## 1 Introduction  
In recent years, Earth system science has attracted wide attention. Considerable effort has been made to understand and model the Earth's climate. Previous studies have revealed that there is a high degree of anisotropy for reflected light and some climate parameters such as the Bond albedo are of bidirectional property [1; 2; 3]. Sufficient angular sampling is beneficial to the inversion of atmospheric parameters. For example, in the Earth's energy budget field, the development of the CERES has greatly enriched the angular sampling coincident with other sensors for Earth's energy budget [4; 5; 6; 7]. In this sense, an Earth observation platform with rich angular sampling can provide additional angular sampling coincident with current satellite constellations, which will greatly advance our understanding of Earth's climate. Since the 1960s, various Earth observation platforms have been developed. Among them, the Earth-orbiting satellites such as low orbit satellites (LEO satellites), geostationary orbit satellites (GEO satellites), and DSCOVR satellite located at the Sun-Earth Lagrange L1 point (L1 satellite) have provided us with a large amount of valuable Earth observation multiangle data [8]. Their angular samplings are different with respect to the observation geometry [9]. The first LEO satellites were expected to provide nadir views for a given terrestrial surface. Since then, the development of multiangle sensors has improved their angular sampling capabilities [10]. Due to the limitation of orbital altitude, they mainly provide data on a smaller spatial scale. To this extent, GEO and L1 satellites can achieve hemisphere-scale Earth observation. Since the GEO satellites and the Earth are in a relatively fixed geometry, their viewing angles remain unchanged [11; 12]. Contrariwise, the L1 satellite is located at the L1 point of the Sun-Earth Lagrange, where it can continuously observe the sunward side of the Earth [13]. This means that the angles of the incoming light and the viewing direction often coincide for a given terrestrial surface. Furthermore, a new miniaturized satellite named CubeSat has been proposed and developed as a standard platform for technology demonstration and scientific instrumentation in recent years [14]. It will provide the opportunity to enable missions that a larger satellite could not accomplish due to its advantages of development cost and time [15]. The construction of a satellite constellation has the potential to provide us with high temporal resolution global multiangle observations, which will benefit the observation of Earth's highly dynamic system, such as the weather system [16; 17]. For example, a constellation of CubeSats in GEO and a special polar-geosynchronous type orbit named Molinya can acquire real-time global natural color imagery on a time-scale of 5 min, which will allow for significant multiangle coverage [16]. In addition to the existing Earth observation platforms, a Moon-based platform is proposed as a novel Earth observation platform to acquire continuous multiangle and hemispheric-scale observation data [18; 19; 20]. It has specific angular sampling characteristics due to its peculiar observation geometry [21]. First, because of the large distance between the Earth and Moon, the sensor can observe the Earth disc, and thus, the adjacent observed points in the hemispherical view have continuous observational angles. Second, such a long-term observation is conducive to the acquisition of multiangle data, which is important for revealing the anisotropic characteristics of terrestrial surfaces. Third, the variable orbital inclination of the lunar orbit will lead to the movement of lunar nadir, which not only benefits global coverage, particularly for polar regions but also increases the diversity of angular combinations. In this sense, a Moon-based platform can complement additional angular sampling observations coincident with existing satellites observations, which will enhance the Earth-based sensor constellation.