chang'e-3-6 [2] - [6]. However, as well-known, the penetrating radar provides a high-resolution subsoil image only once the collected data are processed properly. In this frame, an open issue is the design of imaging approaches based on reliable mathematical models of signal propagation and diffraction in stratified media (air/soil), whose electromagnetic characteristics are typical of the planetary environment of interest. Furthermore, another relevant issue is the capability of exploiting the increased information content offered by multi-antenna systems collecting data by using more than one transmitting and receiving antenna.

This contribution deals with two imaging approaches for multi-antenna penetrating radar systems, which face the imaging in a stratified medium as a linear inverse scattering problem. The approaches exploit two different ray-based propagation models: Interface Reflection Point (IRP) model and  Equivalent Permittivity (EP) model. These models were previously proposed for single transmitter single receiver penetrating radar system [7], and adopted to process Chang'E-4 Lunar Penetrating Radar data [8]. Specifically, a performance analysis comparing the approaches in terms of reconstruction capabilities and computational burden will be presented at the conference. It is worth pointing out that the performance analysis in terms of resolution supports the definition of the penetrating radar system requirements for a given soil, while considering the size of the objects to be detected. Furthermore, computational efficiency is essential to move towards real time imaging.

References

[1] PRORIS Consortium (2024), PRORIS – Programma di Ricerca Spaziale di Base, INAF–CNR Joint Program. Available at: https://www.proris.it

[2] Ip, W.-H., et al. Preface: The Chang’e-3 lander and rover mission to the Moon. Res. Astron. Astrophys. 14, 1511, 2014.

[3] Jia, Y. et al. The scientific objectives and payloads of Chang’E− 4 mission. Planet. Space Sci. 162, 207–215, 2018.

[4] Li, C. et al. The Moon's farside shallow subsurface structure unveiled by Chang'E-4 Lunar Penetrating Radar, Science Advances, 6 (9), 2020.

[5] Su, Y. et al. Hyperfine Structure of Regolith Unveiled by Chang’E-5 Lunar Regolith Penetrating Radar. IEEE Trans. Geosci. Remote Sens. 60, 1–14 (2022)

[6] Li, C. et al. Nature of the lunar farside samples returned by the Chang’E-6 mission. Natl. Sci. Rev. nwae328, 2024.

[7] Catapano, I. et al. Contactless ground penetrating radar imaging: State of the art, challenges, and microwave tomogra-phy-based data processing. IEEE Geoscience and Remote Sensing Magazine, 10.1: 251-273, 2021.

[8] Soldovieri, F. et al. Microwave tomography for Lunar Penetrating Radar data processing in Chang'e 4 mission. Scientific Reports, 15(1):5219, 2025.