JP2006168461A - Orbit control device for formation flight satellite - Google Patents

Abstract

PROBLEM TO BE SOLVED: In the orbit control of a plurality of satellites, a method for realizing a relative orbit suitable for a cross track interferometry SAR and an along track interferometry SAR has not been studied so far. Cross track interferometry SAR could not be realized.
SOLUTION: The orbital coordinate system is defined with the point that moves in the orbital period on the reference orbit determined from the observation mission as the origin, and multiple satellites flying in the range of several tens of kilometers from the origin fly on the origin. The satellite to be operated is a reference satellite, and satellites other than the reference satellite are moved relative to the orbital plane of the reference orbit using the relative orbital motion with respect to the orbital coordinate system.
[Selection] Figure 1

Description

  The present invention relates to a remote sensing system using an artificial satellite.

  Conventionally, in remote sensing systems that remotely measure the earth, planets, etc. from artificial satellites, multiple satellites that are separated from each other by means such as being mounted on multiple satellites or mounted on a single giant satellite. A system for acquiring multidimensional observation information that cannot be obtained by observing the surface of the earth and the planet from a single observation device is being studied. There are various observation devices such as an optical sensor, a passive radio wave sensor, and an active radio wave sensor, and these sensors have directivity in order to observe the ground surface from the orbit.

In general, when observing a combination of a plurality of observation sensors having directivity, the observation resolution improves as the distance in the direction orthogonal to the observation direction, that is, the base line is increased. Therefore, it is expected to realize a high resolution that cannot be realized by one huge satellite by mounting the observation apparatus on a plurality of satellites and appropriately controlling the distance between the satellites. When defining a trajectory coordinate system consisting of the direction of travel, geocentric direction, and out-of-plane direction with a point that moves in the trajectory cycle on the reference trajectory as a starting point, especially within a range of about several tens of kilometers from the origin of the trajectory coordinate system In a satellite that flies in accordance with orbital motion, the orbit when viewed in the orbital coordinate system (hereinafter referred to as a relative orbit) is elliptical on a plane that is fixed in the orbital coordinate system (hereinafter referred to as a relative orbital plane). It is known to draw. A system that uses this property to fly while appropriately controlling the distance between a plurality of satellites is called a formation flying satellite.

Here, as the direction in which the distance between the satellites is three-dimensionally separated, with respect to the orbital movement direction of the satellite, the traveling direction (also called the along track direction), the geocentric direction (or the vector direction is opposite, but the orbit radius In three directions: a direction (also referred to as a direction), and a direction perpendicular thereto (also referred to as an out-of-plane direction or a cross-track direction). In particular, a plane stretched in the traveling direction and the out-of-plane direction is called a local horizontal plane.

  When the distance between satellites is taken in the along-track direction, the next satellite passes on the same ground surface as the first satellite passes, according to the orbital motion. If this property is used, the same target on the ground surface can be observed again after a short time, so that it can be used to observe the moving speed of a moving object.

When observing the same target on the ground surface that is located between the satellites in the cross-track direction, if the terrain near the target is inclined, it can be observed from a different direction with respect to the inclination. Can be used for observation of slope or altitude.
On the other hand, if the observation direction of the observation apparatus is the geocentric direction, the base line cannot be extended even if the distance between the satellites is taken in the altitude direction, so that it does not contribute to improvement in observation accuracy. Therefore, there is no special requirement for the distance between observation devices in the altitude direction.

  For example, the measurement of signals from the same target by a plurality of observation devices composed of optical sensors or passive radio wave sensors and obtaining the target point by the principle of triangulation is called stereoscopic vision. In addition, measuring a single target and extracting difference information from the interference signal with a plurality of observation devices configured by one active radio wave sensor, SAR (Synthetic Aperture Radar), is called an interferometry SAR. .

By arranging multiple satellites with SAR in close parallel orbits and observing the same target at the same time, cross-track interferometry SAR is realized, or multiple satellites with SAR are installed A system that realizes an along track interferometry SAR that is arranged close to the orbit and observes the moving speed of a moving object by observing the same target again after a short time is disclosed (for example, Patent Document 1). ).

  Further, among the relative orbits of the satellites, a relational expression between the orbital elements is calculated such that the shape projected on the local horizontal plane or the relative orbital plane is a circle (for example, Non-Patent Document 1).

JP 09-116151 (page 2, right column 9 to page 3, left column 33 lines, FIGS. 9 and 10) S. Nakasuka, et al, "Study on the Relative Motion and Orbit Design for Clustered Satellites", 1994 ISAS 4th Workshop on Astrodynamics and Flight Mechanics, pp. 88-92 (1994)

  In Patent Document 1, the cross track interferometry SAR is described that a plurality of satellites are arranged in close parallel orbits. However, the cross track direction is an out-of-plane direction, and the center of the orbit surface is set. It is possible to locally arrange a trajectory parallel to the cross-track direction due to the two constraints of the earth coming, but it is not possible to arrange a trajectory parallel to the cross-track direction over the entire trajectory .

On the other hand, it is possible to arrange concentric circular orbits around the earth and to arrange orbits parallel to the radial direction of the orbit. However, since the orbit radius is different, the orbit period uniquely determined from the orbit radius is also different.If orbit control is not performed, the relative distance between satellites increases with time, and the orbital coordinate system It cannot stay within the range of several tens of kilometers from the origin.

As described above, a method for realizing a relative trajectory suitable for the cross track interferometry SAR and the along track interferometry SAR as described in Patent Document 1 has not been studied so far.

  Non-Patent Document 1 shows a relational expression between trajectory elements such that the shape projected onto the local horizontal plane or relative trajectory plane is a circle. In conventional satellites, it was customary to observe the geocentric direction, that is, near the point just below the satellite, like an optical sensor, so if the shape projected on the local horizontal plane is a circle, place multiple satellites on that circle. By doing so, there is an advantage that the base line length becomes almost constant during one orbit of the orbit.

However, in an active radio wave sensor such as SAR, the observation direction of the sensor may have a large offset with respect to the point directly below the satellite, that is, the geocentric direction. Also, when the target is far from the point directly below the satellite, the acquisition direction is increased by directing the observation direction of the optical sensor to the target, that is, imaging with a large offset with respect to the geocentric direction. there is a possibility.

In such a case, because the observation direction and the geocentric direction are misaligned, the base line length greatly changes between orbits in a satellite arrangement where the shape projected onto the local horizontal plane is a circle as in the past. Cause problems. There have been no studies on how to realize such a relative orbit according to the observation direction of the sensor.

  The present invention has been made to solve the above-described problems. The observation direction of an observation sensor such as an optical sensor or a radio wave sensor including a SAR, a cross track interferometry SAR, an along track interferometer, and the like. It is an object of the present invention to obtain a trajectory control device for a formation flying satellite that provides a relative orbit in accordance with a metric SAR or a stereoscopic view.

  A trajectory control device for a formation flying satellite according to the present invention defines an orbital coordinate system with a point moving on a reference orbit determined by an observation mission in an orbital period as an origin, and the formation flight within a range of about several tens of kilometers from the origin. A plurality of satellites that fly on the origin as reference satellites, and use satellites other than the reference satellites in an out-of-plane direction of the reference orbital plane by utilizing relative orbital motion with respect to the orbital coordinate system. Make relative movement.

  The orbit control device for a formation flying satellite according to the present invention selects a plurality of satellites whose relative orbits, which are orbits determined with respect to the orbital coordinate system, are on the same plane in the orbital coordinate system. When configuring a set, a plurality of sets are provided.

  Further, the orbit control device for a formation flying satellite according to the present invention sets a projection plane determined from an observation mission, and sets an inclination of the relative orbital plane with respect to a local horizontal plane determined by an advancing direction and an out-of-plane direction of the orbital coordinate system, It is set from the inclination of the projection plane with respect to the local horizontal plane and the axial length ratio of the ellipse drawn by projecting the relative orbit of each satellite onto the projection plane.

  According to the present invention, by performing relative movement of the remaining satellites in the out-of-plane direction, it is possible to perform cross-track interferometry SAR with a large inter-satellite distance in the cross-track direction with respect to the reference satellite. This is a remarkable effect that has never been seen before.

  Further, according to the present invention, when a pair is formed by satellites having relative orbits on the same plane, by providing a plurality of sets, the observation direction of the observation sensor can be changed, for example, around the traveling direction. The ratio of the major axis length to the minor axis length of the ellipse drawn by the relative trajectory on the projection plane obtained by rotating the local horizontal plane within the range of −45 degrees to +45 degrees is a range required from the observation mission, for example, 1 or more 2 or less, it is possible to squeeze out, and there is an unprecedented remarkable effect.

  According to the present invention, the projection plane is defined according to the observation direction of the sensor, and the relative trajectory plane is determined according to the axial length ratio of the ellipse drawn by the relative trajectory on the projection plane. There is an unprecedented remarkable effect that the orbit can be realized.

Embodiments of the present invention will be described below with reference to the drawings.
Embodiment 1 FIG.
FIG. 1 is a schematic diagram showing a relative trajectory according to Embodiment 1 of the present invention. The orbit of satellite # 1 orbiting the earth 1 is defined as the reference orbit 2, the point moving on the reference orbit in the orbit period is defined as the origin 3, and the traveling direction, the out-of-plane direction, and the geocentric direction have X, Y, and Z axes. An orbital coordinate system 4 is defined. Assume that the center of mass of satellite # 1 (symbol 5) coincides with the origin 3. Satellite # 2 (symbol 6) has an offset corresponding to the length of the base line in order to establish a base line in the X direction in this orbital coordinate system, and in the Y direction in one orbit, as in 6b-6a-6b-6c-6b. Performs reciprocating relative motion. Since the relative motion of the satellite # 2 in the Y direction uses the relative orbital motion with respect to the satellite # 1, as long as each satellite maintains the Kepler motion, control for maintaining the orbit is unnecessary. The relative motion of the satellite # 2 in the Y direction is a simple vibration symmetric about the X axis on the XY plane.

  According to such a configuration, the component in the along track direction of the base line is not changed when satellite # 2 returns to the vicinity of the X axis according to the relative movement in the Y direction without requiring control for maintaining the orbit. Since it is larger than the component in the cross track direction, the along track interferometry SAR is performed, and when the satellite # 2 moves greatly in the out-of-plane direction, the component in the cross track direction of the base line is larger than the component in the along track direction. Therefore, cross-track interferometry SAR can be performed.

  FIG. 2 shows an implementation example of the along track interferometry SAR and the cross track interferometry SAR according to the present embodiment. The horizontal axis in FIG. 2 represents the position of satellite # 1 in the orbit and returns to the initial position at 360 degrees. The vertical axis represents the angle between the direction of the satellite # 2 viewed from the satellite # 1 and the X axis in the local horizontal plane, that is, the azimuth angle of the satellite # 2 viewed from the satellite # 1. Along track interferometry SAR is performed when the azimuth angle is 45 degrees or less, that is, when the main baseline can be secured in the along track direction, and cross track interferometry SAR is performed within an azimuth angle of 45 degrees or more. That is, the case where the main base line can be secured in the cross track direction will be described as an example. In this case, in the configuration of two satellites, when the amplitude B of the relative motion in the Y direction with respect to the offset A in the X direction is selected as B = sqrt (2) · A for the satellite # 2, as shown in FIG. Along track interferometry SAR can be performed in half a circle, and cross track interferometry SAR can be performed in the remaining half circle. Here, sqrt (·) is a function that takes a square root.

  FIG. 3 shows another implementation example of the along track interferometry SAR and the cross track interferometry SAR according to the present embodiment. In the case of the configuration of three satellites, for satellite # 3, an offset A is taken in the -X direction, the amplitude of the relative motion in the Y direction is selected as B = sqrt (2) · A, and the phase of the relative motion in the Y direction is If the relative motion of # 2 is shifted by 90 degrees, as shown in FIG. 3, the orbital half-round is performed along track interferometry SAR with satellite # 2 and simultaneously with cross track interferometry SAR with satellite # 3. The remaining half-round can perform cross-track interferometry SAR with satellite # 2 and simultaneously perform along-track interferometry SAR with satellite # 3.

  In FIG. 1, although there is no restriction on the phase of the relative motion in the Y direction, for the reasons described below, at the ascending and descending points where the orbital plane of satellite # 1 intersects the equatorial plane of the earth, satellite # 2 It is desirable to select the phase so that the relative motion in the out-of-plane direction is maximized. The reason for this will be described below.

A remarkable natural disturbance in the Earth's orbit is the flatness of the Earth. It is known that due to this disturbance, the raceway surface rotates by an angle ΔΩ per orbit (Equation (1)).
ΔΩ = 3πJ2 R ^ 2 cosi / {a ^ 2 * (1−e ^ 2) ^ 2} (1)
Here, J2 is the flatness of the earth (about 0.001), R is the equator radius of the earth, i is the orbit inclination angle, a is the orbital length radius, e is the orbit eccentricity, and ^ is a power.

When the satellite # 2 performs relative motion in the out-of-plane direction, generally, both the orbit inclination angle i and the rising intersection red longitude Ω have different values from the satellite # 1. If the orbit inclination angle i is different from the equation (1), the amount of rotation of the orbital surface is different and the difference increases with time, so control for maintaining the orbit is necessary.

Therefore, it is desirable to select the phase so that the relative motion in the out-of-plane direction of the satellite # 2 is maximized at the ascending and descending points of the satellite # 1. At this time, the orbital inclination angle i of the satellite # 2 is the same as that of the satellite # 1, and ΔΩ in the equation (1) is the same between the satellite # 1 and the satellite # 2, and the orbit for compensating for the difference between the two. The control for holding is not required.

With the configuration of the first embodiment described above, along with the relative movement in the out-of-plane direction without requiring control for maintaining the orbit, along with the along track interferometry when the remaining satellite returns to the traveling direction axis. SAR can be performed, and cross-track interferometry SAR can be performed when the satellite moves greatly in the out-of-plane direction.

Embodiment 2. FIG.
FIG. 4 is a schematic diagram showing a relative trajectory according to the second embodiment of the present invention. For convenience of explanation, the relative orbits of satellites # 1 (symbol 7a), # 2 (symbol 7b), and # 3 (symbol 7c) are on the same plane to form a set # 1 (symbol 7), and satellite # Assume that the relative trajectories of 4 (symbol 8a), # 5 (symbol 8b), and # 6 (symbol 8c) are on a plane symmetric with respect to the set # 1 and constitute the set # 2 (symbol 8 ). .

FIG. 5 is a schematic diagram showing the trajectory drawn on the projection plane by the relative trajectory of the formation flight consisting of these six satellites. (A), (b), and (c) show the projection plane according to the change of the observation direction. Corresponding to XY plane (local horizontal plane), XY plane rotated by -40 degrees around X axis as projection plane, XY plane rotated by +40 degrees around X axis as projection plane To do. For each case, the trajectory plane and the trajectory on the projection plane in the trajectory coordinate system are shown on the right side of each figure. Each satellite goes around the relative orbit in one cycle of the reference orbit. When the latitude argument is used as the phase of the reference orbit, the positions on the relative orbits of the satellites # 1, # 2, and # 3 when the latitude argument is 0 degrees are indicated by o and a satellite number is attached. The positions of the satellites # 4, # 5, and # 6 at the same time on the relative orbit are indicated by * and a satellite number is attached.

For explanation, when the phase of the reference trajectory (also referred to as a latitude argument) is 0 degree, 45 degrees,..., 360 degrees, the point on the relative orbit drawn on the projection plane corresponding to each time is a satellite. Tie between. For example, the line segment connecting the satellite # 1 and the satellite # 2 represents the baseline of the observation sensor mounted on the satellite # 1 and the satellite # 2. Since the projection plane is obtained by rotating the XY plane around the X axis, the intersection line between the projection plane and the XY plane coincides with the X axis, that is, the along track direction. On the other hand, the direction perpendicular to the X axis on the projection plane is a direction obtained by combining the cross track direction and the altitude direction, and is referred to as a pseudo cross track direction here. Of the plurality of base lines, the base line that extends most in the along track direction is called the along track base line, and the length of the along track direction component is called the along track base line length. Similarly, among the plurality of base lines, the base line that extends most in the pseudo cross track direction is called a pseudo cross track base line, and the length of the pseudo cross track direction component is called a pseudo cross track base line length.

In FIG. 5, the horizontal axis direction represents the along track direction, and the vertical axis direction represents the pseudo cross track direction. For example, in (a), the along track base length when the latitude argument is 0 degrees is given by a line segment connecting satellite # 6 and satellite # 5. The pseudo cross track base line length is the length of the pseudo cross track direction component of the line segment connecting satellite # 3 and satellite # 1, or the pseudo cross track direction component of the line segment connecting satellite # 4 and satellite # 2. Given in length. If there is a long line segment in the horizontal axis direction in the figure, the main component of the base line is in the along track direction, which is suitable for missions such as along track interferometry SAR. If there is a long line segment in the vertical axis direction of the figure, the main component of the base line is in the pseudo cross track direction, which is suitable for missions such as cross track interferometry SAR.

Since the base line length and the resolution are inversely proportional as represented by stereoscopic vision, if the along track base line length and the pseudo cross track base line length are equal, the resolution obtained in the along track direction and the pseudo cross track direction The obtained resolution becomes equal and isotropic observation data is obtained. Specific numbers depend on the observation mission, but generally speaking, anisotropy of about twice is acceptable. Therefore, in order to perform a plurality of missions with any combination of satellites, the ratio of the pseudo cross track baseline length to the along track baseline length is within the range required for the observation mission, for example, at least 0.5 times. It is desirable to be within the range of 2 times.

In the example of FIG. 5, the ratios of the pseudo cross track base line length to the along track base line length are 0.86 to 0.88 and 0.93 respectively during one orbit of the track. It changes in the range of 0.95, 0.93-0.95. That is, when the inclination of the projection plane is changed to three types of −40 degrees, 0 degrees, and +40 degrees with respect to the local horizontal plane, the ratio of the base line lengths to 1 of 0.86 to 0.95 in any case. It is within the close range.

FIG. 6 is a schematic diagram showing a relative orbit when the relative orbits of all six satellites are on the same plane for comparison, and is for explaining the effect of the second embodiment of the present invention. . Specifically, satellites # 1 (symbol 9a), # 2 (symbol 9b), and # 3 (symbol 9c) draw the same relative orbits as in FIG. 4, and satellite # 4 (symbol 9d) and satellite # 5 (symbol 9e). ), Satellite # 6 (symbol 9f) is located at the midpoint of satellite # 2 and satellite # 3, the midpoint of satellite # 3 and satellite # 1, and the midpoint of satellite # 1 and satellite # 2 on the same orbital plane. To do.

FIG. 7 is a schematic diagram showing the trajectory drawn on the projection plane by the relative trajectory of the formation flight by the set # 3 (symbol 9 ) consisting of the six satellites, and (a), (b), and (c) are the projection planes. It is a locus on the projection surface when the direction is changed. In FIG. 7, (a), (b), and (c) respectively, the ratios of the pseudo cross track base line length to the along track base line length during one orbit of the track are 0.86 to 1.16, 0.38 to 0. 51, and changes in the range of 0.94 to 1.26. Therefore, in (b), a sufficient base line cannot be secured in the pseudo cross track direction. Thus, unlike the second embodiment, when only one set is configured with satellites having relative orbits on the same plane, there may be cases where a sufficient baseline cannot be secured in the pseudo cross track direction depending on the observation direction.

With the configuration of the second embodiment described above, it is possible to draw a relative trajectory that does not greatly change the ratio of the pseudo cross track base length to the along track base length even when the observation direction is changed.

Embodiment 3 FIG.
FIG. 8 is a schematic diagram showing the relationship between the relative trajectory plane and the projection plane according to the third embodiment of the present invention. Here, the case where the relative orbits with respect to the reference orbit of a plurality of satellites are on the same plane is considered. This plane is defined as a relative track surface 14, and the relative track surface 14 is inclined by an angle θ with respect to the Y axis, including the X axis.

Reference numeral 15 denotes a projection plane that is set so as to be convenient for observation by a plurality of satellites. For example, the projection plane is orthogonal to the observation direction of each satellite as shown in the figure. This projection plane is inclined by an angle φ with respect to the Y axis, including the X axis. The movement of the satellite on the projection plane draws an ellipse determined by orbital mechanics when natural disturbance and control force do not act on the satellite. This ellipse makes a round in one cycle of the satellite orbit, and has a main axis in the X-axis direction, which is the traveling direction of the satellite, and in a direction perpendicular thereto. The axial length ratio is obtained by the following equation (2).

Therefore, from the satellite observation mission, for example, take the projection plane perpendicular to the observation direction of each satellite, set the motion trajectory on the projection plane, and determine the inclination angle θ of the relative orbital plane to satisfy the above formula Then, a desirable motion can be realized on the projection plane. For example, in order to realize along-track interferometry SAR and cross-track interferometry SAR alternately in one orbital cycle with two satellites, the axial length ratio of the ellipse to the specified projection plane inclination φ It is only necessary to determine the inclination θ of the relative orbital plane so that becomes approximately 1. When the axial length ratio = 1, the inclination angle θ of the relative track surface is given by the following two formulas (3) and (4).

The method of determining the inclination of the relative orbital plane is not limited to the above, but it is convenient for the observation mission by setting the projection plane and setting the elliptical motion on the projection plane, for example, the axis of the ellipse on the projection plane The relative raceway surface can be determined so that the length ratio is 0.5 or more and 2 or less.

According to the configuration of the third embodiment described above, an optimal relative orbit in the observation direction of the satellite can be realized.

It is a schematic diagram which shows the relative track | orbit by Embodiment 1 of this invention. It is an implementation example of the along track interferometry SAR and the cross track interferometry SAR according to the first embodiment of the present invention. It is another implementation example of the along track interferometry SAR and the cross track interferometry SAR according to the first embodiment of the present invention. It is a schematic diagram which shows the relative track | orbit by Embodiment 2 of this invention. It is a schematic diagram which shows the trajectory which the relative trajectory of the formation flight by Embodiment 2 of this invention draws on a projection surface. It is a schematic diagram which shows the relative track | orbit for the comparison of FIG. FIG. 6 is a schematic diagram illustrating a trajectory drawn on a projection plane for comparison with FIG. 5. It is a schematic diagram which shows the relationship between the relative track surface and projection surface of the formation flight by Embodiment 3 of this invention.

Explanation of symbols

1 Earth, 2 reference orbit, 3 origin, 4 reference coordinate system, 5 satellite # 1, 6 satellite # 2, 7 set # 1, 8 set # 2, 9 set # 3, 14 relative orbital plane, 15 projection plane.

Claims (5)

A satellite that defines the orbital coordinate system with the point that moves in the orbital period on the reference orbit determined from the observation mission as the origin, and that flies in the range of several tens of kilometers from the origin. Orbit of a formation flying satellite, wherein a satellite other than the reference satellite is caused to move relative to the orbital plane of the reference orbit using a relative orbital motion relative to the orbital coordinate system. Control device. The phase of the relative motion of both satellites is set so that the relative distance of the satellite other than the reference satellite relative to the reference satellite in the out-of-plane direction becomes maximum at the ascending and descending points of the orbital plane of the reference orbit. The orbit control device for a formation flying satellite according to claim 1. An orbital coordinate system is defined with the point moving on the reference orbit determined from the observation mission in the orbital period as the origin, and multiple satellites flying in a range of about several tens of kilometers from the origin to the orbital coordinate system. When a plurality of satellites whose relative orbits, which are determined orbits, are on the same plane in the orbital coordinate system to form a set, a plurality of such sets are provided. Orbit control device. The orbital coordinate system is defined with the point that moves in the orbital period on the reference orbit determined from the observation mission as the origin, and the orbital coordinates of multiple satellites selected from multiple satellites flying in the range of several tens of kilometers from the origin When the relative trajectory that is the trajectory to the system is on the relative trajectory plane that is the same plane, a projection plane determined from the observation mission is set, and the relative to the local horizontal plane determined by the traveling direction and the out-of-plane direction of the trajectory coordinate system. The inclination of the orbital plane is set from the inclination of the projection plane with respect to the local horizontal plane and the axial length ratio of the ellipse drawn by projecting the relative orbit of each satellite onto the projection plane. Orbit control device. 5. The orbit control apparatus for a formation flying satellite according to claim 4, wherein the inclination of the relative orbital plane with respect to a local horizontal plane is determined so that the axial length ratio of the ellipse is 1 with respect to the inclination of the projection plane. .

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