remote sensing Article Effects of Solar Invasion on Earth Observation Sensors at a Moon-Based Platform Hanlin Ye 1, Wei Zheng 1,*, Huadong Guo 2,3, Guang Liu 2,3 and Jinsong Ping 3,4 1 Qian Xuesen Laboratory of Space Technology, China Academy of Space Technology, Beijing 100094, China; [email protected] 2 Key Laboratory of Digital Earth Science, Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100094, China; [email protected] (H.G.); [email protected] (G.L.) 3 University of Chinese Academy of Sciences, No.19(A) Yuquan Road, Shijingshan District, Beijing 100049, China; [email protected] 4 Key Laboratory of Lunar and Planetary Exploration, National Astronomical Observatories, Chinese Academy of Sciences, No.20 Datun Road, Chaoyang District, Beijing 100101, China * Correspondence: [email protected]; Tel.: +86-10-6811-1077 Received: 5 October 2019; Accepted: 23 November 2019; Published: 25 November 2019 Abstract: The solar invasion to an Earth observation sensor will cause potential damage to the sensor and reduce the accuracy of the measurements. This paper investigates the effects of solar invasion on the Moon-based Earth observation sensors. Different from the space-borne platform, a Moon-based sensor can be equipped anywhere on the near-side of the Moon, and this makes it possible to reduce solar invasion effects by selecting suitable regions to equip sensors. In this paper, methods for calculating the duration of the Sun entering of the sensor’s field of view (FOV) and the solar invasion radiation at the entrance pupil of the sensor are proposed. By deducing the expressions of the proposed geometrical relationship between the Sun, Earth, and Moon-based platform, it has been found that the key parameter to the effects of solar invasion is the angle between the Sun direction and the line-of-sight vector. Based on this parameter, both the duration and radiation can be calculated. In addition, an evaluation approach based on the mean value and standard deviation has been established to compare the variation of solar invasion radiation at different positions on the lunar surface. The results show that the duration is almost the same wherever the sensor is placed in the permanent Earth-observation region. Further, by comparing the variation of solar invasion radiation at different positions on the near-side of the Moon, we suggest that equipping sensors on the mid–high latitude regions within the permanent Earth-observation region will result in less solar invasion affects. Keywords: solar invasion; Moon-based Earth observation platform; geometric modeling; remote sensing 1. Introduction With the development of space science and technology, Earth observation systems have been established gradually [1,2]. Various Earth observation platforms, including air-borne and space-borne platforms, have played important roles in many fields and are organized as systems to provide numerous datasets of our living planet. In recent years, the Moon, as the only natural satellite of the Earth, has gained great interest as a potential Earth observation platform [3–6]. The first Moon-based sensor can be dated back to 1972, when Apollo 16 sent men to the Moon for the fifth time [7]. A far-ultraviolet camera was operated on the lunar surface, and imagery of the terrestrial atmosphere and the geocorona was obtained [8]. Forty-one-years later, in 2013, an extreme Remote Sens. 2019, 11, 2775; doi:10.3390/rs11232775 www.mdpi.com/journal/remotesensing Remote Sens. 2019, 11, 2775 2 of 17 ultraviolet camera (EUVC) onboard the Chang’E-3 (CE-3) lander observed the Earth’s plasmasphere from the lunar surface [9]. In recent years, many countries and organizations have initiated programs to set up a base on the Moon, which includes establishing an Earth observation platform [10]. Some pioneer research has been carried out to investigate the Moon-based Earth observations, including scientific goals [3,11], observation geometry analysis [12–15], and Moon-based synthetic aperture radar (SAR) parameters [4,5,16]. According to their research conclusion, a Moon-based Earth observation platform is characterized by longevity, integrity, stability, and uniqueness. Equipping sensors on the lunar surface can be used to monitor the Earth-space environment, the dynamics of solid Earth, the Earth’s radiation budget at the top of atmosphere, and other issues of large-scale phenomena of the Earth. Further, a Moon-based Earth observation platform could be complementary to the existing Earth observation systems. For the Moon to serve as an Earth observation platform, it is essential to evaluate its observation geometry. Compared to traditional Earth observation platforms, equipping sensors on the lunar surface has some special characteristics in observation geometry. First, since the distance between the Earth and the Moon is very large, a sensor installed on the lunar surface could observe nearly half of the Earth, which could extend the existing Earth observations to longer time scales and larger space scales [12]. Additionally, it is possible to realize integrative measurements for the Earth [11]. Second, combining the orbit of the Moon and the seasonal change in relative Earth orientation, a Moon-based platform can realize simultaneous and integral observation of the high latitudes, which will help carry out the contrastive study of the polar regions [17]. Third, due to the vast places of the lunar surface, different kinds of sensors can be installed. They can work together and acquire data from the Earth’s surface to the plasmasphere simultaneously. Many scholars have conducted research on the observation geometry. He et al. [18] simulated the Moon-based extreme ultraviolet images and demonstrated that equipping sensors on the regions where they can observe the Earth is beneficial for acquiring high quality images. Ren et al. [12] proposed simulation technologies of Moon-based Earth observations and evaluated the line-of-sight condition to the Earth. In addition to the consideration of the line-of-sight condition to the Earth, Ye et al. [13] studied the pointing error of a Moon-based sensor and noted that the mid–high latitude regions on the lunar surface are suitable places to equip Earth observation sensors. Guo et al. [14] analyzed the errors of the exterior orientation elements of Moon-based sensors. They also suggested that equipping sensors on the mid–high latitude region helps to reduce the pointing error. As an Earth observation platform, the requirements of acquiring high-quality data need to be considered. Solar invasion for a sensor refers to the sunlight hitting the lens of the sensor directly, which may damage the sensor, reduce the quality of the measurements, and ultimately affect the accuracy of the data [19–21]. When observing the Earth from the lunar surface, the effect of solar invasion also exists. As shown in Figure1, the Sun moves into the FOV of the sensor. At that moment, the Sun moves to the other side of the Earth and the lens of the sensor will be directly exposed to the Sun. From the experiences of space-borne platforms, the effects of the solar invasion depend on the geometric relationships between the Sun, the Earth, and the platforms. The solar invasion does not seem to be an issue in the Sun-synchronous orbit because the satellites will pass over the Earth’s surface at the same local mean solar time [22]. It is an orbit that always maintains the same relationship with the Sun. Another common one is the geostationary orbit. Since the geostationary orbit is a circular orbit above the Earth’s Equator and follows the direction of the Earth’s rotation, the solar invasion will occur near midnight at local time daily, which might lead to the damage of the sensors. A method to solve this problem is to avoid the solar invasion by adjusting the satellite attitude [19]. When observing the Earth from a Moon-based platform, since the relative position of the Sun, the Earth, and the Moon is unfixed, the sensor’s line-of-sight vector would be close to the Sun direction at some time. Such a complex geometrical relationship will no doubt complicate the evaluation of the solar invasion effects. Remote Sens. 2019, 11, 2775 3 of 17 Remote Sens. 2019, 11, x FOR PEER REVIEW 3 of 17 FigureFigure 1.1. Schematic representing thethe solarsolar invasion.invasion. 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