Developments and Prospects of Ground-Based Deep-Space Radar

‍ ‍Ground-based deep-space radar， the most powerful method for post-discovery， remote physical and dynamical characterization of near-Earth objects， is an important method for studying solar system celestial bodies such as the moon， near-Earth asteroids， and terrestrial planets. The advantages of ground-based deep-space radar include all-day and all-weather， accurate ranging， high-resolution imaging. Ground-based deep-space radar can obtain the physical characterizations including orbit， rotation， topography of the objects， prevent newly discovered objects from being lost， reduce uncertainties on orbital elements by orders of magnitude， and greatly improving impact probability estimates compared to optical-only datasets， which is crucial for near-Earth asteroids defense and planetary science. The key facilities for ground-based deep-space radar are the 305-meter Arecibo Observatory （decommissioned in 2020 after two cable failures） in Puerto Rico and 70-meter Goldstone Solar System Radar （GSSR） in California， which played a critical and unique role in the tracking and characterization of near-Earth objects in past decades. This paper introduces the background of ground-based deep-space radar， summarizes the development status of ground-based deep-space radar system， and reviews remarkable historical contributions of ground-based deep-space radar detection， such as high-resolution imaging of the surface of the Moon， Venus， Mercury and Mars， identifications of water ice deposits in polar regions on the Moon and Mercury， precise measurements of the orbits and shapes of near-Earth asteroids， rapid assessment of impact effect in near-Earth asteroid defense mission. However， existing ground-based deep-space radars are centralized radar systems with a single antenna， which have some inherent limitations including limited detection range and high failure rate due to the physical limits of antenna size and transmitter power. This paper introduces the innovative concept of the distributed aperture deep-space radar system， which consists of dozens of radar units. Each radar unit has separate antenna， transmitter and receiver. The distributed aperture deep-space radar can be equivalent to a large-scale radar via coherent signal processing technology that aligning the phases of electromagnetic waves transmitted by each radar. It can overcome physical limits of centralized radar system and is vividly called “China compound eye” radar because its configuration is similar with compound eyes of insects. Furthermore， the recent developments of distributed aperture ground-based radar system are introduced. A scaled-down system composed of four 16-meter radars is completed and taken into use. Imaging experiments to the Moon have been conducted on the basis of the scaled-down system and successfully verified the feasibility of distributed aperture deep-space radar concept. Recent 2D/3D imaging results of the famous Tycho crater on the Moon are shown in this paper. Finally， the future developments of ground-based deep-space radar are considered. We believe that the distributed aperture deep-space radar system will play a critical role in planetary science and planetary defense in future.