Li Mingyuan et al. - IEEE TGRS & RS: Research on Lunar Imaging Technology for Ground-Based Radar

Ground-based radar is one of the methods of effective remote sensing of the Moon and even other celestial bodies in the solar system, and can provide a lot of valuable information about the physical properties of the lunar regolith, such as the abundance of rocks on the lunar surface and subsurface, the thickness of the regolith, iron and titanium content, the distribution of impact melts, and the large-scale topographic map of the front. At present, the lunar imaging of ground-based radar is mainly carried out by the two radars of the United States, Arecibo and Haystack, and the relevant work has been less due to the lack of suitable equipment and related experimental design and data processing experience in China. Recently, there have been some preliminary studies on the lunar detection of ground-based radar in China, such as the continuous wave signal dual-base lunar exploration test based on the Kashgar Deep Space Station and the Kunming 40-meter radio telescope, and the Buck code pulse signal spontaneous self-collection lunar exploration experiment based on qujing incoherent scattering radar, but there are still many problems to be solved in terms of experimental design and algorithm research.

In view of the above problems, the incoherent scattering radar project team of the Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, used the recently completed Sanya incoherent scattering radar to conduct preliminary lunar imaging experiments, experimental design and data processing algorithm research. The project team used two emission waveforms -13-bit buck codes and linear frequency modulation pulses for lunar imaging experiments, and developed the following three key technologies: (1) the imaging core algorithm adopts the distance Doppler imaging algorithm commonly used in synthetic aperture imaging; (2) in order to avoid the side-lobe effect of the 13-bit buck code matching filter, a side-lobe filter is used; (3) in order to solve the problem of "north-south blur", the doppler north-south hemisphere stitching imaging technology on the front of the moon is adopted.

The study yielded two types of imaging results: a mosaic image of the whole northern and southern hemispheres of the lunar front and a local area image (Figure 1). The distance resolution of the imaging results is 500 meters and the azimuth resolution is 1.2 kilometers. The results prove the feasibility and reliability of lunar imaging based on Sanya radar, and accumulate experience in experimental design and data processing of lunar imaging on ground-based radar, which makes it possible to use continental-based radar for lunar geology research in the future.

Figure 1 (a) Imaging of the northern and southern hemispheres of the lunar front;

The study also found that although the distance Doppler algorithm traditionally used is computationally efficient, when it is necessary to extend the coherence accumulation time to obtain higher azimuth resolution, if the coherence accumulation time is too long, the defocusing phenomenon will occur, resulting in image blur.

In view of this problem, the project team proposed to apply the backward projection algorithm in the reverse synthetic aperture imaging technology to focus on imaging the moon. The backward projection algorithm is a time domain algorithm that can theoretically achieve focused imaging of each pixel in any long coherent accumulation time, but the defect of the algorithm is that the algorithm is highly complex, so this paper applies the algorithm to the local area of the moon for imaging experiments. The project team used a linear frequency modulation pulse to transmit right-handed circularly polarized electromagnetic waves, receive left-handed circular polarization echoes, and used the distance Doppler algorithm and backward projection algorithm to image and compare the Pythagorean crater area (Figure 2).

The results show that the backward projection algorithm can be well applied in the ground-based radar lunar imaging experiment and has a certain focusing performance, but the focusing performance needs to be further quantified to the degree of image quality improvement, which is the content that needs to be paid attention to in the follow-up work; in addition, the backward projection algorithm is inefficient, and it is necessary to develop a rapid algorithm to improve the calculation efficiency.

Figure 2 The left column is the imaging map of the Pythagorean impact crater area, and the right column is a magnified view of the red rectangular box area in the left column. (a, b) distance from the Doppler algorithm imaging map, coherence accumulation time of 4 minutes; (c, d) distance from The Doppler algorithm imaging map, coherence accumulation time of 8 minutes; (e, f) backward projection algorithm imaging map, coherence accumulation time of 8 minutes

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The research results were published in the international academic journals IEEE Transactions on Geoscience and Remote Sensing and Demote Sensing. The research was co-funded by the Strategic Pilot Science and Technology Project (Category B) of the Chinese Academy of Sciences (XDB4100000), the Meridian Project and the National Natural Science Foundation of China (41427901).

1. Li Mingyuan, Le Xin'an*, Wang Yonghui, Wang Junyi, Ding Feng, Juha Vierinen, Zhang Ning, Wang Zhongqiu, Ning Baiqi, Zhao Biqiang, Zeng Lingqi. Moon Imaging Technique and Experiments Based on Sanya Incoherent Scatter Radar [J]. IEEE Transactions on Geoscience and Remote Sensing, 2022, 60: 1-14. DOI: 10.1109/TGRS.2022.3167156

2.Li Mingyuan,Le Xin'an*,Ding Feng,Ning Baiqi,Wang Junyi,Zhang Ning,Luo Junhao,Huang Lijia,Wang Yonghui,Wang Zhongqiu. Focused Lunar Imaging Experiment Using the Back Projection Algorithm Based on Sanya Incoherent Scatter Radar [J]. Remote Sensing，2022, 14: 2048. DOI: 10.3390/rs14092048

Editor: Chen Feifei

Proofreader: Jiang Xuejiao Zhou Xingxing