Terrain Observation by Beam Steering Mode for Lunar Wide-swath Imaging with Earth-based Radar

Lunar wide-swath imaging with Earth-based radar is of great significance for lunar scientific research. Conventional Earth-based radars with high frequencies and large-aperture antennas usually operate in the spotlight mode, resulting in a relatively narrow imaging swath. With the increasing demand for large-scale mapping of the lunar surface, it is necessary to develop a kind of wide-swath imaging mode. Hence, this paper proposes a novel imaging mode, namely terrain observation by beam steering (TOBS) mode. The key feature of TOBS mode is to steer the antenna beam at a non-uniform angular speed along the geographical orientation of the scene swath during the data acquisition. The key techniques are: 1) a dynamic beam control method is proposed to achieve uniform azimuth resolution; 2) a variable PRF design method is proposed for effective data acquisition; 3) an improved ground Cartesian back-projection (GCBP) algorithm based on affine geographic coordinate (AGC) system is proposed for efficient TOBS mode imaging. This paper reports the first demonstration of the TOBS mode lunar imaging with an Earth-based radar prototype system. A scene swath of about 750 km has been imaged utilizing the TOBS mode with an azimuth resolution of about 25 m, and the effectiveness of the proposed method is successfully validated. Unlike the traditional "mosaic mode", where multiple separate observations are required to obtain a long swath image leading to reduction in efficiency and enhanced complexity of data processing. The TOBS mode provides a new technical approach for lunar wide-swath imaging utilizing large-aperture and high-frequency Earth-based radars, translating to high-resolution lunar image datasets.