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Claims  |
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What is claimed is:
1. An attitude determination system for an artificial satellite, comprising:
an image processor for processing star images which are observation values of stars sensed at predetermined time points by a star sensor mounted on said artificial satellite to arithmetically determine direction vectors of said observed stars;
a rotation estimator for estimating a rotational motion between an attitude of said artificial satellite at a predetermined time point and an attitude of said artificial satellite at another time point;
an elongation estimator, having an output of said rotation estimator and an output of said image processor as its inputs, which estimates elongations between the direction vectors of said observed stars on the basis of said estimated rotational
motion;
a star identifier for establishing correspondences between a plurality of the stars for which relation of said elongations has been determined and stars on a star catalog; and
an attitude angle estimator for estimating an attitude angle of said artificial satellite on the basis of result of the identification of the stars,
wherein said rotation estimator uses the star images on a plurality of image screens respectively photographed at different image pick-up time points by said star sensor.
2. An artificial satellite attitude determination system according to claim 1,
wherein said rotation estimator means is adapted to estimate an average motion of the star images on a plurality of image screens generated at image pick-up time points close to one another, to thereby establish correspondences among individual
star images on a plurality of image screens generated at image pick-up time points remote from one another on the basis of said estimated average motion of the stars, for thereby estimating the rotational motion of said artificial satellite during a
period intervening between the successive image pick-up time points.
3. An artificial satellite attitude determination system according to claim 1,
wherein said attitude angle estimator is adapted to arithmetically determine candidates for the attitude angle at a current time point on the basis of estimated value of the rotational motion estimated by said rotation estimator, while said star
identifier is adapted for providing a number of candidates for the identification of the stars on said star catalog for said observed star images, and
wherein on the basis of difference between the position of the star image as picked up by said star sensor and the position of the star image predicted from the star catalog, those of said candidates for the attitude angle or those of said
candidates for the identification which can not be present are sequentially eliminated as said star images are inputted from said star sensor, to thereby estimate the attitude angle of the artificial satellite on the basis of the candidate remaining to
the last.
4. An artificial satellite attitude determination system according to claim 1,
wherein said star identifier is adapted to compare the directions of the observed stars directly with those of the stars on the star catalog to thereby extract the stars for the candidates and arithmetically determine the elongations of the
observed stars and those on the star catalog relevant to said candidate stars, respectively, whereby the candidate stars for which elongations are sufficiently close to one another are outputted as the result of the identification.
5. An artificial satellite attitude determination system according to claim 1,
wherein in the artificial satellite for which attitude angle information is of relatively low accuracy, said star identifier is adapted to identify the corresponding star on the star catalog on the basis of the directions of plural stars observed
by said star sensor by determining a vector of difference between the directions of two of said observed stars and comparing said vector of difference between the directions of the stars on said star catalog.
6. An artificial satellite attitude determination system according to claim 1,
wherein in the identification of the star by comparing the elongations among a plurality of observed stars with elongations among the stars on said star catalog, when differences between the elongations of plural stars on said star catalog as
generated as the candidates for one observed star and the elongations of plural observed stars fall within an allowable error range, said star identifier regards said stars on said star catalog as the candidates for said observed stars to thereby combine
said stars on said star catalog and said observed stars in a set and selects as secondary candidates for the observed stars the stars on the star catalog be longing to said set when said set makes appearance a number of times exceeding a predetermined
value inclusive.
7. An artificial satellite attitude determination system according to claim 1,
wherein when correspondence is established between asset including a plurality of vectors and a set including a plurality of other vectors containing noise as generated during a same rotational motion as said vectors, said attitude angle
estimator estimates a coordinate transformation matrix between said vectors included in said sets, respectively, by first displacing one of said sets close to the other set through appropriate rotational motion and by resorting to a sequential estimating
procedure for the rotational motions linking the vector set as displaced and the other vector set with rotation angle of zero being set as an initial value.
8. An attitude determination system for an artificial satellite, comprising:
image processing means for image-processing star images which are observation values of stars sensed at predetermined time points by a star sensor mounted on said artificial satellite to arithmetically determine direction vectors of said observed
stars;
rotation estimating means for estimating a rotational motion between an attitude of said artificial satellite at a predetermined time point and an attitude of said artificial satellite at another time point;
elongation estimating means, having an output of said rotation estimating means and an output of said image processing means as its inputs, for estimating elongations between the direction vectors of said observed stars on the basis of said
estimated rotational motion;
star identifying mean for establishing correspondences between a plurality of the stars for which relation of said elongations has been determined and stars on a star catalog; and
attitude angle estimating means for estimating an attitude angle of said artificial satellite on the basis of result of the identification of the stars;
wherein said elongation estimating means uses the star images on a plurality of image screens respectively photographed at different image pick-up time points by said star sensor.
9. An artificial satellite attitude determination system according to claim 8,
wherein said elongation estimating means estimates elongations among direction vectors of a plurality of stars which are photographed by said star sensor at unequal image pick-up time points by means of an estimated value of the rotational motion
of said artificial satellite estimated by said rotation estimating means.
10. An artificial satellite attitude determination system according to claim 8,
wherein said star identifying means is adapted to compare the directions of the observed stars directly with those of the stars on the star catalog to thereby extract the stars for the candidates and arithmetically determine the elongations of
the observed stars and those on the star catalog relevant to said candidate stars, respectively, whereby the candidate stars for which elongations are sufficiently close to one another are outputted as the result of the identification.
11. An artificial satellite attitude determination system according to claim 8,
wherein in the artificial satellite for which attitude angle information is of relatively low accuracy, said star identifying means is adapted to identify the corresponding star on the star catalog on the basis of the directions of plural stars
observed by means of said star sensor by determining a vector of difference between the directions of two of said observed stars and comparing said vector of difference between the directions of the stars on said star catalog.
12. An artificial satellite attitude determination system according to claim 8,
wherein the identification of the star by comparing the elongations among a plurality of observed stars with elongations among the stars on said star catalog, when differences between the elongations of plural stars on said star catalog as
generated as the candidates for one observed star and the elongations of plural observed stars fall within an allowable error range, said star identifying means regards said stars on said star catalog as the candidates for said observed stars to thereby
combine said stars on said star catalog and said observed stars in a set and selects as secondary candidates for the observed stars the stars on the star catalog belonging to said set when said set makes appearance a number of times exceeding a
predetermined value inclusive.
13. An artificial satellite attitude determination system according to claim 8,
wherein when correspondence is established between a set including a plurality of vectors and a set including a plurality of other vectors containing noise as generated during a same rotational motion as said vectors, said attitude angle
estimating means estimates a coordinate transformation matrix between said vectors included in said sets respectively, by first displacing one of said sets close to the other set through appropriate rotational motion and by resorting to a sequential
estimating procedure for the rotational motions linking the vector asset as displaced and the other vector set with a rotation angle of zero being set as an initial value.
14. An attitude determination system for an artificial satellite, comprising:
image processing means for image-processing star images which are observation values of stars sensed at predetermined time points by a star sensor mounted on said artificial satellite to arithmetically determining direction vectors of said
observed stars;
rotation estimating means for estimating a rotational motion between an attitude of said artificial satellite at a predetermined time point and an attitude of said artificial satellite at another time point;
elongation estimating means, having an output of said rotation estimating means and an output of said image processing means as its inputs, for estimating elongations between the direction vectors of said observed stars on the basis of said
estimated rotational motion;
star identifying means for establishing correspondence between a plurality of the stars for which relation of said elongations has been determined and stars on a star catalog; and
attitude angle estimating means for estimating an attitude angle of said artificial satellite on the basis of result of the identification of the stars,
wherein said attitude angle estimating means uses the star images on a plurality of image screens respectively photographed at different image pick-up time points by said star sensor.
15. An artificial satellite attitude determination system according to claim 14, wherein said attitude angle estimating means is adapted to arithmetically determine a plurality of candidates for the attitude angle at a current time point on the
basis of an estimated value of the rotational motion estimated by said rotation estimating means, while said star identifying means is adapted to provide a number of candidates for the identification of the stars on said star catalog for said observed
star images, and on the basis of a difference between the position of the star image as picked up by said star sensor and the position of the star image predicted from the star catalog, said candidates for the attitude angle or said candidates for the
identification which cannot be present are sequentially eliminated to estimate the attitude angle of the artificial satellite on the basis of the candidate remaining to last.
16. An artificial satellite attitude determination system according to claim 14,
wherein said star identifying means is adapted to compare the directions of the observed stars directly with those of the stars on the star catalog to thereby extract the stars for the candidates and arithmetically determine the elongations of
the observed stars and those on the star catalog relevant to said candidate stars, respectively, whereby the candidate stars for which elongations are sufficiently close to one another are outputted as the result of the identification.
17. An artificial satellite attitude determination system according to claim 14,
wherein in the artificial satellite for which attitude angle information is of relatively low accuracy, said star identifying means is adapted to identify the corresponding star on the star catalog on the basis of the directions of plural stars
observed by means of said star sensor by determining a vector of difference between the directions of two of said observed stars and comparing said vector of difference between the directions of the stars on said star catalog.
18. An artificial satellite attitude determination system according to claim 14,
wherein the identifications of the star by comparing the elongations among a plurality of observed stars with elongations among the stars on said star catalog, when differences between the elongations of plural stars on said star catalog as
generated as the candidates for one observed star and the elongations of plural observed stars fall within an allowable error range, said star identifying means regards said stars on said star catalog as the candidates for said observed stars to thereby
combine said stars on said star catalog, and said observed stars in a set and selects as secondary candidates for the observed stars the stars on the star catalog belonging to said set when said set makes appearance a number of times exceeding a
predetermined value inclusive.
19. An artificial satellite attitude determination system according to claim 14, wherein when correspondence is established between the set including a plurality of vectors and a set including a plurality of other vectors containing noise as
generated during the same rotational motion as said vectors, said attitude angle estimating means estimates a coordinate transformation matrix between said vectors included in said sets, respectively, by first displacing one of said sets close to the
other set through an appropriate rotational motion and by resorting to a sequential estimating procedure for the to rational motions linking the vector set as displaced and the other vector set with a rotation angle of zero being set as an initial
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an attitude determination system for an artificial satellite for determining attitude of a spacecraft or an artificial satellite on the basis of identification as to whether the stars on an all-sky star catalog
are caught by a star sensor. Incidentally, the term "artificial satellite" is used herein as being equivalent to the term "spacecraft".
2. Description of Related Art
For having better understanding of the concept underlying the present invention, description will first be made in some detail of a conventional artificial satellite attitude determination system. FIG. 13 is a schematic diagram for illustrating
a star identifying operation in a conventional artificial satellite attitude determination system which is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 6100/1985 (JP-A-60-6100). Referring to the figure, reference
numeral 1 designates an artificial satellite equipped with a star sensor (not shown) and traveling or cruising around the earth 2, numeral 3 designates a plurality of stars on the celestial sphere observed by the star sensor mounted on the artificial
satellite 1, numeral 4 designates a star concerned of those observed by the star sensor, numeral 5 designates a circle having a radius corresponding to a maximum value of the angle of field of the star sensor, numeral 6 designates all the stars that are
darker than the star 4 concerned within the circle 5, numeral 7 designates a set including collected star patterns in all of the sky, numeral 8 designates a table containing combinations of the star patterns in all of the sky and all characteristic
quantities, respectively, numeral 9 denotes a table prepared by rearranging the table 8 in the descending order of the characteristic quantities, and numeral 10 denotes a table generated by extracting only the pattern identifying numbers from the table
9. A star catalog can be created on the basis of the contents of the tables mentioned above.
Next, description will turn to the operation of the conventional artificial satellite attitude determination system. The artificial satellite 1 carrying the star sensor travels around the earth 2 while observing a plurality of stars 3 on the
celestial sphere by means of the star sensor. The characteristic quantities required for the star identification are determined in such a manner as mentioned below. For all the stars on the celestial sphere which can be caught by the star sensor, the
circle 5 is depicted around the concerned star 4 assumed as being positioned at the center with a radius corresponding to the maximum value of the angle of field of the star sensor, whereon the stars 6 falling within the circle and darker than the
central star 4 are sorted out. By drawing lines radially from the concerned star 4 located at the center to the individual darker stars 6, a radial line pattern is generated. The star identifying number of the concerned star 4 is affixed to the
generated radial line pattern as the pattern identifying number.
On the basis of the set 7 of the patterns for the stars in all the sky, the table 8 is generated by combining the characteristic quantities and the pattern identifying numbers of all the patterns. In the table 8, the characteristic quantities
and the pattern identifying numbers are rearranged in the descending order of the magnitudes of the characteristic quantities, whereby the table 9 is generated in which the characteristic quantities and the pattern identifying numbers are listed in the
descending order of the characteristic quantities. On the basis of the table addresses of the table 9, approximating functions 11 for representing the characteristic quantities are generated. Additionally, the pattern identifying number table 10
containing only the pattern identifying numbers extracted from the table 9 is created. For the star identification, the pattern generated through the similar procedure for the star which is brightest in the image data obtained through observation by the
star sensor and the functions 11 are matched or collated with each
other, to thereby extract a common or shared portion from a corresponding sub-table of the pattern identifying number table 10. When only one shared portion is extracted, this means that the star identification has been carried out
successfully.
FIG. 14 is a block diagram showing a typical configuration of a conventional artificial satellite attitude determination system equipped with a star sensor. This artificial satellite attitude determination system is comprised of an attitude
propagation unit 12, an attitude updating unit 13 and a time-independent drift estimation module 13a. Heretofore, in the system for determining the attitude of the artificial satellite with the aid of combination of a star sensor 16 and a gyro 14, it
has been a general practice to estimate the drift of the gyro 14 with the precise attitude angle determined on the basis of the output of the star sensor 16. More specifically, the output or observation values of the star sensor 16 are arithmetically
processed by a star sensor processing unit 15 to determine an attitude angle qm of the artificial satellite, whereon a difference qe between the attitude angle qm and an estimated attitude angle qh obtained as the output of the attitude propagation unit
12 is inputted to the attitude updating unit 13 as an estimation error.
In the attitude updating unit 13, the drift .omega.d of the gyro 14 is estimated through the medium of a first-order filter KPO+KIO/s on the presumption that the drift is time-invariable, whereon the estimated value .omega.d is subtracted from
the gyro output .omega.m to obtain an estimated value .omega.h of the attitude angular velocity (also known as the attitude rate) of the satellite. The attitude propagation unit 12 is so designed as to add the estimated value of the attitude angle of
the artificial satellite one sampling cycle before by integrating the estimated value .omega.h of the attitude angular velocity as a function of time and output additionally an estimated value qh of the attitude angle at the current time point.
FIG. 15 is a view for graphically illustrating steady characteristics of attitude angle estimation errors of a navigation filter employed in the conventional attitude determination system. As can be seen in the figure, in the case where the
nominal angular velocity (or nominal attitude rate) of the artificial satellite assumes a constant value, the error involved in estimating the attitude angle of the satellite is substantially equal to zero, indicating the desirable characteristics.
However, when the nominal angular velocity changes periodically as in the case of the artificial satellite in which steering operation is performed about a yaw axis, error in the attitude angle estimation makes appearance significantly in dependence on
the periodical change of the nominal angular velocity.
In the conventional star identifying method known heretofore, identification of the star is performed by collating the characteristic quantities arithmetically determined on the basis of the star images falling within the visual field of the star
sensor at a given time point with the stars on the star catalog. Consequently, the conventional star identifying method is imposed with requirement that the visual field of the star sensor has to be set widely so that a sufficient number of stars
required for the identification make appearance within the visual field without fail or the identification has to be given up at a time point when a sufficient number of stars can not be seen within the visual field.
As the characteristic quantities mentioned above, there are selected such quantities which remain invariable independent of rotation about the optical axis of camera such as the elongation, i.e., an angle formed by the direction vectors of two
stars, an area of a triangle constituted by three stars on an imaging screen of the camera. Thus, the identification which makes much of a relative relation between the predicted star position and the observed star position has not always been realized
when the positions of the stars on the imaging screen of the camera can roughly be predicted from the star catalog as in the case where the attitude value of the artificial satellite can be estimated.
Additionally, when the identification is to be performed by a computer system mounted on the artificial satellite, it is necessarily required that the direction of the sight of the star sensor is known roughly in advance and that the range on the
star catalog to be searched is sufficiently limited. Unless such computational capability can be ensured by the satellite-onboard computer, it is necessary to send the observation data to a ground station where the arithmetic operations for the
identification are carried out, the result of which is then sent back to the artificial satellite.
On the other hand, when the attitude determination of the artificial satellite is performed on the basis of the result of the identification, it is required to determine parameters representing the coordinate transformation or rotational motion
between the star catalog data concerning the direction vectors of the stars in the inertial space and the direction vectors of the stars in the coordinate system fixed to the body of the artificial satellite (hereinafter referred to as the spacecraft
body coordinate system) which is calculated on the basis of the positions of the star images within the visual field of the star sensor.
Heretofore, it has been a conventional practice to determine at first roughly the attitude of the artificial satellite by using the attitude sensor such as an earth sensor or sun sensor and thereafter determine the attitude of the artificial
satellite with high accuracy by resorting to the use of the star sensor, which makes it possible to utilize the rough values with enhanced accuracy for determining the parameters expressing the coordinate transformation or rotational motion mentioned
previously. However, in the case of the artificial satellite which is equipped with neither the earth sensor nor the sun sensor, or when these sensors mounted on the artificial satellite suffer fault and can not be used, the rough value mentioned above
can not be made use of in capturing initially the attitude of the artificial satellite. To cope with such situation, some measures have to be provided for enabling such rough value to be utilized.
On the other hand, when the rough value such as mentioned above is available, it is conventional practice to reduce overhead involved in the calculations required for estimation of the attitude angle by adopting linearization based on the
minute-angle approximation concept. However, when the accuracy of the rough value is low or poor, error due to the approximation can no more be neglected, incurring degradation in convergence in the transient response, giving rise to a problem.
Furthermore, the artificial satellite destined for astronomical and/or terrestrial observation is required to be equipped with the attitude determination system capable of correcting drifts of a gyro with very high accuracy by utilizing the
highly accurate attitude angle information available from the star sensor. However, in the conventional attitude determination system known heretofore, the drift of the gyro is regarded as being a constant or a variable of very large time constant on
the presumption that the nominal angular velocity or attitude rate of the artificial satellite is constant. As a consequence, when the nominal angular velocity of the artificial satellite changes as a function of time lapse, then the time-dependent
variable component of the drift in the gyro can not be estimated with sufficiently high accuracy. Thus, the attitude determination accuracy as realized is not satisfactory.
SUMMARY OF THE INVENTION
In the light of the state of the art described above, it is a general object of the present invention to provide an attitude determination system for an artificial satellite which can solve satisfactorily the problems mentioned above.
Thus, it is an object of the present invention to provide an attitude determination system for an artificial satellite which is capable of performing the star identification without need for the aid of the ground station even when the range of a
star catalog to be searched can not sufficiently be narrowed because the rough value of the artificial satellite are unknown.
It is another object of the present invention to provide an attitude determination system for an artificial satellite which is capable of performing the identification of the star even when the number of stars making appearance within a visual
field of a star sensor in a single image pick-up operation is short of the number of the stars required for the star identification.
Yet another object of the present invention to provide an attitude determination system for an artificial satellite which is capable of arithmetically determining rough value of the attitude of the artificial satellite without resorting to the
use of other attitude sensor such as the sun sensor and/or the earth sensor.
A further object of the present invention to provide an attitude determination system for an artificial satellite, which system is capable of determining the attitude of the artificial satellite with high accuracy while reducing overhead involved
in the arithmetic operations to be executed by a satellite-onboard computer and which can estimate with improved accuracy even the time-dependent variable components contained in the drift in a gyro even when the nominal angular velocity of the
artificial satellite should vary as a function of time lapse.
In view of the above and other objects which will become apparent as description proceeds, there is provided according to a general aspect of the present invention an attitude determination system for an artificial satellite, which system
includes an image processing module for processing star images observed at predetermined time points by a star sensor mounted on the artificial satellite for arithmetically determining direction vectors of the observed stars, a rotation estimating module
for estimating a rotational motion of the artificial satellite between an attitude of the artificial satellite at a predetermined time point and an attitude of the artificial satellite at another time point, an elongation estimating module for estimating
elongations between the direction vectors of plural stars the images of which are picked up at a same time point by means of the star sensor and estimating the elongations between the direction vectors of plural stars the images of which are picked up at
different time points by means of the star sensor on the basis of the estimated rotational motion, a star identifying module for establishing correspondences between a plurality of the stars for which relation of the elongation has been determined and
stars on a star catalog for thereby identifying the stars, and an attitude angle estimating module for estimating an attitude angle of the artificial satellite on the basis of result of the identification of the stars.
With the arrangement of the artificial satellite attitude determination system described above, identification of the stars can be realized even when the number of the stars making appearance within the visual field of the star sensor in a
single-shot image pick-up operation is short of the number required for the identification. Besides, a rough value of the attitude of the artificial satellite can be arithmetically determined without resorting to the aid of other attitude sensor such as
a sun sensor, earth sensor or the like.
In a preferred mode for carrying out the invention, the rotation estimating module may be so designed as to estimate an average motion of the star images on a plurality of image screens generated at image pick-up time points close to one another,
to thereby establish correspondences among individual star images on a plurality of image screens generated at image pick-up time points remote from one another on the basis of the estimated average motion of the stars, for thereby estimating the
rotational motion during a period intervening between the successive image pick-up time points.
By virtue of the arrangement mentioned above, the rotational motion of the artificial satellite can be estimated on the basis of the motions or displacements of the star images on the imaging screen even when information concerning the rotational
motion of the artificial satellite is not furnished in advance. In addition, even when a given one of the stars makes appearance only once on the image screen because the imaging time points are discrete and spaced from one another, correspondence
between the star images appearing on a plurality of the image screens and those on the star catalog can be established with high certainty.
In another preferred mode for carrying out the invention, the attitude angle estimating module may be so designed as to arithmetically determine candidates for the attitude angle at a current time point on the basis of estimated value of the
rotational motion estimated by the rotation estimating module, while the star identifying module may be so designed as to provide a number of candidates for the identification of the stars on the star catalog for the observed star images. On the basis
of difference between the position of the star image as picked up by the star sensor and the position of the star image predicted from the star catalog, those of the candidates for the attitude angle or those of the candidates for the identification
which can not be present are sequentially eliminated as the star images are inputted from the star sensor, to thereby estimate the attitude angle of the artificial satellite on the basis of the candidate remaining to the last.
Owing to the arrangement of the artificial satellite attitude determination system mentioned above, identification of the stars and calculation of rough value of the attitude of the artificial satellite can be carried out without using the output
of other attitude sensor such as the sun sensor, the earth sensor or the like or without relying on the aid of the ground station. Besides, the procedure to this end is sequentially executed upon every star image pick-up operation, load imposed on the
computer can be dispersed.
In yet another preferred mode for carrying out the invention, the star identifying module may be so designed as to compare the directions of the observed stars directly with those of the stars on the star catalog to thereby extract the stars for
the candidates and arithmetically determine the elongations of the observed stars and those on the star catalog relevant to the candidate stars, respectively, whereby the candidate stars for which elongations are sufficiently close to one another are
outputted as the result of the identification.
With the arrangement mentioned above, the identification can be realized with imposed accuracy when compared with the identification carried out by collating only the elongations or only the direction and the brightness.
In still another preferred mode for carrying out the invention and directed to in the artificial satellite for which attitude angle information is of relatively low accuracy, the star identifying module may be so designed as to identify the
corresponding star on the star catalog on the basis of the directions of plural stars observed by means of the star sensor and determining vector of difference between the directions of two of the observed stars for comparing the vector of difference
between the directions of the stars on the star catalog.
With the above-mentioned arrangement, the identification can be realized with improved accuracy when compared with the identification carried out by collating only the elongations or only the direction and the brightness
The present invention may be carried out in another preferred mode in which in the identification of the star by comparing the elongations among a plurality of observed stars with elongations among the stars on the star catalog, when differences
between the elongations of plural stars on the star catalog as generated as the candidates for one observed star and the elongations of plural observed stars fall within an allowable error range, the star identifying module may regard the stars on the
star catalog as the candidates for the observed stars to thereby combine the stars on the star catalog and the observed stars in a set and selects as secondary candidates for the observed stars the stars on the star catalog belonging to the set when the
set makes appearance a number of times exceeding a predetermined value inclusive.
Owing to the above arrangement, overhead involved in the calculations required for the identification can be reduced, which in turn means that the capacity of the memory incorporated in the computer can be decreased.
In yet further preferred mode for carrying out the invention, it is proposed that when correspondence is established between a set including a
plurality of vectors and a set including a plurality of other vectors containing noise as generated during a same rotational motion as the vectors, the attitude angle estimating module may be so designed as to estimate a coordinate
transformation matrix between the vectors included in the sets, respectively, by first displacing one of the sets close to the other set through appropriate rotational motion and by resorting to a sequential estimating procedure for the rotational
motions linking the vector set as displaced and the other vector set with rotation angle of zero being set as an initial value.
By virtue of the arrangement of the artificial satellite attitude determination system mentioned above, the solution for the estimation of the rotational motion can be determined with high accuracy without increasing overhead involved in the
arithmetic operation. Accordingly, by employing a navigation filter designed to this end, it can be ensured that the estimated values of the attitude angle in the transient phase converge speedily.
According to another aspect of the invention, there is provided an attitude determination system for an artificial satellite which may be so designed that a navigation sensor signal outputted from a navigation sensor is subjected to series
development operation with an orbital angular velocity of the artificial satellite and a frequency corresponding to an integral multiple of the orbital angular velocity and that a time-dependent variable component is estimated as a combination of two
status quantities represented by a frequency component and a first order derivative thereof for each of the frequency components.
With the arrangement of the artificial satellite attitude determination system mentioned above, the time-invariable components and the frequency components contained in the navigation signal can dynamically be separated, which is advantageous in
that the transient response of the navigation filter becomes gentle while the sensitivity to the observation noise in the steady state can be lowered. Thus, the attitude determination system ensuring high accuracy or certainty can be provided.
According to a further aspect of the invention, there is provided an attitude determination system for an artificial satellite. A plurality of navigation aiding satellites travel around the earth along different orbits, respectively, for aiding
navigation of artificial satellites, wherein in a given one of the artificial satellites, message signals sent from all the operating navigation aiding satellites and carrying information concerning positions and velocities of the navigation aiding
satellites are received and decoded, whereon attitude of the given one artificial satellite is roughly estimated on the basis of the position information of the navigation aiding satellites corresponding to the received message signals and the position
information of the operating navigation aiding satellites which does not correspond to any one of the received message signals.
With the above arrangement of the attitude determination system, rough value of the attitude angle of the artificial satellite can be estimated even when the information of the attitude sensor such as a sun sensor, earth sensor or the like is
unavailable.
The above and other objects, features and attendant advantages of the present invention will more easily be understood by reading the following description of the preferred embodiments thereof taken, only by way of example, in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the course of the description which follows, reference is made to the drawings, in which:
FIG. 1 is a schematic block diagram showing a system configuration of a star sensor processing unit incorporated in an artificial satellite attitude determination system according to a first embodiment of the present invention;
FIG. 2 is a view showing schematically a configuration of a rotation estimating module in the artificial satellite attitude determination system according to a second embodiment of the present invention;
FIG. 3 is a view for illustrating operation of an inter-star correspondence establishing submodule of the artificial satellite attitude determination system according to the second embodiment of the invention;
FIG. 4 is a view for illustrating operation of an inter-image correspondence establishing submodule of the artificial satellite attitude determination system according to the second embodiment of the invention;
FIG. 5 is a flow chart for illustrating operations of a star identifying module and an attitude angle estimating module of the artificial satellite attitude determination system according to a third embodiment of the invention;
FIG. 6 is a flow chart for illustrating operation of a star identifying module of the artificial satellite attitude determination system according to a fourth embodiment of the invention;
FIG. 7 is a flow chart for illustrating operation of a star identifying module of the artificial satellite attitude determination system according to a fifth embodiment of the invention;
FIG. 8 is a flow chart for illustrating operation of a star identifying module of the artificial satellite attitude determination system according to a sixth embodiment of the invention;
FIG. 9 is a flow chart for illustrating operations of an attitude angle estimating module and a rotational motion arithmetic submodule in the artificial satellite attitude determination system according to a seventh embodiment of the invention;
FIG. 10 is a block diagram showing a system configuration of an attitude determination system for an artificial satellite in which a gyro and a star sensor are used in combination according to an eighth embodiment of the invention;
FIG. 11 shows graphical diagrams for illustrating exemplary numerical values of estimated drift of the gyro employed in the artificial satellite attitude determination system according to the eighth embodiment of the invention;
FIG. 12 is a flow chart for illustrating a procedure of estimating roughly the attitude of an artificial satellite by using a message signal from a navigation aiding satellite;
FIG. 13 is a schematic view for illustrating operation of a star identifying module of a conventional artificial satellite attitude determination system;
FIG. 14 is a block diagram showing a typical configuration of the conventional artificial satellite attitude determination system equipped with a gyro and a star sensor in combination; and
FIG. 15 is a view for graphically illustrating exemplary numerical values of estimated drift of the gyro employed in the conventional artificial satellite attitude determination system.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Now, the present invention will be described in detail in conjunction with what is presently considered as preferred or typical e | | |
