Abstract:
A system for, and method of recovering a solar-powered spacecraft from an anomaly that renders the attitude of the spacecraft unknown includes maintaining a power-safe attitude by switching between two orthogonal axes using solar panel current sensors. The system and method may also include simultaneously determining spacecraft attitude using a star sensor. The system is applicable to spacecraft operating in a solar wing-stowed configuration.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of U.S. patent application Ser. No. 10/910,430, entitled, “STAR-TRACKER-BASED ATTITUDE DETERMINATION FOR SPINNING SPACECRAFT,” filed on Aug. 3, 2004, which is in turn a continuation-in-part of U.S. patent application Ser. No. 10/727,764, entitled, “UNIFIED SENSOR-BASED ATTITUDE DETERMINATION AND CONTROL FOR SPACECRAFT OPERATIONS,” filed on Dec. 3, 2003, both of which are owned by the assignee of the present application, and the entirety of both of which are hereby expressly incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention is generally directed to satellite attitude and power acquisition systems and methods, and, more particularly, to satellite attitude and power acquisition systems and methods that are applicable to satellites in a solar wing-stowed configuration.  
         [0004]     2. Description of the Related Art  
         [0005]     Transporting a spacecraft from the ground to a destination orbit is an integral and crucial part of any spacecraft mission. For example, to insert a spacecraft into a geosynchronous orbit, a launch vehicle typically injects the spacecraft into a low-altitude parking orbit. The spacecraft then performs transfer orbit operations to transfer the spacecraft from the parking orbit to a destination orbit. The transfer orbit is usually performed by firing a liquid apogee motor (LAM) with the spacecraft spinning around a LAM axis to stabilize the spacecraft and to even the thermal and power conditions, or by firing a combination of LAM and xenon-ion propulsion (XIP) thrusters. Once the spacecraft has completed its transfer orbit, it then may enter in-orbit testing and on-station operation.  
         [0006]     From cradle to grave, the spacecraft may go through the following phases of operations: separation, transfer orbit operation (including coasting, spin speed change, reorientation and LAM burn), deployment (including antennas, reflectors, solar wings, radiators), acquisition (including power acquisition and attitude acquisition), in-orbit test (including antenna mapping), on-station operation (including normal pointing, momentum dumping, station keeping and station change), and a deorbiting operation.  
         [0007]     Typically, spacecraft, such as communication satellites, use multiple separate sets of sensors and control algorithms for different phases of operations. For example, different sets of sensors and/or control algorithms may be used for attitude determination and control for bi-propellant spinning transfer orbit operations versus those that are used for on-station operations. The use of different sensors, attitude determination, and attitude control methods for spinning transfer orbits and on-station operations, respectively, increases the spacecraft weight, sensor and processor complexity, as well as the development cost for spacecraft attitude determination and control systems.  
         [0008]     Spinning transfer orbit operations for spacecraft typically may be performed by ground-assisted attitude determination using a spinning earth sensor and a spinning sun sensor set. The measured leading edge and trailing edge of the earth detected by the earth sensor and the measured TOA (time of arrival) of the sun detected by the sun sensor collected and relayed periodically to a ground station. Typically, at least one orbit pass is dedicated to this data collection. A ground orbital operator may then run a ground attitude determination algorithm using these inputs and ephemeris-computed sun and earth positions to determine the spin axis attitude of the spacecraft. This spin axis attitude (the spin phase being still undetermined) is then uploaded to the spacecraft. Next, on-board software may use this spin axis attitude together with the spin phase measured by the spinning sun sensor to complete the 3-axis attitude determination for subsequent spacecraft reorientation or liquid apogee motor (LAM) burn. Power acquisition or sun acquisition are typically performed with a wide-field-of-view sun sensor, an orthogonal set of narrow-field-of-view slit sun sensor, or an orthogonal set of spinning slit sun sensor.  
         [0009]     On-station spacecraft operations typically use different sensors, such as a staring earth sensor assembly (STESA) and a wide field of view (WFOV) sun sensor assembly (SSA), and/or a star tracker for attitude determination; and WFOV sun sensor for power acquisition or sun acquisition. Thus, the sensors used for transfer orbit operations may lie dormant for the entire time that the spacecraft is on station. The number of sensor types used and the number of sensors used, increase the hardware and development cost, increase weight and launch cost, and complicate the mission operation. In addition, some spacecraft have configurations and equipment that may make it difficult in some situations to provide a clear field of view for some sensors, such as, for example, a WFOV SSA, which spans a diamond of about 120×120 deg.  
         [0010]     The present invention is directed to overcoming one or more of the problems or disadvantages associated with the prior art.  
       SUMMARY OF THE INVENTION  
       [0011]     In accordance with one aspect of the invention, a method of acquiring a power-safe state for a spacecraft in a spinning transfer orbit is provided. The method includes maintaining a steady spin for a stowed solar wing configuration, sensing panel current from exposed solar panels, and synchronizing an attitude quaternion of the spacecraft with the sensed panel current to position the spin axis for power safety. The steady spin may be a z-spin or x-spin.  
         [0012]     In accordance with another aspect of the invention, a method is provided of acquiring a power-safe state for a wing-stowed spacecraft, with one or more exposed solar panels in a spinning transfer orbit. The method includes a first rotation phase that includes maintaining a steady spin about a first axis that is substantially orthogonal to a direction that is normal to at least one of the exposed solar panels, sensing panel current from at least one of the exposed solar panels, and if the sensed solar panel peak current does not exceed a threshold current, stopping the first rotation at the peak current of one of the solar panels. The method may further include entering a second rotation phase, the second rotation phase including maintaining a steady spin about a second axis that is substantially orthogonal to the direction that is normal to at least one of the exposed solar panels, and that is substantially orthogonal to the first axis, and sensing panel current from at least one of the exposed solar panels, and if the sensed solar panel peak current does not exceed a threshold current, stopping the second rotation at the peak current of one of the solar panels, and entering the first rotation phase. The first axis may be the z-axis of the spacecraft, and the second axis may be the x-axis of the spacecraft. The first rotation phase and the second rotation phase may use thrusters and/or reaction wheels for attitude control. The first rotation and the second rotation may select a rotation direction and a rotation rate such that the reaction wheels will not be saturated. The first rotation phase and the second rotation phase may use thrusters to dump momentum to facilitate attitude control by the reaction wheels. The peak current may be determined by a peak current finding algorithm. Stopping the rotation at peak current may be performed by resetting the commanded and estimated attitude quaternions to identity at peak current.  
         [0013]     In accordance with another aspect of the invention, a method of acquiring a power-safe attitude for a wing-stowed spacecraft in a spinning transfer orbit that has one or more exposed solar panels is provided. The method includes maintaining a steady spin about a first axis that is substantially orthogonal to a direction that is normal to at least one of the exposed solar panels, acquiring spacecraft attitude using one or more star trackers, and maneuvering the spacecraft to a predetermined power-safe attitude after the spacecraft attitude has been acquired.  
         [0014]     According to yet another aspect of the invention, a method of acquiring a power-safe state for a wing-stowed spacecraft in a spinning transfer orbit that has one or more exposed solar panels is provided. The method includes entering a first rotation phase that includes maintaining a steady spin about a first axis that is substantially orthogonal to a direction that is normal to at least one of the exposed solar panels, acquiring spacecraft attitude using one or more star trackers, sensing panel current from at least one of the exposed solar panels, and if the sensed solar panel peak current does not exceed a threshold current, stopping the first rotation at the peak current of at least one of the solar panels and entering a second rotation phase, the second rotation phase including maintaining a steady spin about a second axis that is substantially orthogonal to the direction that is normal to at least one of the exposed solar panels, and that is substantially orthogonal to the first axis, and maneuvering the spacecraft to a predetermined power-safe attitude after the spacecraft attitude has been acquired.  
         [0015]     In accordance with still another aspect of the invention, a spacecraft attitude control system is provided for a spacecraft having one or more solar panels. The spacecraft attitude control system includes a control processor, a set of actuators operatively connected to the control processor, and at least one solar array current sensor operatively connected to the control processor and to at least one solar panel. The control processor is programmed to perform the steps of placing the spacecraft in a first rotation phase that includes maintaining a steady spin about a first axis that is substantially orthogonal to a direction that is normal to at least one of the solar panels, sensing panel current from at least one of the exposed solar panels, and if the sensed solar panel current does not exceed a threshold current, stopping the first rotation at the peak panel current, and entering a second rotation phase. The second rotation phase includes maintaining a steady spin about a second axis that is substantially orthogonal to the direction that is normal to at least one of the exposed solar panels, and that is substantially orthogonal to the first axis. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     Objects, features, and advantages of the present invention will be become apparent upon reading the following description in conjunction with the drawing figures, in which:  
         [0017]      FIG. 1  is a diagram that illustrates various exemplary spacecraft orbits about the Earth;  
         [0018]      FIGS. 2A-2C  are side views of a spacecraft that may incorporate the invention;  
         [0019]      FIG. 3  is a diagram that illustrates an example of a modular attitude control system architecture;  
         [0020]      FIG. 4  is a flow diagram illustrating processing steps that may be used for attitude determination from star tracker data;  
         [0021]      FIG. 5  is a block diagram illustrating an example of computer software units that may be used for transfer orbit and on-station attitude determination;  
         [0022]      FIG. 6  is a block diagram illustrating further detail of an example of an attitude determination system that may be used for both transfer orbit and on-station operations;  
         [0023]      FIG. 7  is a flow diagram illustrating an example of three-axis stabilized controller for coasting operations in a transfer orbit;  
         [0024]      FIG. 8  is a block diagram further illustrating an example of a liquid apogee motor burn overturning torque feedforward control system and method;  
         [0025]      FIG. 9  is a block diagram illustrating a power/stellar acquisition system and method for recovering a spacecraft in a power-safe fashion from an anomaly during a transfer orbit;  
         [0026]      FIG. 10  is a diagram illustrating a spacecraft in a power safe state;  
         [0027]      FIG. 11  is a diagram illustrating a spacecraft and designating regions in which the position of the sun relative to the spacecraft for either power safe or not power safe states;  
         [0028]      FIG. 12  is a diagram illustrating a configuration in which a spacecraft is not in a power safe state;  
         [0029]      FIG. 13  is a graphic illustration of a synchronization of a spacecraft quaternion using solar panel current, in order to determine when a spacecraft is in a power-safe state;  
         [0030]      FIG. 14  is a block diagram, similar to that of  FIG. 5 , illustrating further detail of another example of an attitude determination system that may be used for both transfer orbit and on-station operations;  
         [0031]      FIG. 15  is a block diagram, similar to that of  FIG. 6 , illustrating an example of a stellar attitude acquisition system; and  
         [0032]      FIG. 16  is a flow diagram illustrating an example of stellar attitude determination in a spinning transfer orbit. 
     
    
     DETAILED DESCRIPTION  
       [0033]     With reference initially to  FIG. 1 , a spacecraft  30 S with its solar wings in a stowed position is depicted in a first transfer orbit  32  about the earth  34 . Also depicted in  FIG. 1  are a launch path  36 , a parking orbit  38 , and a second transfer orbit  40 . The first transfer orbit  32 , the parking orbit  38 , and the second transfer orbit  40  may all have a common perigee point indicated at  42 . The first transfer orbit  32  shares an apogee point indicated at  44 , that is the same altitude as a geosynchronous orbit at  46 . The second transfer orbit  40  has an apogee at  48  that is greater in altitude than the geosynchronous orbit  46 . As indicated by the reference numeral  30 D, the spacecraft in the second transfer orbit and in the geosynchronous orbit  46  may have solar wings  50  deployed and extending beyond a main portion  52  of the spacecraft  30 D. As shown in  FIGS. 2A-2C , the spacecraft  30 D may include a primary star tracker  54  and a redundant star tracker  56 , an optional gyro device, such as an inertial reference unit (IRU), and may carry any suitable payloads such as, for example, a set of communication antennas  58  that may be mounted on or near a positive yaw face  60  of the spacecraft  30 D.  
         [0034]     Now referring to  FIG. 3 , a spacecraft attitude control system architecture, generally indicated at  62 , may include one or more star trackers  54 , and may also include inertia measurement units  64 , as well as solar array current sensors  66  that provide inputs to a spacecraft control processor  68 . The spacecraft control processor  68  may be used to command many spacecraft systems such as, for example, a spot beam pointing mechanism  70 , a crosslink pointing mechanism  72 , a pitch/yaw magnetic torquer rod  74 , a roll magnetic torquer rod  76 , and a set of three or more reaction wheels  78  (that may be arranged in a pyramid configuration) by providing commands for wheel torque and/or wheel speed. In addition, the spacecraft control processor may provide commands to a solar wing positioner (SWP) and solar wing drive  80 , as well as thrusters and engines such as, for example, a liquid apogee motor engine  82 , bi-propellant thrusters  84 , and bi-propellant latch valves  86 .  
         [0035]     A unified attitude sensor set, generally indicated at  87 , for multiple phases of spacecraft operations can be a plurality of star trackers  54 . More than one star tracker  54  can be installed for failure redundancy and potential intrusion from bright objects, such as the sun, the moon and the earth. The star trackers are used to determine spacecraft attitude and derive spacecraft rate.  
         [0036]     Alternatively, the unified attitude sensor set  87  can be a plurality of star trackers  54  and inertia measurement units  64  (such as gyros) for multiple phases of spacecraft operations. The spacecraft attitude, rate and acceleration are determined by use of a Kalman filter using star tracker and gyro measurement data. Gyro parameters can also be calibrated by star tracker measurement in the Kalman filter. As a further alternative, the unified attitude sensor set  87  can further be a plurality of star trackers, in addition to gyros and, or solar panel current sensors. Star tracker and gyro data may be used to determine spacecraft attitude, rate and acceleration, and calibrate gyro parameters via a Kalman filter. The solar panel current sensors may be used to validate the acquired stellar attitude after a loss-of-attitude anomaly, to maneuver the wing-stowed spacecraft  30 S for power safety, and to position the solar wings  50  for power safety for wing-deployed spacecraft.  
         [0037]     With reference to  FIG. 4 , a control software system may include computer software units (CSUs) such as star tracker processing (STP) CSU  88 , that provides input to both a star measurement and steering (SMS) CSU  90  and a stellar attitude acquisition (SAA) CSU  92 . The SMS CSU  90  provides input to a pre-Kalman processor (PKP) CSU  94 , and the SAA CSU  92  and the PKP CSU  94  both provide input to an attitude determination (ATD) CSU  96 .  
         [0038]     Now referring to  FIG. 5 , showing exemplary software units that may be used for transfer orbit and on-station attitude determination, a hemispherical inertial reference unit (HIRU) sensor processing CSU  98  operates in parallel with the star tracker processing CSU  88  using data from the star tracker unit  54 , in providing attitude data to the ATD CSU  96 . In addition, a flight star catalog (FSC)CSU  100  provides data to the SMS CSU  90  and the SAA CSU  92 . In a scenario in which the attitude is lost and needs to be initialized during both transfer orbit and on-station operations, the STP CSU  88  may provide input directly to the SAA CSU  92  for attitude acquisition and initialization, whereas during nominal transfer orbit and on-station operations, the data from the STP CSU  88  may be provided to the SMS CSU  90  which in turn provides residual data to the PKP CSU  94  for preprocessing and subsequent handoff to the ATD CSU  96 . The unified sensor architecture and attitude determination/control method can also be used to perform other typical spacecraft operations, such as separation, deployment, station keeping, and deorbiting.  
         [0039]     With reference to  FIG. 6 , the ATD CSU  96  is shown in further detail, to include a Kalman filter attitude update CSU  130 , a three axis propagation CSU  132 , a rate/acceleration update CSU  134 , and a time-match circular buffer CSU  136 . The three axis propagation CSU  132  propagates the previous attitude to real time to a propagated attitude using delta angle measured by the inertia measurement units  64  during the time interval. It may also propagate spacecraft rate, acceleration and other dynamic states. As shown in  FIG. 6 , a separate path is used where there is a loss of attitude in which the stellar attitude acquisition CSU  92  provides inputs to the Kalman filter attitude update CSU  130 . On the other hand, for nominal transfer orbit operations and nominal on-station operations, the PKP CSU  94  provides input to the Kalman filter attitude update CSU  130 . In other words, the Kalman filter attitude update CSU  130  updates the propagated attitude (in the three axis propagation CSU  132 ) by attitude residual (derived from star tracker measurements) either from the PKP CSU  94  or the SAA CSU  92 .  
         [0040]     A wheel-gyro wobble and nutation controller (WGWANC) is typically not effective at slow spin rates. However, a 3-axis stabilized controller can perform WGWANC control function by making the momentum in ECI as the attitude steering target as depicted in  FIG. 7 .  
         [0041]     Now referring to  FIG. 7 , a flow diagram for providing three-axis stabilized control during a coasting operation in a bi-propellant transfer orbit is generally indicated at  138 . At block  140 , the spacecraft angular momentum unit vector, {right arrow over (m)}, is determined in earth centered inertial (ECI) coordinates. {right arrow over (m)} is inertially fixed up to the slow precession by environmental torque. At block  142 , the designated spacecraft spin axis {right arrow over (z)}, is determined in the spacecraft body coordinates. The designated spacecraft spin axis can be any axis in the spacecraft body, but is usually the z-axis or x-axis in a typical spacecraft mission. Next, at block  144 , a set of allowable power safe attitudes relative to ECI is determined, for example, attitudes having a sun polar angle of 90±20 deg. Next, at block  146 , a steering attitude, q cmd , is determined. q cmd  is the attitude for which the spin axis is aligned as closely as possible with the momentum vector in ECI coordinates, but constrained to being in the set of allowable power safe attitudes, A. I.e., if we define ECI {right arrow over (z)}  to be the designated spacecraft spin axis, as determined in the ECI frame, assuming a spacecraft attitude of q cmd , then q cmd  is the attitude which satisfies: 
 
 q   cmd : max(|&lt;{right arrow over ( m )}, ECI {right arrow over (z)} &gt;|) such that  q   cmd   ⊂A  
 
 where &lt;.,.&gt; is a mathematical symbol for the inner product, or dot product, of two vectors. 
 
         [0042]     If power safety can be maintained, the steering law of q cmd  max (|&lt;{right arrow over (m)},  ECI {right arrow over (z)}&gt;|) for the spacecraft  30 S will have a steering attitude such that the designated spin axis is aligned with the momentum vector. The control law will command wheel torque in a direction which is perpendicular to both the designated axis and the momentum vector (i.e., {right arrow over (m)}× ECI {right arrow over (z)} direction) to bring the two vectors to be co-aligned. This is the 3-axis stabilized controller replacing the existing WGWANC control law.  
         [0043]     Thus, the three-axis stabilized controller can perform WGWANC-like control functions in a slow-spin transfer orbit operation. The benefit of this steering law for the steering attitude is that it reduces the reaction wheel activities and power consumption. A derivative of this steering law is by maximizing the difference between the power received from solar panel and the power consumed by the reaction wheels  78 .  
         [0044]     During a bi-propellant transfer orbit, the spacecraft  30 S may be deliberately spun at a low rate (e.g., from about 0.3 to about 1.5 deg/sec), to remain within the Star Tracker tracking rate limit (e.g., &lt;3.0 deg/sec in sensor frame), and such that 3-axis stabilized controls can be used in lieu of the WGWANC controllers. The nominal spin rate may be set at only one-tenth of the STA tracking rate limit so that it will remain below the STA tracking rate limit, even after an unexpected thruster failure that spins up the spacecraft. The 3-axis stabilized controller has the option to use the momentum vector in Earth-centered inertial (ECI) coordinates as the z-axis target, similar to WGWANC controllers.  
         [0045]     The above steering law is merely an example, with more steering laws introduced below. The steering law can be derived by optimizing the reaction wheel momentum storage capability, given a steering attitude within the power safe attitude set. This will lead to placing the spin axis to where the environmental torque effect is a minimum and the reaction wheel pyramid has the maximum margin for momentum storage. The momentum accumulated due to environmental torques may be dumped whenever necessary, for example, in the subsequent reorientation or burn maneuvers.  
         [0046]     Momentum accumulation in a transfer orbit is less than 10 N-m-s in 12 hours in low perigee if the spin axis is parallel to the perigee velocity direction (based on data from Thuraya D1 spacecraft), and less than 4 N-m-s for any attitude after the perigee is raised to be over 500 Km (also based on data from Thuraya D1 spacecraft). Thrusters, such as bi-propellant thrusters  84 , may be used to dump momentum during reorientation or during a LAM burn. A momentum dumping operation may be performed at each burn, but the wheel momentum envelope is capable of storing momentum for multiple days. Existing flight software supports filtered (slow) wheel momentum dumping.  
         [0047]     The steering attitude can be optimized to be closer to the next LAM burn attitude (e.g., by minimizing the separation angle to the next LAM burn) to reduce next reorientation time and fuel consumption for the next LAM burn. This steering law may be used to place the coasting attitude as close as is practical to the next burn attitude as possible. The steering attitude may be set to maximize the difference between the power received by the solar panel and the power consumed by heaters, or to maximize power received by solar panel minus power consumed by heaters minus power consumed by the reaction wheels  78 . The steering attitude can also be an optimization of the combination of the aforementioned objectives. The steering law above can also be a rate-only steering law if there is no intention to control the spin phase. In general, the optimal steering attitude may not be fixed over time, and may be a time-varying attitude trajectory.  
         [0048]     The LAM overturning torque during a LAM burn is fixed in the spacecraft body frame. The magnitude is proportional to the LAM force and the moment arm between LAM force and the spacecraft center-of-mass. One potential advantage of slow spinning is lower LAM turn on/off transients (although the gyroscopic stiffness is lower at low spin rate, the thruster firing phase lag is also reduced). The transient is mainly due to the time lag in the acceleration estimation. By reducing the time constant of the acceleration estimation loop, one can generally reduce the transients. Furthermore, by re-initializing the estimated acceleration to an a priori value, either based on pre-launch LAM alignment survey or based on previous burn acceleration estimate, the transients can be virtually subdued.  
         [0049]     With reference to  FIG. 8 , a Thruster Controller (THC) computer software unit (CSU)  148  determines LAM burn window opening and closing times, and provides them to a LAM burn sequencer or ascending mode sequencer (ASM) CSU  150 . The LAM burn may also use 3-axis stabilized control at a slow-spin rate, and may use thrusters and/or the reaction wheels  78  to make attitude corrections during the LAM burn. LAM burn on-off transients may be reduced by estimating the overturning torque, and then feeding-forward the overturning torque in the form of an acceleration estimate to the ATD CSU  96 . This estimated acceleration due to the overturning torque can be stored in the ASM CSU  150 , and may be used to re-initialize the acceleration at the start of burn, and to reset the acceleration to zero at the end of the burn. In addition, the spacecraft  30 S may be reoriented prior to each coasting operation and prior to each LAM burn, for example, to maximize solar power during coasting, as noted above, and/or to minimize fuel needed for attitude control during each LAM burn.  
         [0050]     The timing for the LAM burn estimated acceleration re-initialization is as follows:  
         [0051]     Based on a pre-launch survey of LAM orientation and estimated center-of-mass and spacecraft inertia, an a priori estimated acceleration of LAM overturning torque, a 3×1 vector in unit of rad/sec/sec, is computed and stored in the ASM CSU  150 .  
         [0052]     When the LAM burn software window is open and the LAM is to fire, the ASM CSU  150  may reinitialize the estimated acceleration in the ATD CSU  96  to the value stored in the ASM CSU  150  to immediately compensate for the LAM overturning torque to reduce the turn on transient.  
         [0053]     When the LAM burn is about to end, the ASM CSU  150  may store the estimated acceleration from the ATD CSU  96  for use in the next LAM burn. Note that this end condition is very close to the initial condition for the next LAM burn.  
         [0054]     When the LAM stops firing, the ASM CSU  150  may immediately reinitialize the estimated acceleration in the ATD CSU  96  to zero to reduce the LAM turn off transient.  
         [0055]     A simultaneous power and stellar attitude acquisition sequencer may be provided for the bi-propellant spinning transfer orbit operation (when the solar wings  50  are stowed, using exposed solar panel currents). The sequencer may maintain a steady spin, and then configure and command the stellar attitude acquisition in parallel in the background processing. A steady spin may be achieved by stabilizing the spacecraft at a controlled rate determined by then estimated rate and momentum. At this point, spacecraft attitude may be initialized to an inaccurate, or even arbitrary, value. Inaccurate attitude does not affect spacecraft stabilization under this rate-only control law. Alternatively, a steady spin may be achieved by a 3-axis stabilized controller. Again, inaccurate attitude does not affect spacecraft stabilization either, as the control law may be referenced to an arbitrary inertial frame, offset to the ECI frame by the attitude error. The above spacecraft stabilization, controls and maneuvers can be achieved by the use of thrusters, reaction wheels, or a combination of thrusters and reaction wheels (e.g., use thrusters to dump the momentum to be within the wheel control authority, then use reaction wheels for attitude control).  
         [0056]     The sequencer may also synchronize the attitude estimate of the spacecraft  30 S with the panel current such that, for example, the identity attitude (spacecraft frame, at the current time, coaligns with the inertial frame) is synchronized with the panel peak current corresponding to the present sun polar angle, and an attitude corresponding to the identity attitude rotated through 90 degrees about the spacecraft spin axis is synchronized with the zero panel current. Therefore, by resetting the commanded and estimated attitude quaternions at the peak panel current, one can control the spacecraft to an identity attitude and bring the sun to the plane formed by the solar panel normal vector and the spin axis; so the exposed solar panels will be illuminated by the sun, providing maximum panel current for power safety.  
         [0057]     To provide a power safe, 3-axis stellar attitude acquisition for the wing-deployed spacecraft  30 D (solar wings  50  deployed, without the need of a sun sensor assembly (SSA)), a stellar attitude acquisition procedure may first stabilize the spacecraft  30 D, for example, by simultaneously performing a slow rotisserie maneuver for power safety and using STA attitude acquisition to acquire the spacecraft attitude. When the wing is deployed, a simple rotisserie maneuver at an appropriate rate along any axis perpendicular to the wing-rotation-axis can maintain power/thermal safety indefinitely (momentum safety can also be assured provided a solar tacking algorithm is in place). For non-XIP spacecraft, with the potential of high momentum due to faulty thruster stuck-on (an event classified as highly improbable in failure mode analysis), the reaction wheels  78  may be saturated if there are only 3 reaction wheels left, then a WGWANC-like controller is needed. The WGWANC-like controller may align the spacecraft momentum vector with the spacecraft spin axis, and reaction wheel momentum bias can be commanded in the same direction (super-spin) to reduce the spin rate to suit stellar attitude acquisition, or in the opposite direction (counter-spin) to intentionally increase the spin rate.  
         [0058]     Various examples of procedures for power/attitude acquisition in bi-propellant phase for the wing-stowed spacecraft  30 S (solar wing stowed, no SSA) will now be described:  
         [0059]     Wing current synced power acquisition: Owing to the slow spin, the spacecraft momentum after a failure is usually within the reaction wheel momentum envelope. If not, thrusters can be used in an open-loop or closed-loop control to dump the spacecraft momentum, such that the residual momentum is small enough for attitude control to be performed by reaction wheels. The controller will also select favorable spin direction and spin rate such that the reaction wheels will not be saturated. With reference to  FIG. 9 , after processor, sensors and actuators initialization at block  152 , at block  154  the z-axis is captured using a rate-only controller, the Stellar Attitude Acquisition (SAA)  92  is initiated after the spacecraft is stabilized, and the spacecraft  30 S may transition to a z-spin configuration, as indicated at block  156 . The spacecraft  30 S may then maintain a steady z-spin configuration as indicated at block  156 , by rate-only control using the reaction wheels  78  or thrusters, use the wing current sensor to measure the peak current and to detect the spin phase when the peak current occurred. If the peak current is over the power-safe threshold, the spacecraft  30 S is power safe and can remain in this state indefinetely. If the peak current is too low for power safety (e.g., sun to spin axis separation angle less than 70 deg), a maneuver may be performed (block  158 ) to bring the spacecraft  30 S to an x-spin configuration, as indicated at block  160 .  
         [0060]     As indicated in  FIG. 10 , the sun polar angle  162  when the spacecraft  30 S reaches the power-safe z-spin configuration will be within 90±20 deg. The sun polar angle is the separation angle between the spin axis and the line-of-sight to the sun.  FIG. 11  illustrates regions in which the position of the sun  164  relative to the spacecraft  30 S for either power safe or not power safe states.  FIG. 12  illustrates a configuration in which the spacecraft  30 S is not in a power safe state; As shown in the plots of  FIG. 13 , if the spacecraft is not power safe, the controller may detect peak current  166 , memorize or reset the attitude quaternion  168  at the peak current, and transition to x-spin. Peak current detection can be performed by various peak current finding algorithms. One algorithm is to perform a best sinusoidal fit to the measured sinusoidal panel current (panel current is sinusoidal in spin phase). Another algorithm is to find the largest value of the absolute value of the filtered panel current. By resetting the commanded and estimated attitude quaternions to identity at the peak current, the controller will stop the rotation at an attitude that the sun provides peak current (the magnitude of the peak current depends on sun polar angle. The maximum possible magnitude occurs when the sun polar angle is 90 degrees). The sun is therefore in the plane spans by the current spin axis and the panel normal vector. Since the next spin axis is orthogonal to the plane, the next spin axis is therefore also orthogonal to the sun, or a sun polar angle of 90 degrees, that provides the maximum possible panel current for power safety.  
         [0061]     Stellar attitude acquisition may be performed in parallel with power acquisition. Examples of stellar attitude acquisition and power acquisition systems and methods may be found in U.S. Pat. No. 6,470,270, issued to Needelman et al. on Oct. 22, 2002, and U.S. Pat. Nos. 6,571,156 and 6,588,708, issued to Wang et al. on May 27, 2003 and Jul. 8, 2003, respectively, all of which are owned by the assignee of the present application, and all of which are hereby expressly incorporated by reference herein.  
         [0062]     A wing current based, quaternion triggered, sun-spin-axis precession (reorientation) using a thruster may also be used for the spacecraft  30 S to reach a power safe attitude. An appropriate algorithm may be used to precess the spin axis either toward or away from the sun-line until power is maximized. The same 3-axis stellar attitude acquisition may be performed simultaneously to acquire the attitude.  
         [0063]     In addition, one may run the attitude acquisition mode as above, using the reaction wheels  78  or a thruster to stop the spin or to spin at slow rate, and simultaneously command 3-axis stellar attitude acquisition to acquire the attitude. This may be accomplished by budgeting battery power margin (for example, a Boeing GEM spacecraft currently has about 6 hours and typical Boeing BS702 spacecraft have about 15 hours of battery life after a failure) to allow sufficient time for attitude acquisition (&lt;0.5 hours), and then slewing to the desired power safe spin attitude. Two such attitudes are available, one is the ecliptic normal direction (normal direction of the earth orbit around the sun) with an achievable sun polar angle of 90 degrees, and the other is the equatorial normal (north/south direction of the Earth, also the north/south direction in the ECI frame). with sun polar angle in the range of 90±23.5 degrees for power safety. The following computed quaternion command, q cmd  will maneuver the spacecraft designated spin-axis to be equatorial normal for power safety: the equatorial normal in ECI is a unit vector of  ECI n=[0 0 1] T , let the spacecraft spin axis in ECI be  ECI z, then the slew direction and magnitude is v= ECI z× ECI n where the x is the vector cross product, and the slew quaternion is  
         q   slew     =       v        v          ⁢       sin   ⁡     (     0.5   ⁢       sin     -   1       ⁡     (        v        )         )       .           
 
 The commanded quaternion is computer by q cmd =q B     —     ECI ⊕q slew , where q B     —     ECI  is the spacecraft attitude estimated by stellar attitude acquisition algorithm, and the ⊕ is the quaternion product. 
 
         [0064]     Still further, one may use a binary halving method to find the maximum-power spin-axis in x/z plane using thrusters. This is a systematic trial and error method to find the spin axis in x/z plane that is perpendicular to the sun line at that instant.  
         [0065]     Acquired stellar attitude monitored with wing current threshold can be performed as follows:  
         [0066]     Let {right arrow over (s)} ECI  be the sun unit vector in the ECI frame, then {right arrow over (s)} B =C ECI   B {right arrow over (s)} ECI  is the sun unit vector in the body frame, where C ECI   B  is the attitude determined by the gyro and the star tracker.  
         [0067]     Let {right arrow over (u)} n  and {right arrow over (u)} s  be the normal unit vectors for north and south solar panels, and let I max  be the panel current when the sun is perfectly normal to the panel. Then, the predicted north panel current is I n =I max  ({right arrow over (u)} n ·{right arrow over (s)} B ), and the predicted south panel current is I s =I max ({right arrow over (u)} n ·{right arrow over (s)} B ).  
         [0068]     Let I measured  be the measured panel current from the Integrated Power Controllers (IPC), then the panel current residual is 
 
I measured,n −I n  
 
I measured,s −I s  
 
         [0069]     A small panel current residual is an indication that the acquired stellar attitude is correct. On the other hand, if the residual is too large, the stellar attitude reacquisition may need to be restarted.  
         [0070]     The bi-propellant transfer orbit can be performed with low spin or no spin at all (a substantially zero spin rate), using 3-axis stabilized controller. This will make the transfer orbit no different from on-station as far as attitude determination and control is concerned, and allow the spacecraft  30 S to have a modular and unified attitude determination and attitude control for both transfer orbit and on-station operations.  
         [0071]      FIG. 14 , shows exemplary software units that may be used for stellar attitude acquisition. The SAA CSU  92 , performing initial attitude acquisition without any prior attitude knowledge, may be initiated in the coasting phase under a rate-only controller or a WGWANC controller wherein change of the attitude knowledge does not affect the controlled attitude. The lost-in-space algorithm module  170 , may be, for example, but not limited to, that disclosed in U.S. Pat. No. 6,470,270 and U.S. Published Patent Application Number US 2004/0098177, both of which are hereby incorporated by reference herein. The lost-in-space algorithm module  170  may generate an initial attitude estimate, that may be inaccurate, and may use two consistent consecutive attitudes computed by measured star positions to verify that the attitude is acquired successfully.  
         [0072]     The SAA CSU  92  may include command data buffers  172 , adapted to store, send, and receive data to and from the STP CSU  88 , ATD CSU  96 , and the lost-in-space algorithm module  170 , as indicated in  FIG. 14 . The SAA CSU may also include a bias/attitude refinement (BAR) algorithm module  174  that may be used in conjunction with a bias correction gain computation module  176  and/or an attitude correction gain computation module  178  to provide gyro bias correction and/or attitude correction data, respectively, to the ATD CSU  96 .  
         [0073]     With reference to  FIG. 15 , the ATD CSU  96  may include a Kalman filter attitude update module  130 , a three axis propagation module  132 , a rate/acceleration update module  134 , and a time-match circular buffer module  136 .  
         [0074]     The acquired attitude may be checked with a computed wing current residual (measured wing current-predicted wing current using the acquired attitude) threshold. The attitude acquisition may be performed under a power safe condition where north/south solar panels constantly sweep through the sun at each revolution.  
         [0075]     Once acquired, the attitude may be updated continuously so long as the attitude residual (stellar attitude-gyro propagated attitude) is below a computed threshold on a per update basis. The attitude may be continuously checked by the wing current residual to protect globally against potential attitude error that can not be detected on a per update basis. If the attitude is accurate, the flight software will be able to predict when to see the current on which panel at what magnitude. The 3-axis attitude may be maintained by the gyro and tracker, and ready for the reorientation or the LAM burn at any time. Gyro bias estimate convergence is a prerequisite for the reorientation and LAM burn. Loss of attitude, if occurring after an anomaly, may lead to autonomously re-initiating the stellar attitude acquisition method. Loss of attitude during reorientation or LAM burn may lead to autonomously disabling the stellar attitude update and relying only on the gyro propagated attitude.  
         [0076]     A hemispherical inertial reference unit (HIRU) sensor processing CSU  98  may operate in parallel with processing of data from the star tracker unit  54 , in providing attitude data to the ATD CSU  96 . In addition, the flight star catalog (FSC)CSU  100  provides data to the lost-in-space algorithm module  170  and the BAR algorithm module  174 .  
         [0077]      FIG. 16  depicts an exemplary procedure  200  for stellar attitude determination in a spinning transfer orbit:  
         [0078]     Step  1  of the stellar attitude determination procedure  200  may include: stabilizing the spacecraft as indicated at block  202 . This may be done using a rate-only controller, a 3-axis stabilized controller and/or a WGWANC controller such that the spacecraft is power safe and the rate is below star tracker rate limit. Attitude error does not affect stabilization as only the relative attitude or the rate is used by the controller.  
         [0079]     Step  2  may include acquiring attitude using star tracker data, as indicated at block  204 . This may include: acquiring the stellar attitude using star tracker measurements. This operation may use data from a single star tracker, multiple star trackers, one Inertial Measurement Unit (IMU), or multiple IMU&#39;s. The methods using such data may be chosen from suitable attitude acquisition algorithms, such as those which use star matching, multiple star matching, and multiple consistently determined attitudes.  
         [0080]     Step  3  may include validating acquired stellar attitude, as indicated at block  206 . This may include: validating the acquired attitude by comparing subsequently determined stellar attitude, and/or subsequent star residuals, checking against solar panel current, checking against sun sensor measurement, and/or checking against any available sensors. If the acquired attitude is not validated, the operation of acquiring stellar attitude using star tracker data may be repeated  
         [0081]     Step  4  may include propagating attitude to real time as indicated at block  208 : periodically propagate the acquired attitude (or a previous attitude) to real time. The propagation may use IMU data, and/or star tracker estimated rate.  
         [0082]     Step  5  may include updating attitude using star residuals as indicated at block  210 : the propagated attitude may then be periodically updated by residuals from new star measurements or from newly determined stellar attitude. This update is typically only performed when the residual is within tolerable threshold. The residuals may also be used to update IMU parameters, such as gyro bias, scaling factor and misalignments.  
         [0083]     Step  6  may include monitoring the attitude, as indicated at block  212 . This may include: monitoring the attitude using difference to a subsequent determined stellar attitude, subsequent star residuals, checking against solar panel current, checking against a sun sensor measurement, and/or checking against any available sensors. It also may include checking the trending of updated IMU parameters to see if they are within a predetermined bound. As indicated at block  214 , if any of the above checks failed for a predetermined duration, returning to step  2  to reacquired attitude.  
         [0084]     Step  7  may include maneuvering the spacecraft, as indicated at block  216 : with attitude passing monitoring criteria above, one may perform spacecraft spin speed change, reorientation to a desired attitude, and may perform LAM burn for orbit change. If attitude monitoring fails to meet the desirable criteria, one may suspend the stellar update and only perform the gyro propagation. At the end of maneuver, step  2  may be repeated to reacquire the attitude.  
         [0085]     Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed herein and in the accompanying claims. For example, although the embodiments are illustrated in spacecraft applications, the invention can be used in space planes and other vehicles.