Lunar orbiter

Abstract

To enable observation of the moon with an artificial satellite.SOLUTION: A moon orbiting satellite 100 orbits around the moon 191 by flying along an inclined elliptical orbit. The moon orbiting satellite 100 includes a synthetic aperture radar and a propulsion device. The synthetic aperture radar observes a portion of the South Pole by irradiating a range from the front side of the South Pole point to the tip of the South Pole point with the electric wave every time the moon orbiting satellite 100 passes through the vicinity of the South Pole. The propulsion device rotationally moves an orbital surface of the inclined elliptical orbit around a south-north axis the moon 191 by applying the propulsive power to the moon orbiting satellite 100 every time the moon orbiting satellite 100 passes above the equator of the moon 191.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] This disclosure relates to lunar orbiters. [Background technology]

[0002] Lunar exploration plans in various countries are accelerating, and there are also plans to build a Gateway, which will serve as a base for orbiting the moon. There is a plan to build a lunar base in the south polar region of the moon, and it is necessary to create a topographical map of the antarctic region. In areas that reflect sunlight, it is possible to use optical observation equipment to obtain two images with different line-of-sight vectors (stereo pair images). The stereo pair images can then be used to generate a DEM. DEM is an abbreviation for Digital Elevation Model. However, in the Antarctic, optical observations using reflected sunlight are difficult to create topographical maps due to insufficient light, and long shadows often lead to false positives when matching corresponding points in stereo pair images.

[0003] A method for creating topographical maps without using optical observation equipment is to use a synthetic aperture radar. By using a synthetic aperture laser, it is possible to create topographical maps without depending on sunlight conditions. However, when a satellite orbiting the moon in a polar orbit performs squint observation using a synthetic aperture laser, the polar regions become blind spots due to the off-nadir angle of view when passing over them, making it impossible to monitor the crucial polar regions.

[0004] US Pat. No. 5,399,433 discloses a system for observing a target area on the Earth using a constellation of observation satellites. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2008-126876 A Summary of the Invention [Problem to be solved by the invention]

[0006] The present disclosure aims to enable observation of the moon by artificial satellites. [Means for solving the problem]

[0007] The lunar orbiting satellite of the present disclosure orbits the moon in an inclined elliptical orbit. The lunar orbiting satellite comprises a synthetic aperture radar and a propulsion device; the synthetic aperture radar, each time the lunar orbiting satellite passes near one of the south pole and the north pole of the moon, irradiates a range from just before a pole of the one of the polar regions to beyond the pole of the one of the polar regions with radio waves to observe a part of the one of the polar regions; The propulsion device applies a thrust to the lunar orbiting satellite to rotate the orbital plane of the inclined elliptical orbit around the north-south axis of the moon every time the lunar orbiting satellite passes above the lunar equator. Effect of the Invention

[0008] According to the present disclosure, observation of the moon by artificial satellite becomes possible. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing a lunar orbiting satellite 100 according to the first embodiment. [Diagram 2] FIG. 1 is a configuration diagram of a lunar orbiting satellite 100 according to a first embodiment. [Diagram 3] FIG. 2 is a diagram showing an observation state from a lunar orbiting satellite 100 in the first embodiment. [Figure 4] 3A and 3B are diagrams showing how the lunar orbiting satellite 100 in the first embodiment performs observations from orbits in different directions. [Diagram 5] FIG. 2 is a diagram showing a coordinate system of the lunar orbiting satellite 100 in the first embodiment. [Figure 6] FIG. 2 is a diagram showing a state in which the lunar orbiting satellite 100 in the first embodiment has changed its attitude. [Figure 7] 3A and 3B are diagrams showing how the lunar orbiting satellite 100 in the first embodiment performs observations from orbits in different directions. [Figure 8] FIG. 13 is a diagram showing a lunar orbiting satellite 101 according to the second embodiment. [Figure 9] FIG. 11 is a configuration diagram of a lunar orbiting satellite 101 according to a second embodiment. [Figure 10] FIG. 11 is a diagram showing the relationship between the moon 191, the earth 192, and the sun 193 in the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] In the embodiments and drawings, the same or corresponding elements are denoted by the same reference numerals. Descriptions of elements denoted by the same reference numerals as those of elements already described will be omitted or simplified as appropriate.

[0011] Embodiment 1 The lunar orbiting satellite 100 will be described with reference to FIGS. In the following explanation, "South Pole" and "North Pole" may be read interchangeably. The South Pole and the North Pole are two polar regions. Polar regions are also called polar regions. The South Pole and the North Pole are two poles.

[0012] ***Configuration Description*** FIG. 1 shows a lunar orbiter 100. Lunar orbiter 100 is an artificial satellite that orbits the moon 191. The dotted line indicates the orbit of the lunar orbiter 100. The dashed line indicates the axis passing through the South Pole and the North Pole (north-south axis). The dashed dotted line indicates the equator.

[0013] The configuration of the lunar orbiting satellite 100 will be described with reference to FIG. The lunar orbiting satellite 100 is equipped with a synthetic aperture radar 111, a propulsion device 112, an attitude control device 113, a communication device 114, and a satellite control device 115.

[0014] The synthetic aperture radar 111 is an observation device that uses radio waves. The observation target is the surface of the moon 191 (especially the antarctic and arctic). The synthetic aperture radar 111 is also called SAR. The synthetic aperture radar 111 observes an object by irradiating the object with radio waves and receiving the radio waves reflected from the object. The distance to the object is measured based on the time from when the radio waves are irradiated to when they are received. By processing the data (observation data) obtained by the synthetic aperture radar 111, an image (image data) showing the observation target is obtained. The lunar orbiting satellite 100 may include a device (data processing device) that processes the observation data to generate image data. The data processing device includes a processing circuit.

[0015] The propulsion device 112 is a device that provides propulsive force to the lunar orbiting satellite 100, and changes the speed of the lunar orbiting satellite 100. A specific example of the propulsion device 112 is an electric propulsion device. For example, the propulsion device 112 is an ion engine or a Hall thruster. By applying a thrust force to the lunar orbiting satellite 100 in a particular direction, the orientation of the orbital plane of the lunar orbiting satellite 100 can be adjusted.

[0016] The attitude control device 113 is a device for controlling the attitude elements of the lunar orbiting satellite 100. Specific attitude elements are the attitude and the angular velocity. The attitude control device 113 changes each attitude element to a desired direction. Alternatively, the attitude control device 113 maintains each attitude element in a desired direction. The attitude control device 113 includes an attitude sensor, an actuator, and a controller. The attitude sensor is a gyroscope, an earth sensor, a sun sensor, a star tracker, a thruster, a magnetic sensor, etc. The actuator is an attitude control thruster, a momentum wheel, a reaction wheel, a control moment gyro, etc. The controller controls the actuator according to the measurement data of the attitude sensor or various commands from the ground system. The attitude control device 113 can be used as a device for changing the viewing direction of the synthetic aperture radar 111.

[0017] The communication device 114 is a device for communicating with a ground system or other satellites. For example, the communication device 114 receives various commands from a ground system, and also transmits observation data (or image data) to the ground system.

[0018] The satellite control device 115 is a computer that controls each device of the lunar orbiting satellite 100 and includes a processing circuit. For example, the satellite control device 115 controls each device according to various commands from the ground system.

[0019] The processing circuit will now be described. The processing circuitry may be dedicated hardware or may be a processor that executes a program stored in a memory. In the processing circuit, some functions may be realized by dedicated hardware and the remaining functions may be realized by software or firmware, i.e. the processing circuit may be realized by hardware, software, firmware or a combination thereof. The dedicated hardware may be, for example, a single circuit, multiple circuits, a programmed processor, parallel programmed processors, an ASIC, an FPGA, or a combination of these. ASIC is an abbreviation for Application Specific Integrated Circuit. FPGA is an abbreviation for Field Programmable Gate Array.

[0020] ***Explanation of Operation*** The operation of the lunar orbiting satellite 100 will be described with reference to FIGS. FIG. 3 shows a lunar orbiter 100 observing the Antarctic. "H" indicates the altitude (orbital altitude) of the orbit at which the lunar orbiting satellite 100 flies. "Φi" indicates the inclination angle (orbital inclination angle) of the orbit in which the lunar orbiting satellite 100 flies. "θ" indicates the off-nadir angle of the radio wave emitted from the synthetic aperture radar 111. "θmin" is the minimum off-nadir angle. "θmax" is the maximum off-nadir angle. The shaded area indicates the area irradiated with radio waves when the synthetic aperture radar 111 irradiates radio waves.

[0021] The lunar orbiter 100 flies in an inclined elliptical orbit around the moon 191. When the lunar orbiting satellite 100 passes near the Antarctic, the synthetic aperture radar 111 irradiates radio waves toward the ground surface of the Antarctic and receives the reflected waves. In other words, when the lunar orbiting satellite 100 passes the point closest to the Antarctic among the points on the inclined elliptical orbit, the synthetic aperture radar 111 observes a part of the Antarctic from the orbital altitude H. The range of the radio waves is from just before the South Pole to beyond the South Pole. That is, at an off-nadir angle θmin, the radio waves are emitted in front of the South Pole. At an off-nadir angle θmax, the radio waves are emitted farther than the South Pole.

[0022] When the lunar orbiting satellite 100 passes near the right ascension on the inclined elliptical orbit, the propulsion device 112 operates. In other words, when the lunar orbiting satellite 100 passes above the equator, the propulsion device 112 operates. This causes the normal vector of the orbital plane to rotate in the direction of the longitude of the moon 191. In other words, the orbital plane rotates around the north-south axis. By repeating this process, the orbital plane rotates 360 degrees around the north-south axis.

[0023] Figure 4 shows the observations from two inclined elliptical orbits with 180 degree differences around the north-south axis. Every time the lunar orbiting satellite 100 passes over the equator, the propulsion device 112 operates. And every time the lunar orbiting satellite 100 passes near the South Pole, the synthetic aperture radar 111 emits radio waves to perform observations. In this way, 360 degrees around the South Pole are observed.

[0024] FIG. 5 shows the coordinate system of the lunar orbiter 100. The Z-axis indicates the direction from the lunar orbiter 100 to the center of the moon 191 (center direction). The X-axis indicates the flight direction of the lunar orbiter 100. The Y-axis indicates a direction perpendicular to the Z-axis and the X-axis.

[0025] The attitude control device 113 rotates the attitude of the lunar orbiting satellite 100 by 180 degrees around the Z axis. In other words, the attitude control device 113 rotates the attitude of the lunar orbiting satellite 100 by 180 degrees with the central direction of the moon 191 as the axis. As a result, the direction of the radio waves emitted from the synthetic aperture radar 111 rotates 180 degrees around the Z axis. In other words, the observation direction rotates 180 degrees around the center direction of the moon 191 as an axis.

[0026] FIG. 6 shows a lunar orbiter 100 observing the North Pole. When the lunar orbiting satellite 100 passes near the North Pole, the synthetic aperture radar 111 irradiates radio waves toward the ground surface of the North Pole and receives the reflected waves. In other words, when the lunar orbiting satellite 100 passes the point closest to the North Pole among the points of the inclined elliptical orbit, the synthetic aperture radar 111 observes a part of the North Pole from the orbit altitude H. The range of radio waves is from just before the North Pole to beyond the North Pole. In other words, at an off-nadir angle θmin, the radio waves are emitted in front of the North Pole. At an off-nadir angle θmax, the radio waves are emitted farther than the North Pole.

[0027] When the lunar orbiting satellite 100 passes near the right ascension on the inclined elliptical orbit, the propulsion device 112 operates. In other words, when the lunar orbiting satellite 100 passes above the equator, the propulsion device 112 operates. This causes the normal vector of the orbital plane to rotate in the direction of the longitude of the moon 191. In other words, the orbital plane rotates around the north-south axis. By repeating this process, the orbital plane rotates 360 degrees around the north-south axis.

[0028] Figure 7 shows observations from two inclined elliptical orbits with 180 degree differences around the north-south axis. Every time the lunar orbiting satellite 100 passes over the equator, the propulsion device 112 operates. And every time the lunar orbiting satellite 100 passes near the North Pole, the synthetic aperture radar 111 emits radio waves to perform observations. In this way, 360 degrees around the North Pole are observed.

[0029] ***Advantages of the First Embodiment*** The first embodiment employs an inclined orbit. This provides the flight position with an offset angle equal to the orbital inclination angle when passing over the Antarctic. Then, on the condition that the Antarctic is located within the range of off-nadir angles θ (between the minimum and maximum angles) starting from the southernmost point of the orbital plane, it is possible to obtain observation information in a strip extending in the flight direction and including the Antarctic. Furthermore, when passing the equatorial plane of the Moon 191, the propulsion device 112 operates and the orbital plane rotates around the north-south axis of the Moon 191 in the longitude direction of the Moon 191. This makes it possible to obtain topographical map information from multiple directions by shifting the position in the longitude direction. In other words, it is possible to obtain observation information in a belt-like shape including the Antarctic. When observing a high crater using the synthetic aperture radar 111, it is not possible to obtain information on the far side in the direction of the radio wave irradiation. However, as the orbital plane rotates around the moon's north-south axis, it becomes possible to obtain information on the far side.

[0030] ***Explanation of the first embodiment*** If the satellite is equipped with an optical observation device, stereo pair images are acquired by the optical observation device, and the stereo pair images are viewed stereoscopically to create a lunar topographical map. However, in the polar regions near the North or South Pole, the angle of incidence of sunlight is shallow, making the polar regions dark and casting long shadows. This makes it easy to misidentify corresponding points between stereo pair images, making it difficult to create topographical maps of the polar regions. When a satellite is equipped with a synthetic aperture radar, it is possible to generate a two-dimensional image without relying on sunlight. In this case, the two-dimensional image is generated by applying distance measurement technology. In distance measurement technology, radio waves are emitted, the time it takes for the emitted radio waves to be reflected by an object and returned is measured, and the distance to the object is calculated based on the measured time. When a synthetic aperture radar receives a reflected radio wave directly, the strength of the received radio wave becomes excessive, so it is common to irradiate the radio wave with an offset angle called an off-nadir angle. If a polar orbit is adopted to produce topographical maps of the polar regions, the polar regions become blind spots due to off-nadir angles, and images of the polar regions cannot be produced. The first embodiment employs an inclined orbit. When passing near the Antarctic, radio waves are emitted toward the Earth's surface from an orbital altitude H. At this time, at an off-nadir angle θmin, the radio waves are emitted in front of the Antarctic. At an off-nadir angle θmax, the radio waves are emitted farther away than the Antarctic. Furthermore, when the lunar orbiting satellite 100 passes over the equator, the propulsion device 112 operates and the normal vector of the orbital plane rotates in the longitude direction of the moon 191. As a result, every time the lunar orbiting satellite 100 flies over the Antarctic, the location where the radio waves are irradiated in high latitude regions including the Antarctic changes. Then, by operating the synthetic aperture radar over multiple orbits, a topographical map of the Antarctic can be created. The attitude control device 113 rotates the attitude of the lunar orbiting satellite 100 by 180 degrees around an axis in the central direction of the moon 191. This makes it possible to create a topographical map of the area near the North Pole in the same way as a topographical map of the South Pole.

[0031] Embodiment 2 Regarding the lunar orbiting satellite 101, the points different from the first embodiment will be described with reference to FIG. 8 to FIG.

[0032] ***Configuration Description*** FIG. 8 shows a lunar orbiter 101. Lunar orbiter 101 is an artificial satellite that orbits the Moon 191.

[0033] The configuration of the lunar orbiting satellite 101 will be described with reference to FIG. The lunar orbiter 101 includes an optical observation device 116 , a propulsion device 112 , an attitude control device 113 , a communication device 114 , and a satellite control device 115 . In other words, the lunar orbiter 101 is equipped with an optical observation device 116 instead of the synthetic aperture radar 111 .

[0034] The optical observation device 116 is an observation device that uses light. The observation target is the surface of the Moon 191 (especially the south pole and the north pole). An image (image data) showing the observation target is obtained by processing data (observation data) obtained by the optical observation device 116. Alternatively, the optical observation device 116 outputs image data.

[0035] ***Explanation of Operation*** The operation of the lunar orbiting satellite 100 will be described with reference to FIG. FIG. 10 shows the relationship between the Moon 191, the Earth 192 and the Sun 193. The Moon 191 rotates on its axis and revolves around the Earth 192. The Earth 192 rotates on its axis and revolves around the Sun 193.

[0036] The lunar orbiter 101 flies in a polar orbit and orbits the Moon 191. The polar orbit is an orbit that passes over the South Pole of the Moon 191 and the North Pole of the Moon 191. The propulsion system 112 operates to provide thrust to the lunar orbiter 101 each time the lunar orbiter 101 passes over the equator of the moon 191 . As a result, the orbital plane of the polar orbit rotates around the north-south axis. Specifically, the normal vector of the orbital plane of the lunar orbiter 101 rotates around the north-south axis of the moon 191 in synchronization with the orbital period of the earth 192. In other words, the propulsion device 112 is controlled and operated so that the polar orbit of the lunar orbiting satellite 101 becomes a sun-synchronous orbit.

[0037] *** Description of the Example *** Based on the normal vector of the orbital plane where the solar altitude is highest on the surface of the moon, the normal vector of the orbital plane of the lunar orbiting satellite 101 maintains a rotated state within a range of 22.5±7.5 degrees counterclockwise in the longitude direction as viewed from the north pole of the moon 191 (see Figure 10). In other words, the normal vector of the orbital plane of the lunar orbiter 101 maintains a state of rotation within a range of 15 degrees to 30 degrees counterclockwise from the reference vector as viewed from the north pole of the Moon 191. The reference vector is the normal vector of the orbital plane with the highest solar altitude. The orbital plane of the lunar orbiter 101 corresponds to the orbital plane of LST10:30±30 minutes for an Earth orbiter.

[0038] However, the normal vector of the orbital plane of the lunar orbiting satellite 101 may be maintained rotated within a range of 15 degrees to 30 degrees clockwise from a reference vector as viewed from the north pole of the Moon 191.

[0039] ***Explanation of the second embodiment*** In a lunar orbiting satellite equipped with an optical observation device, the optical observation device obtains information on the observation target by utilizing the reflection of sunlight. Therefore, when observing the topography of the lunar surface with the optical observation device in the lunar orbiting satellite, it is expected that sunlight will be irradiated at an appropriate incidence angle. However, there is a concern that the optical observation device may be damaged if sunlight is directly irradiated on the optical observation device in the lunar orbiting satellite that flies in a polar orbit of the moon while pointing toward the center of the moon. A sun-synchronous orbit is an orbit in which the orbital plane rotates in sync with the Earth's revolution. In one year, while the Earth revolves around the Sun, the orbital plane of a sun-synchronous orbit rotates around the Earth once. Therefore, the angle of incidence of sunlight on the orbital plane of a sun-synchronous orbit is always constant. For example, the orbital plane of a satellite that flies at 12:00 LST will maintain 12:00 LST throughout the year. In other words, a satellite that crosses the equator at noon will cross the equator at noon every day throughout the four seasons. LST is an abbreviation for Local Sun Time. When an Earth-orbiting satellite equipped with an optical observation instrument flies at 12:00 LST, sunlight enters the aperture of the optical observation instrument just before and after the sun's shadow. Also, when an Earth-orbiting satellite crosses the equator at noon while pointing at a highly reflective object such as the ocean, there is a risk that the optical observation instrument will be damaged by the directly reflected sunlight. Therefore, a sun-synchronous orbit of LST10:30 or LST13:30 is often applied to Earth-orbiting satellites equipped with optical observation instruments. Because the solar altitude is high, the Earth's surface is bright, direct reflection by the ocean can be avoided, and direct viewing of the sun before and after the sun's shadow can be avoided. If we take the normal vector of the orbital plane of LST12:00 as the reference, the normal vector of LST10:30 is rotated 22.5 degrees clockwise in the longitude direction as seen from the North Pole. Also, the normal vector of LST10:30 is rotated 22.5 degrees counterclockwise in the longitude direction as seen from the North Pole. Therefore, it is reasonable to adopt a sun-synchronous orbit for the lunar orbiting satellite 101, just like the earth-orbiting satellite. In a sun-synchronous orbit, the angle of incidence of sunlight with respect to the orbital plane is always constant. The condition for realizing a sun-synchronous orbit for the lunar orbiting satellite 101 is that the orbital plane of the lunar orbiting satellite 101 revolves around the moon 191 in synchronization with the revolution period of the earth 192. As the moon 191 revolves around the earth 192, its orbital plane rotates in sync with the revolution of the earth 192. This makes it possible to realize a sun-synchronous orbit for the lunar orbital satellite 101.

[0040] By selecting the orbital altitude and orbital inclination based on the effect of the gravity bias caused by the pear-shaped configuration, the orbit of an Earth-orbiting satellite can be synchronized with the sun relying solely on natural phenomena. However, when the shape of the moon is close to a perfect sphere, it is difficult to synchronize the orbit of a lunar orbiting satellite with the sun by relying solely on natural phenomena. Therefore, the propulsion device 112 applies a propulsive force to the lunar orbiting satellite 101 every time the lunar orbiting satellite 101 passes over the equator of the Moon 191. As a result, the orbital plane of the polar orbit rotates and moves around the north-south axis of the Moon 191. Specifically, the normal vector of the orbital plane of the lunar orbiting satellite 101 rotates around the north-south axis of the Moon 191 in synchronization with the revolution period of the Earth 192. In other words, the propulsion device 112 operates under control so that the polar orbit of the lunar orbiting satellite 101 becomes a sun-synchronous orbit.

[0041] ***Supplementary explanation of implementation form*** Each embodiment is an example of a preferred embodiment, and is not intended to limit the technical scope of the present disclosure. Each embodiment may be implemented in part or in combination with other embodiments. [Explanation of symbols]

[0042] 100 Lunar orbiter, 101 Lunar orbiter, 111 Synthetic aperture radar, 112 Propulsion system, 113 Attitude control system, 114 Communications system, 115 Satellite control system, 116 Optical observation system, 191 Moon, 192 Earth, 193 Sun.

Claims

1. It is a lunar orbiting satellite that orbits the moon in an inclined elliptical orbit. A synthetic aperture radar; A propulsion device; a satellite control device that, each time the lunar orbiting satellite passes near one of the lunar south pole and the lunar north pole, causes the synthetic aperture radar to irradiate radio waves to a range from just before the pole of the one polar region to beyond the pole of the one polar region, thereby observing a part of the one polar region, and that, each time the lunar orbiting satellite passes above the lunar equator, causes the propulsion device to apply a propulsive force to the lunar orbiting satellite, thereby rotating and moving the orbital plane of the inclined elliptical orbit around the north-south axis of the moon; A lunar orbiting satellite equipped with

2. The lunar orbiting satellite is equipped with an attitude control device, The satellite control device rotates the attitude of the lunar orbiting satellite by 180 degrees around an axis in the direction of the center of the moon, and causes the attitude control device to rotate the attitude of the lunar orbiting satellite by 180 degrees around an axis in the direction of the center of the moon, and causes the synthetic aperture radar to irradiate radio waves to a range from just before the pole of the other polar region to beyond the pole of the other polar region every time the lunar orbiting satellite passes near the other polar region of the South Pole and the North Pole, thereby observing a part of the other polar region.

2. The lunar orbiting satellite of claim 1.

Images