METHOD, DEVICE AND COMPUTER PROGRAM PRODUCT FOR DETERMINING THE POSITION OF A SPACECRAFT IN SPACE

Abstract

A method for determining the position of a spacecraft in space, includes cyclically÷ repeating steps of capturing distorted star images; processing the distorted star images to form distorted star group data; storing the distorted star group data; determining a current rotation rate by comparing the distorted star group data of two consecutive cycles; transmitting the current rotation rate to a position control system; and/or the following steps are carried out: processing the distorted star images of a current cycle to form rectified star group data; determining position information by matching the rectified star group data with star group catalog data which is carried along; transmitting the position information to the position control system. A method for determining the position of a spacecraft in space, taking into account known system parameters of an optical system, includes: coding star group catalog data with n = 3...4 stars [x.sub.n, y.sub.n, z.sub.n], which are visible in an image field, into representative focal-plane coordinates; forming a scaling-, translation-, and rotation-invariant star group code on the basis of [xPiX,yPiX]n; or coding star group catalog data with n = 3...4 stars [x.sub.n,y.sub.n, z.sub.n], which are visible in an image field, into representative tangent and/or angular coordinates [tan(a),tan(β)].sub.n. The invention further relates to a device for carrying out such methods and to a computer program product for carrying out such methods.

Claims

1-15. (canceled)

16. A method for determining the position and orientation of a spacecraft in space, the method comprising at least one of: A) cyclically repeatedly acquiring distorted star images with at least one star camera; processing the distorted star images of a current cycle with a computer to form rectified star group data; determining position and orientation information by matching the rectified star group data with star group catalog data stored in a database; and transmitting the position and orientation information to a position control system of the spacecraft; or B) performing one of the following: i) coding star group catalog data for a star group having 3 to 4 stars that are visible in an image field, wherein each star is defined by 3 coordinates, into representative focal-plane coordinates, and forming a scaling- invariant, translation- invariant, and rotation-invariant star group code based on the focal plane coordinates; or ii) coding star group catalog data for a star group having 3 to 4 stars that are visible in an image field, wherein each star is defined by 3 coordinates, into at least one of representative tangent coordinates or representative angular coordinates.

17. The method of claim 16, further comprising: cyclically repeatedly performing: processing the acquired distorted star images on the computer to form distorted star group data, and storing the distorted star group data; determining a current rotation rate of the spacecraft by comparing the distorted star group data of two consecutive cycles; and transmitting the current rotation rate to the position control system.

18. The method of claim 16, wherein at least one of: the database in which the star group catalog data is stored is carried onboard the spacecraft; or the computer is carried onboard the spacecraft.

19. The method of claim 16, wherein: the star group catalog data includes data on group stars, on star groups, and on a vector index tree; the data on the star groups include identification vectors and reference data; and the vector index tree relates to the identification vectors of the star groups.

20. The method of claim 16, wherein: the database in which the star group catalog data is stored is carried onboard the spacecraft; and star catalog data are carried onboard the spacecraft and is used with additional star data to determine the position and orientation of the spacecraft.

21. The method of claim 16, further comprising statistically filtering the position and orientation information.

22. The method of claim 21, wherein the position and orientation information is filtered over at least one of several cycles, or over several star cameras.

23. The method of claim 16, wherein the at least one star camera includes at least one rolling shutter star camera.

24. The method of claim 17, wherein processing the distorted star images comprises processing the images with the aid of the at least one star camera to form the distorted star group data.

25. The method of claim 17, wherein: the at least one star camera has image elements; and several mutually-adjacent image elements are combined to form an image element module, in order to increase a rotation rate limit.

26. The method of claim 16, further comprising: detecting different image fields using one or more star cameras in a combined manner.

27. The method of claim 16, wherein at least one of: the method is carried out with the aid of at least one separate processor device; or the method is carried out with the aid of a processor device of the spacecraft.

28. A device for determining the position and orientation of a spacecraft in space from repeatedly acquired distorted star images, the device comprising: at least one star camera configured for acquiring the distorted star images; and at least one processor device; wherein the at least one processor device comprises at least one of: a) a first processing block configured for cyclically repeatedly: acquiring distorted star images, processing the distorted star images to form distorted star group data, and storing the distorted star group data, and a second processing block for determining a current rotation rate by comparing the distorted star group data of two consecutive cycles; or b) a third processing block for processing the distorted star images of a current cycle to form rectified star group data, and a fourth processing block for determining position and orientation information by matching the rectified star group data with star group catalog data stored in a database.

29. The device of claim 28, wherein the at least one star camera is a rolling shutter camera.

30. The device of claim 28, comprising: a plurality of star cameras and a separate processor device for each star camera; or a common processor device for a group of star cameras.

31. The device of claim 28, wherein the at least one processor device is at least one of: a processor device separate from the spacecraft; or a processor device of the spacecraft.

32. A computer program product comprising program code stored on a non-transient, computer-readable medium, the program code, when executed by a computer, causing the computer to carry out the method of claim 16.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the principles of the invention.

[0036] FIG. 1 depicts a block diagram of a gyro-less position determination system for highly dynamic spacecraft,

[0037] FIG. 2 depicts a block diagram of a gyro-less position determination system for highly dynamic spacecraft, with only a rotation rate measurement,

[0038] FIG. 3 depicts a block diagram of the position determination system for less dynamic spacecraft,

[0039] FIG. 4 depicts a block diagram of a gyro-less position determination system for highly dynamic spacecraft, with an extra-precise position measurement,

[0040] FIG. 5 schematically illustrates star groups distorted by a rolling shutter,

[0041] FIG. 6 schematically illustrates identification parameters in star group data,

[0042] FIG. 7 is a flowchart of a processing in lost-in-space conditions,

[0043] FIG. 8 schematically illustrates an application of a robust rotation rate and position determination with a star sensor by means of selected star group catalog coding, and

[0044] FIG. 9 is a flowchart of a robust rotation rate and position determination with a star sensor by means of selected star group catalog coding.

DETAILED DESCRIPTION

[0045] FIG. 1 shows a block diagram of a gyro-less position determination system 1 for highly dynamic spacecraft. A star camera 2 of the gyro-less position determination system 1, which is designed as a rolling shutter camera, acquires distorted star images 4 of the starry sky in time with the star sensor cycle and delivers them to a processor device 3 for evaluation. There, star vectors are determined from the star images 4 in a processing block 5 for star group generation, which star vectors characterize the position of all stars in the image and are processed further to form star group data 6. Similarly to the usual star images, star groups consist of three, four, or more stars and can be derived according to various principles. What is decisive is the property of the star groups that they can be unambiguously found in the entire starry sky with the aid of their star group parameters. Like the image, the star group data 6 are rolling-distorted and cannot be reliably used to search in star group catalog data 14. However, they can be identified in the quantity of the likewise-distorted star group data of the previous cycle. The distorted star group data 6 of the current image are therefore temporarily stored for the next cycle as distorted star group data of the previous cycle. If it is the very first cycle, i.e., lost-in-space conditions are present, there are no distorted star group data of the previous cycle. In this case, a current rotation rate 8 cannot be immediately calculated in the processing block 7. A rotation rate of zero angular degrees per second is in this case used as a fixed initial value. In subsequent cycles after the initialization, the current rotation rate can always be estimated in the processing block, for generating the current rotation rate 8. For this purpose, at least one star group from the current image is found again in the previous image. The determination of the current rotation rate from the matched distorted star groups is possible using various methods. In some variants of star groups, a direct calculation of the rotation rate from the star group parameters is possible. The use of the delta-quaternion estimation is always possible by statistical optimization of the rotation matrix, which brings the two sets of star vectors of the matched star groups as well as possible to congruence. The known QUEST algorithm can be used for this optimization.

[0046] The current rotation rate 8 determined in this way is, on the one hand, used for the geometric correction of the distorted star group data 6 in the processing block 10 for rolling shutter correction and is, on the other hand, output directly to the position control system 9 of the spacecraft. The current rotation rate 8 can be reliably measured from the second cycle on, even for very high rotation rates with an unstable axis of rotation, and is available to the spacecraft for applications with high rotation rates and low accuracy requirements. One such application of importance is the emergency with a fast-wobbling spacecraft, which can be stabilized again with the aid of the current rotation rate 8.

[0047] The processing block 10 for rolling shutter correction converts the star position of the stars of the star group data 6 into a coordinate system of the star sensor as it would come about in a snapshot without rolling shutter. The conversion takes place in relation to a well-defined reference time that is freely selectable in the star sensor cycle. For example, the middle of the exposure time of the first image line can be selected as the reference time. By means of a model of the rolling shutter, the time of its acquisition in relation to the reference time can, as a function of the number of the image line of the star, be determined as a time difference. Stars in the first image line have a difference of zero at the reference time mentioned. For each star of the groups, the current rotation rate 8 results in a rotation of its star vector, which puts the star vector into the position it had at the reference time. Stars from the first image line are not rotated at all; the stars from the last image line are subjected to the maximum rotation corresponding to the maximum time difference. The resulting rectified star group data 11 can now be found with success in the star group catalog data 14. This second matching of star groups takes place in the processing block 12 for generating the final position information. As a result of the matching, star vectors are then present both in the coordinate system of the star sensor at the reference time and in the inertial coordinate system of the catalog. This enables the calculation of the position information 13, consisting of the rotational position and precise rotation rate, according to known methods. In this case, a calculation of the position information 13 directly from the star group parameters or the use of the matched star vectors are possible. As a function of the actual current rotation rate, the catalog identification can also already work under lost-in-space conditions in the very first cycle, without measured current rotation rate. The rotational position part of the position information is thus available as of the first or second cycle. In the case of rotations below the previous rotation rate limit of conventional star sensors, the rotational position is immediately available, so that known disadvantages can be avoided.

[0048] The gyro-less position determination system can be adapted to special requirements of a mission, wherein the required resources are optimized according to the adaptation.

[0049] FIG. 2 shows a block diagram of a gyro-less position determination system 1 a for highly dynamic spacecraft with only a rotation rate measurement. Here, all components of the system that are not required for the measurement of the current rotation rate 8 can be omitted. The remaining components do not require any modification, so that a high degree of modularity of the system is achieved.

[0050] FIG. 3 shows a block diagram of the position determination system 1 b for less dynamic spacecraft. Here, the system components for determining the current rotation rate 8 and for the processing block 10 for rolling shutter correction are omitted - likewise in a modular manner. If the system is to be upgraded for higher requirements, corresponding extensions are possible, as shown in FIG. 4 for the case of the gyro-free position determination system for highly dynamic spacecraft with extra precise position measurement.

[0051] The measurement accuracy can be increased by using additional star catalog data 15. With the number of matched stars, the accuracy of both the rotational position measurement and the rotation rate measurement is improved. The number of the matched stars is now no longer limited, as with conventional star sensors, by the capacity of the tracking of stars in image windows. It can potentially be extended to all detectable stars in the image. The number of the measured stars thereby increases, e.g., to about 50 to 100, compared to 16 stars tracked in windows. This leads to a significant increase in the measurement accuracy.

[0052] FIG. 5 shows two examples of star groups distorted by the rolling shutter. The non-distorted star group 17 is also shown for comparison. The non-distorted star group 17 consists of the three stars 16. The star group 19 with the stars 18 demonstrates the distortion by a rotation about the optical axis of the star camera, while the star group 21 with the stars 20 shows the distortion by a rotation about the vertical image axis of the star camera. The image ratios correspond to a typical case with a square star camera image field 22 of 25 angular degrees, a rolling shutter delay of 100 milliseconds between the first and the last image lines in an 8 Hz star sensor cycle. The first image line is located at the top in the image. In the case of an assumed size of the image matrix of 1,000 image elements times 1,000 image elements, a distortion of about 80 image elements for a rotations around the image axis and of about 34 image elements for a rotation about the optical axis of the star camera results in the last image line in each case at a rotation rate of 20 angular degrees per second. FIG. 5 reflects the geometric proportions of the distortion for these cases. As a function of the rotation rate and the axis of rotation, corrections for the star positions result, which can be compensated for only with the rotation corresponding to all three components of the current rotation rate 8, as they are carried out in the processing block 10. Mere shifts are not sufficient.

[0053] For the star groups, four stars are preferably used. In the case of the use of three stars per group, there is theoretically already a clear identification of the group, but very small errors in the determination of the star position are required for this purpose, which cannot always be ensured in practice. The star group data include the identification parameters, combined in an identification vector. Furthermore, they can include reference data which are used for the rotational position and rotation rate calculation after successful identification. For large sets of star group data, such as star group catalog data 14, index trees can additionally be included to accelerate the search.

[0054] FIG. 6 shows identification parameters in star group data. For star calculations, data relating to the unit sphere with unit vectors and solid angles are preferably used. The star group considered consists of four stars 23, the position of which is represented as a direction by the corresponding star vector on the unit sphere. The star vectors of the stars in the image of the star camera are determined by the position of the star in the image and the optical imaging parameters -especially the focal length and possibly further calibration parameters. In the example shown in FIG. 6, the identification parameters are calculated as follows: First, the two stars with the greatest angular separation are selected; they are called primary stars. The spherical primary axis 24 of the star group is the circular segment on the unit sphere that connects the two stars with the greatest separation angle. The length of the primary axis corresponds to the maximum separation angle. The unit vector to the center of the primary axis is used as a position vector 25 of the group. The spherical secondary axis is perpendicular to the primary axis and has its origin in the position vector of the group. With these two axes, the angular coordinates of the two remaining stars of the group, called secondary stars, are then defined in a local group coordinate system. The group coordinate system is two-dimensional, with the angular coordinates perpendicular to the primary axis 26 and the angular coordinates parallel to the primary axis 27. In order to make the group identification independent of the image scale of the star camera, or to perform a recalibration of the focal length in orbit, the angular coordinates of the secondary stars can additionally be normalized to the angular size of the primary axis. After identification, changes in scale, e.g., as a result of thermal effects in the objective, can then be compensated for by evaluation of the non-normalized angles. In this case, the variant, independent of the image scale, of the identification is to be used with the angles normalized to the primary axis.

[0055] The four values of the angular coordinates (primary axes 26, 27) of the two secondary stars form the four-dimensional identification vector of the star group. The indices of the four group stars in the index list of the stars detected in the image are the reference data. The global star group catalog with the star group catalog data 14 is calculated with the same method on the ground, and is carried along in flight. The indices of the four stars correspond to a continuous numbering of all group stars in the star group catalog data 14. The index tree, which is likewise calculated on the ground, is preferably designed as a vector index tree for the specified identification vector. In addition to the star group catalog data 14, additional star catalog data 15 are carried along for the variant of the gyro-less position determination system 1c for highly dynamic spacecraft with an extra precise position measurement.

[0056] In the case of star detection with windows, the typical star sensor is not suitable for very high rotation rates for geometric reasons, and possible modifications of the sensor configuration must be considered. An improvement that is easy to implement results from the enlargement of the image field. In a star camera with a square image field of 40 angular degrees, only about 1,000 stars are required in the catalog for the star groups. The result is a rotation rate limit of 14 angular degrees per second with an image exposure of 100 milliseconds. Even larger image fields result in a further increase in the theoretical rotation rate limit. The approximate uniform distribution of the stars assumed here is practically not present. For this reason, in the design of the star catalog and of the star group catalog for high rotation rates, the star distance must be considered in particular. Stars with near neighbors must not be used for the catalog. Since all stars are detected in the image of the star camera - even the unsuitable ones - a moderately increased processing capacity in the processing block 7 of the star group generation additionally results. Overall, the desired rotation rate limit of 20 angular degrees per second for the emergency regime in a simple, typical star sensor with window detection can be achieved with the aid of an image field enlargement, an adapted catalog design, and increased computing power.

[0057] Newer star sensors no longer work with star windows, but with star clusters. Clusters are groups of coherent, bright image elements, which are extracted as objects from the image.

[0058] In the continuation of the measurement of rotational position and rotation rate with the method of the invention in a continuous sequence, a measurement is possible in each star sensor cycle, regardless of how the rotation rate below the limit changes. In the first measurement under lost-in-space conditions, it can happen that, at high rotation rates, a measurement result is available only in the second cycle. The cause of the possible failure of the very first cycle is the missing star image from the previous cycle.

[0059] FIG. 7 shows a flowchart of a processing with lost-in-space conditions in the very first star sensor cycle. In the very first cycle, the current rotation rate for rolling correction is set to zero. This can take place by direct assignment of the zero value, or by using the data of the current cycle instead of the star groups of the previous cycle. With a zero rotation rate, the rolling shutter correction does not change the data. A measurement result in the first cycle will therefore only be available if the rotation rate leads to tolerable distortions. At a rotation rate of 1 angular degree per second and an extension of the star group over half the image field, a distortion of about 2 image elements occurs in the typical star sensor. 2 image elements of geometric errors generally result in failure of the identification. Nevertheless, in practice, rotation rates of 1 to 4 angular degrees per second can be tolerated for the star identification in a typical star sensor. This is because the rolling distortion can act partially as a rotation and enlargement. These components of the distortions are compensated for by the identification algorithms.

[0060] In the flow chart according to FIG. 7, there is a success check of the matching of the rectified star groups to the catalog. It is in particular required for the very first processing cycle. A success check of the matching of the star groups from the current and the last image is not required. The matching of the distorted star groups of two consecutive images works for all relevant usage scenarios of the star sensor. During a rotation about the image axes, the time differences of the star detection do not change. In this most favorable case, there is no change in the rolling distortion from image to image. The greatest change in the rolling distortion exists during the rotation about the optical axis of the star camera. It is less than one third of the image element at a rotation rate of 20 angular degrees per second, and is thus not critical.

[0061] FIG. 8 shows an application of a robust rotation rate and position determination with a star sensor by means of selected star group catalog coding. FIG. 9 shows a flowchart of robust rotation rate and position determination with a star sensor by means of selected star group catalog coding.

[0062] As derived from the method according to FIG. 8 and FIG. 9, the known properties of a calibrated star sensor, such as optics focal length, detector pixel size and number, are included in the star group coding. The arrangement of the star group code in preferably binary search trees enables very fast star group identifications. The method was optimized and verified on real measurement data. This proposal enables real-time star identifications in each individual and independent measuring cycle (<100 ms). That is to say, as soon as a star group (3 ... 4 stars) is detected, an identification and position calculation can take place. If this takes place in directly consecutive cycles or with known time intervals, rotation rate, direction of rotation and rotation acceleration, or rate estimation can be performed even for very high rotation rates. The conventional attitude tracking can be supported with this method - in particular, at high rates and accelerations - which leads to an increase in the robustness of the position measurement. These properties are a valuable extension of the functionality of a star sensor.

[0063] The word “may” refers in particular to optional features of the invention. Accordingly, there are also further developments and/or embodiments of the invention which additionally or alternatively have the respective feature or the respective features.

[0064] If necessary, isolated features can also be selected from the combinations of features disclosed in the present case and can be used in combination with other features to delimit the subject matter of the claim, while resolving a structural and/or functional relationship that may exist between the features.

[0065] While the present invention has been illustrated by a description of various embodiments, and while these embodiments have been described in considerable detail, it is not intended to restrict or in any way limit the scope of the appended claims to such de-tail. The various features shown and described herein may be used alone or in any combination. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative example shown and described. Accordingly, departures may be made from such details without departing from the spirit and scope of the general inventive concept.

TABLE-US-00001 Reference signs 1 Position determination system 1a Position determination system 1b Position determination system 1c Position determination system 2 Star camera 3 Processor device 4 Star image 5 Processing block 6 Star group data 7 Processing block 8 Rotation rate 9 Position control system 10 Processing block 11 Star group data 12 Processing block 13 Position information 14 Star group Catalog data 15 Star catalog data 16 Star 17 Star group 18 Star 19 Star group 20 Star 21 Star group 22 Image field 23 Star 24 Primary axis 25 Position vector 26 Primary axis 27 Primary axis 28 Start of measurement, cycle number=0 29 Calculation of the star vectors from the image 30 Calculation of the star groups from the star vector 31 Cycle number=0 32 Yes 33 No 34 Current rotation rate=0 35 Matching of the current and the previous star groups 36 Calculation of the current rotation rate 37 Rolling shutter correction of the star group 38 Matching of the corrected and the catalog star groups 39 Sucessful match? 40 No 41 Yes 42 Notification of a measurement error to the satellite 43 Final calculation of rotational position and rotation rate, and notification to the satellite 44 Cycle number=cycle number+1 45 Execution of the method according to the invention, duration < 100 ms (real-time) 46 Detect (x, y) 47 Identify 48 Calculate position 49 Detector, binning stage 1, 2, 4 50 # of objects? 51 Identify 52 Rate exists 53 Yes 54 Rolling shutter correction 55 No 56 Image star group code(s) 57 Catalog star group mempry 58 Star group ID? 59 No 60 Next cycle 61 Yes 62 Store position of cycle N 63 Guide star catalog 64 N-1 positions exist? 65 No 66 Yes 67 Calculate rate and direction