As a part of China’s Chang’e-4 lunar far side mission, two lunar microsatellites for low frequency radio astronomy, amateur radio and education, Longjiang-1 and Longjiang-2, were launched as secondary payloads on 20 May 2018 together with the Queqiao L2 relay satellite. On 25 May 2018, Longjiang-2 successfully inserted itself into a lunar elliptical orbit of 357 km × 13,704 km, and became the smallest spacecraft which entered lunar orbit with its own propulsion system. The satellite carried the first amateur radio communication system operating in lunar orbit, which is a VHF/UHF software defined radio (SDR) designed for operation with small ground stations. This article describes and evaluates the design of the VHF/UHF radio and the waveforms used. Flight results of the VHF/UHF radio are also presented, including operation of the radio, performance analysis of downlink signals and the first lunar orbit UHF very-long-baseline interferometry (VLBI) experiment.

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Small solar system bodies, such as asteroids and comets, have held important keys to understanding the origin and evolution of the solar system. As a result, the study of these celestial bodies has garnered worldwide attention for many years. The knowledge of the 3D shape of small solar system bodies is fundamental for determining their global physical properties (volume, density, rotational parameters), choosing the landing site, and observing their interior structure.There are three kinds of methods that can be used for reconstructing 3D global shapes of small solar system bodies. The first method is based on Light Detection and Ranging (LiDAR) (Mukai et al., 2007;Zuber et al., 2000). But LiDAR can only achieve high resolutions when scanning at close range. The second method is based on optical images (Pajusalu & Slavinskis, 2019; R. Wang et al., 2020). It's the most accurate method and the measurement range can be larger than LiDAR. However, the performance of camera is severely limited by the illumination condition. The third method is using radio waves, for example, radar tomography and synthetic aperture radar (SAR) techniques. The resolution of SAR is restricted by the transmit bandwidth and synthetic aperture length (Soumekh, 1999), so it can overcome the above problems of long range and poor illumination conditions. Furthermore, different from rocky moons and planets whose interior structure can be investigated by seismic waves, the small solar system bodies have much looser structure, such as a rubble-pile, and the use of seismic waves is limited. At present, the interior of a small solar system body can only be recovered through the detection of radio waves.In recent years, with the development of space instrument technology, many spaceborne radars have been used in the Moon and major planet exploration missions. Computed radar tomography (CRT) approach, SAR technique, and ground penetrating radar system, etc., have been used and achieved rich results (C. Li et al., 2020;Neish et al., 2011;Simpson et al., 2006). Different from the Moon and major planets, small solar system bodies are much smaller in size and have weaker gravity. Moreover, as shown by (Warner et al., 2009), most small solar system bodies spin rapidly, causing relative motion between the radar and the target in multiple directions, which

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