Abstract:
A vehicle ( 12 ) including a control system ( 18 ) is used for controlling vehicle attitude or angular velocity ( 38 ). The processor ( 24 ) is coupled to a star sensor or tracker ( 22 ) and a memory ( 30 ) that may include a star catalog ( 32 ), and an exclusion list ( 36 ). The exclusion list ( 36 ), a list of stars to be temporarily excluded from consideration when determining attitude or angular velocity or relative alignment of star sensors or trackers, is calculated on-board. Such a calculation prevents the necessity for a costly, periodic, ground calculation and upload of such data. By manipulating the star catalog, or sub-catalogs derived from said catalog, based upon the exclusion list ( 36 ), measurements of such excluded stars are prevented from corrupting the attitude or angular velocity or alignment estimates formulated on board.

Description:
FEDERAL RESEARCH STATEMENT  
       [0001]     This invention was made with Government support. The Government has certain rights in this invention. 
     
    
     BACKGROUND OF INVENTION  
       [0002]     The present invention relates generally to attitude or angular velocity or sensor alignment estimate adjustment for a vehicle, and more particularly, to algorithms involving attitude or angular velocity or sensor alignment determination, using star position measurements. The present invention allows identification of stars whose position measurements are suspect, due to the presence of nearby bright objects, in order to exclude such measurements from such determination.  
         [0003]     Satellites and other vehicles are in widespread use for various purposes including scientific research and communications. Many scientific and communications missions, however, cannot be accurately fulfilled without consistently monitoring and controlling the 3-axis attitude and angular velocity of the vehicle. In many applications, the vehicle must be oriented to transmit signals in particular directions or to receive signals from specifically located sources. Furthermore, in such a situation, the vehicle angular velocity must be such so as to maintain the desired orientation, over time. Without accurate control over vehicle 3-axis attitude and angular velocity, the transmission or reception of such signals is hindered and at times impossible.  
         [0004]     Such control requires systems for 3-axis attitude and angular velocity determination, which generally include one or more star trackers and a 3-axis gyroscope. During normal operation, star trackers or star sensors provide attitude-related information and the 3-axis gyroscope is needed to provide angular velocity information. As there are inherent, and time-varying, errors from star trackers, star sensors, and gyros, it is often necessary to constantly estimate such errors, in order to compensate for them. One common method of doing so is to correlate star tracker or sensor position measurements of stars with known positions of the same stars, as listed in a star catalog, or database. Discrepancies between the measured and predicted positions allow direct estimation of tracker error, and indirect estimation of gyro error. Knowing such errors allows estimation of attitude or angular velocity, or refinement of existing estimates. Furthermore, if there are multiple star trackers or star sensors on-board, such correlations allow determination of the alignment of such trackers or sensors, with respect to each other; such determination yields greater accuracy in future attitude and angular velocity estimates.  
         [0005]     Stellar Inertial Attitude Determination (SIAD) algorithms employ a carefully designed star catalog or database for selection and identification of stars tracked by star trackers or star sensors. The current known art in star selection for star trackers mainly concentrates on generating star catalogs with certain properties. Typically, one method of locating entries in a star catalog that correspond to stars in a tracker field of view (FOV) uses a standard binary search that is sorted by declination only. This method is not very efficient because it involves searching through hundreds of entries to find the stars that are located within the FOV.  
         [0006]     Various other methods involve generation of multiple overlapping sub-catalogs that contain stars for a specific FOV in the sky. At any point in time, the stars within a star tracker&#39;s FOV will reside in one or more of these sub-catalogs. Each star catalog entry that is represented in a FOV sub-catalog has been determined, using on-board processing, to be in the tracker FOV.  
         [0007]     In a SIAD algorithm, the entries corresponding to stars intruded by bright objects such as planets, asteroids, or comets, need to be excluded from the star catalog or sub-catalog, i.e., excluded from consideration by the algorithm. This is so that bright objects are not mistaken for stars, or the light from the objects does not corrupt the measurements made by the star sensor. Traditional object-based catalog entry exclusion is performed on the ground. These stars intruded by planets or other bright objects are excluded from a revised copy of the on-board star catalog. The revised star catalog is then uploaded to the spacecraft control processor (SCP). The uploading is time-consuming and may easily be interrupted. This may cause delays in the proper attitude determination or errors in the on-board version of the catalog.  
         [0008]     It would therefore be desirable to provide a system and method for excluding affected stars from consideration by the attitude or angular velocity or sensor alignment determination algorithms and procedures, without the disadvantages of processing the information on the ground.  
       SUMMARY OF INVENTION  
       [0009]     The present invention reduces the problems associated with uploading star exclusion information to a spacecraft by calculating the planet- or object-driven exclusion on the spacecraft.  
         [0010]     In one aspect of the invention, a method of controlling a spacecraft includes calculating a bright object stayout zone onboard the spacecraft, determining whether a star is in the stayout zone, excluding the star from the catalog or sub-catalog, when the star is intruded upon by a bright object, and determining a vehicle inertial attitude or angular velocity or sensor alignment, in response to the revised catalog or sub-catalog.  
         [0011]     In a further aspect of the invention, a vehicle includes an attitude or angular velocity control system, a star tracker having a field of view, and a star catalog memory having a star catalog stored therein. The star catalog has a plurality of entries, each entry having an associated flag therewith.  
         [0012]     The spacecraft further includes an exclusion list memory. A processor is coupled to the attitude or angular velocity control system, the star tracker, the star catalog memory, and the exclusion list memory. The processor calculates a stayout zone onboard the spacecraft, determines a plurality of stars in the stayout zone, excludes the catalog entries corresponding to the stars from the catalog or sub-catalog, determines a spacecraft inertial attitude or angular velocity or star sensor relative alignment in response to the revised catalog or sub-catalog, and controls the spacecraft attitude or angular velocity in response to the estimated spacecraft inertial attitude or angular velocity.  
         [0013]     One advantage of the invention is that the processing may take place onboard the spacecraft. This eliminates problems that can occur and the uploading time due to calculations being performed on the ground.  
         [0014]     Other aspects and advantages of the present invention will become apparent upon the following detailed description and appended claims, and upon reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0015]      FIG. 1  is a perspective view of a set of vehicles (depicted as a satellite system, in accordance with the preferred embodiment of the present invention) for determining stars in a star tracker field of view;  
         [0016]      FIG. 2  is a block diagram of an apparatus for determining stars in a star tracker field of view in accordance with the preferred embodiment of the present invention; and  
         [0017]      FIG. 3  is a diagrammatic view of a star catalog.  
         [0018]      FIG. 4  is a flow chart of a first embodiment of a method for determining stars in a star tracker field of view in accordance with the preferred embodiment of the present invention.  
         [0019]      FIG. 5  is a flow chart of a second embodiment of a method for determining stars in a star tracker field of view in accordance with the preferred embodiment of the present invention.  
         [0020]      FIG. 6A  is one example of multiple stayout zones including two circular stayout zones.  
         [0021]      FIG. 6B  is an example of three stayout zones as the multiple stayout zones which are rectangular.  
         [0022]      FIG. 6C  is a third embodiment of a multiple stayout system including three different types of stayout zones including circular, elongated and rectangular.  
         [0023]      FIG. 7  is a flow chart illustrating a method of operating a system using multiple stayout zones.  
     
    
     DETAILED DESCRIPTION  
       [0024]     In the following figures the same reference numerals will be used to identify the same components.  
         [0025]     The present invention is described with respect to a star catalog, or sub-catalog, entry exclusion system for a vehicle. It should be noted that various types of vehicles may benefit from the exclusion system. It should also be noted that the exclusion system is not limited to exclusions due to the effects of planets. Rather, various objects such as planets, asteroids, comets, other extraterrestrial objects, or other vehicles, may be the cause of exclusion. Also, gyros or other information may be used in addition to star catalog/stay out zone calculations.  
         [0026]     Referring to  FIG. 1 , a perspective view of a system  10  for determining stars in a star tracker field of view (FOV) in accordance with one embodiment of the present invention is illustrated. The system  10  is comprised of one or more vehicles  12 . Although not required, the system may also be in communication with a station  14 . Each vehicle  12  includes an apparatus  18  for determining inertial attitude based upon a plurality of stars  20 . The vehicle  12  may be one of various types of vehicles, including satellites and other spacecraft.  
         [0027]     The apparatus  18  is responsible for locating stars  20  within or near a star tracker field of view in order to control the attitude or angular velocity of vehicle  12 . Many scientific and communications missions cannot be accurately fulfilled without consistently monitoring and controlling the vehicle 3-axis attitude or angular velocity. In many applications the vehicle must be positioned to transmit signals in particular directions or to receive signals from specifically located sources. Without accurate control over vehicle 3-axis attitude and angular velocity, the transmission or reception of such signals is hindered and at times impossible.  
         [0028]     Referring to  FIG. 2 , a block diagram of apparatus  18  for determining stars  20  within or near a star tracker field of view  28  in accordance with one embodiment of the present invention is illustrated. Apparatus  18  includes a star tracker or star sensor  22  and a processor  24 . Apparatus  18  also includes a memory  30  that includes a star catalog  32 , and an exclusion list  36 . The processor  24  may also be coupled to an attitude or angular velocity control system  38 .  
         [0029]     Star tracker  22  is mounted to the vehicle  12  and is coupled to processor  24  in a conventional manner. Star tracker  22  includes a field of view (FOV)  28  and a bore-sight  40  in the FOV  28 . Star tracker  22  is used to sense a plurality of positions, relative to star tracker  22 , of a plurality of stars  20 . Star tracker  22  then generates a plurality of signals corresponding to the plurality of star positions in the FOV  28 . It should be noted that more than one star tracker  22  may be included on a vehicle. The present invention allows determination of multiple star trackers with respect to the others when multiple star trackers are used.  
         [0030]     Processor  24  may be an individual processor or may be comprised of a plurality of processors. For example, star tracker  22  may include its own processor. Likewise, attitude or angular velocity control system  38  may also comprise its own processor such as a spacecraft control processor (SCP). For simplification purposes the processor is illustrated as a single component. Each processor may be microprocessor-based. Processor  24  resides in vehicle  12 . Processor  24  is coupled to star tracker  22  and receives the plurality of signals corresponding to the plurality of star positions in the FOV  28 . Processor  24  provides control logic operative to select at least one of the plurality of signals corresponding to the positions of one or more stars. Processor  24  uses star catalog  32  to determine which stars  20  to select or determine the stars&#39; positions with respect to the Earth-Centered Inertial (ECI) frame.  
         [0031]     Memory  30  is illustrated as a plurality of separate elements. Those skilled in the art will recognize that the memory may comprise a single memory system. It should also be noted that if a microprocessor is used for processor  24  that the microprocessor may include memory therein. The memory may include various types of memory including but not limited to random access memory (RAM).  
         [0032]     The attitude or angular velocity control system  38  may comprise a plurality of thrusters and momentum wheels as will be evident to those skilled in the art. The attitude or angular velocity control system  38  controls the various momentum wheels or thrusters to orient the vehicle in its desired direction.  
         [0033]     Referring now to  FIG. 3 , a simplified database, star catalog or sub-catalog  32  is illustrated. Each entry in catalog or sub-catalog  32  may include associated star information and may also include an include/exclude flag  46 . The include/exclude flag may, for example, be implemented as a binary zero or one. For illustration purposes an “I” for include and an “E” for exclude have been used. The processor  24 , as will be further described below, may be programmed to change the include/exclude flag  46  to a desired state. That is, processor  24  may change the flag  46  to include from exclude or to exclude from include (flagging). Star catalog  32  may also consist of a plurality of information including but not limited to star right ascension and declination angles in a given ECI frame, star instrument magnitude, and other star properties. Star catalog  32  may contain data that is stored using primary and multiple secondary arrays. It should also be noted that various numbers of stars may be included in the catalog.  
         [0034]     Referring to  FIG. 4 , a flow chart of a method of a first embodiment of a method according to the present invention is illustrated. In step  58 , the include/exclude flags associated with each star catalog or sub-catalog entry are set to include. In step  60  a stayout zone, associated with a bright object, is determined. The stayout zone may, for example, be a regular shape such as a rectangular or square field of view. The stayout zone may also be variable in size and may also be round or oblong. The properties of the stayout zone may be a function of the object position and magnitude; they may also depend on various other parameters, such as magnitude and position of the stars within the zone. In step  62  the determination of which stars are inside the stayout zone of the bright object or planet is made. In step  64  the stars to be excluded are listed in the exclusion list. In step  66  the stars in the exclusion list are flagged as excluded by switching the include/exclude flag of the corresponding star catalog or sub-catalog entries to exclude.  
         [0035]     In step  68 , based on star sensor measurements of the positions of stars listed in the star catalog or sub-catalog, and flagged as included, the spacecraft inertial attitude estimate or angular velocity estimate is determined or refined. In step  70 , the attitude is controlled to orient the spacecraft as desired, or the angular velocity is controlled to maintain the desired attitude over time. Other considerations such as vehicle gyros such as 3-D gyros may also be taken into consideration.  
         [0036]     It should be noted that the steps described above may all be performed on the spacecraft. This prevents problems due to timing and errors due to the transmission of the information from a ground station.  
         [0037]     Referring now to  FIG. 5 , the same process described above with respect to  FIG. 4  may be used for multiple planets or objects. N is defined as the number of objects.  
         [0038]     The configuration is nearly identical in that step  58 ′ initializes all star catalog or sub-catalog include/exclude flags to include, step  60 ′ determines a stayout zone for bright object x, where x was set to  1  in step  59 ′, step  62 ′ calculates the stars inside the stayout zone of bright planet/object x, step  64 ′ lists the stars to be excluded in an exclusion list.  
         [0039]     Step  66 ′ excludes the stars in the exclusion list from consideration in attitude or angular velocity estimate formulation by switching include/exclude flags of the corresponding star catalog entries to exclude.  
         [0040]     In step  67 ′, x is replaced with (x+1). In step  67 ″, should the new value of x be less than or equal to N, the procedure returns to step  60 ′, to determine exclusions due to object x. Should the value of x be greater than N, in step  68 ′, the spacecraft attitude or angular velocity or relative star sensor or tracker alignments may be determined after the star catalog exclusions due to the last planet/object N are determined in step  66 ′. Likewise, in step  70 ′, the attitude or angular velocity may be controlled only after the attitude or angular velocity is determined.  
         [0041]     It should be noted that the present invention may be used autonomously to perform realtime exclusion. Also, periodic maintenance for all the planets/objects may be performed or individually performed. The excluded star list may also be stored as part of a whole list or an individual list corresponding to the planet/object.  
         [0042]     As shown in  FIG. 6A , a first stayout zone  80  circular in shape is illustrated. A second stayout zone  82  is concentric with the first stayout zone. As will be further described below, the first stayout zone  80  may have all stars excluded from therein. In stayout zone  82 , only stars that are dimmer than a first threshold may be excluded.  
         [0043]     Referring now to  FIG. 6B , three stayout zones are provided. The center rectangular stayout zone  84 , a second stayout zone  86  that forms an annular band around the first stayout zone, and a third rectangular stayout zone  88  is also illustrated. In the first stayout zone  84 , all the stars may be excluded. In stayout zone  86 , only stars dimmer than a first threshold may be excluded from this annular band. In stayout zone  88  which is also an annular band around stayout zone  86 , only stars that are dimmer than a second threshold may be excluded. As illustrated in  FIG. 6B , each of the stayout zones may be rectangular or square in shape.  
         [0044]     Referring now to  FIG. 6C , three differently shaped stay-out zones may be provided. The first stayout zone  90  corresponds to an area where all stars are excluded therefrom. Stayout zone  90  is elliptical or oval in shape. The elongated shape corresponds to a column within a CCD of the star tracker. A second stayout zone  92  having first stayout zone  90  therein excludes stars that are dimmer than a first threshold. A third stayout zone  94  is elongated in the direction of axis  91 . The stayout zone  94  is rectangular in shape. The rectangle is narrower, in this example, than the width of stayout zone  92 .  
         [0045]     One reason for providing an elongated stayout zone is the deleterious effects of objects on the sensors in the star trackers. An effect known as CCD array blooming forms a highly asymmetric image that typically occurs along the readout column direction of the sensor with a rectangular field of view. Prior knowledge of the intended pointing direction of the star tracker may allow a suitably restrictive yet overly conservative stayout zone in the star catalog. Depending on the application, this region need not guarantee non-interference, but simply reduce the probability of interference. Other uses of non-circular stayout zone shapes is for inherently non-circular objects such as comet tails.  
         [0046]     Further, a spacecraft typically operates in three different scenarios. That is, the satellite may have no attitude knowledge, rough attitude knowledge, or priority attitude knowledge. The present invention is capable of worse scenario no attitude knowledge. The circular stayout zones are typically useful for any application in any of the above mentioned scenarios, particularly no attitude knowledge. Once some attitude knowledge is obtained, the stayout zones may be reduced in one or both horizontal and vertical directions to increase the number of stars available for attitude determination. Rectangular stayout zones or a combination circular and rectangular zones may be performed on orbit. However, these zones are particularly useful for exclusion performed on the ground, when rough attitude information is available. The stars inside the part or whole rectangular field of view of the star sensor can be excluded based on the spacecraft nominal steering profile in order to avoid the CCD array blooming problem. The star catalog may then be uploaded from the ground to the spacecraft.  
         [0047]     Referring now to  FIG. 7 , a method of operating a system using multiple stayout zones is illustrated. In step  100  the boundaries of multiple stayout zones for a particular object is determined. As mentioned above, the multiple stayout zones may be of various shapes, sizes and numbers. The present example is set forth with three stayout zones. However, two, three, four, etc. may be provided. In step  102  if the star is within the innermost stayout zone, step  104  excludes the star from the stayout zone. Step  100  may then be executed for a different bright object.  
         [0048]     Referring back to step  102 , if the star is not within the innermost zone, step  106  determines whether the star is within a second zone outside of the first zone. If the star is outside of the first zone and within the second zone, step  108  is executed. If the magnitude of the star is less than a first threshold in step  108  the star is excluded in step  110 . In step  108 , if the star is not less than a first threshold or after step  110 , step  100  is excluded for a different star for a different object.  
         [0049]     Referring back to step  106 , if the star is not within a second threshold, step  112  determines whether the star is within a third zone outside of the second zone. If the star is within the third zone, step  114  is executed. In step  114 , if the magnitude of the star is less than a second threshold, step  116  excludes the star from the star catalog. In step  114  if the magnitude of the star is not less than the second threshold and after step  116 , step  100  is executed for a different star for a different object.  
         [0050]     Referring back to step  112 , if the star is not within the third zone, step  100  is again executed.  
         [0051]     The above method is suitable for objects of magnitude of +1 or brighter. Typically, objects with a stellar magnitude of +1 or brighter are ignored with a singular circular-stayout zone of 0.5 degrees. Objects that may reach a magnitude of +1 or brighter may include Mars, Jupiter, Venus, Saturn, Uranus, and Neptune. Further, the small central stayout zone provides complete exclusion while a larger region includes stars that were excluded in prior systems. Thus, more stars are available for attitude determination.  
         [0052]     While the invention has been described in connection with one or more embodiments, it should be understood that the invention is not limited to those embodiments. On the contrary, the invention is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the appended claims.