High-Resolution Lunar Imaging Using Stepped-Frequency Waveforms With Earth-Based Radar

High-resolution lunar imaging using Earth-based radar is essential for advancing lunar science and exploration. Stepped-frequency waveforms have emerged as an effective approach to achieve high resolution while alleviating hardware limitations. However, system errors exist in stepped-frequency radar systems, and challenges remain in system error estimation and wideband synthesis, particularly under low signal-to-noise ratio (SNR) conditions and severe range-azimuth coupling. To address these issues, a high-resolution lunar imaging method using stepped-frequency waveforms with Earth-based radar is proposed in this paper. First, system errors are decomposed into intra-sub-band errors and inter-sub-band errors. Then, image-domain system error estimation methods are proposed, which enhance robustness under low SNR and eliminate inter-sub-band space-variant time offsets. The phase gradient autofocus (PGA) algorithm is extended to back-projection (BP) images for intra-sub-band error estimation, and overlapping wavenumber spectrum interferometric method is utilized to estimate the inter-sub-band errors. Finally, an image-domain wideband synthesis method based on spectrum shift is proposed to construct wideband image. Simulation results and experimental results based on an Earth-based radar system validate the proposed algorithm.