Ground-Based Radar 3-D Imaging for the Moon: Motion Error Analysis and Multi-Track Multi-Aperture Joint Autofocus

Ground-based (GB) radar plays an important role in lunar observation. With the synthetic aperture formed by the relative motion between the earth and the moon, GB radar can acquire lunar three-dimensional (3-D) image which can support the scientific research in lunar topography mapping and water ice detection, etc. However, the Earth–Moon motion geometry leads to long observation time span and complex motion trajectories, causing multi-source time-varying errors that degrade imaging quality in synthetic aperture radar (SAR) imaging. To address this issue, this manuscript establishes the error analysis model for multi-source motion errors, and proposes a multi-track multi-aperture joint autofocus method. The main contributions are summarized as follows: First, a theoretical model is developed to analyze the influence mechanism of time-varying errors in 3-D imaging, which quantitatively analyzes range migration error, Doppler phase error, and height migration error. Second, an autofocus method for SAR 3-D imaging in rectangular coordinate system is proposed to simultaneously compensate for these errors, which contains sub-aperture division, echo delay error inversion, and 3-D frequency-domain autofocus. Third, the first attempt of lunar 3-D imaging experiment through the GB radar has been conducted to validate the proposed method, which provides an example for GB radar lunar 3-D imaging.