Abstract:
A system and associated method for compensating for thermal deformation of a spaced-based structure having a spacecraft payload. The system including a beacon source coupled to a first spacecraft for transmitting a first signal and a beacon sensor coupled to a second spacecraft for receiving the first signal and providing measurement data derived from the first signal. At least one attitude sensing device is coupled to the second spacecraft for determining estimated spacecraft attitude data for the second spacecraft. A processor on the second spacecraft is configured to process ephemeris data of the first and second spacecraft, beacon measurement data, estimated spacecraft attitude data, and gimbal angular position data to estimate spacecraft structural deformation.

Description:
RELATED APPLICATION  
       [0001]     This application is a divisional application of U.S application Ser. No. 10/929,844, filed Aug. 30, 2004 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates generally to spacecraft attitude determination and control systems, and more specifically to a method and system for determining spacecraft structural thermal deformation using a space-based beacon for accurate spacecraft pointing.  
       BACKGROUND  
       [0003]     Inaccurate pointing of a spacecraft payload can significantly reduce spacecraft effectiveness. For example, inaccurate pointing of a communication antenna can significantly reduce spacecraft communication throughput as well as reduce the communication coverage.  
         [0004]     Structural thermal deformation is always a major error contributor to spacecraft payload pointing. Accordingly, to achieve high pointing accuracy, the amount of spacecraft structural thermal deformation needs to be accurately determined and compensated.  
         [0005]     As a result, there is a need for achieving high pointing accuracy of a spacecraft by accurately accounting for spacecraft structural thermal deformation.  
       SUMMARY  
       [0006]     A method is disclosed herein for achieving payload pointing accuracy of a spacecraft, such as a communications satellite. The pointing accuracy is achieved by accurately accounting for spacecraft structural thermal deformation. This accuracy is determined by using a spaced-based beacon from another spacecraft. This spaced-based beacon may be a beacon dedicated for use in the present invention. Alternatively, the beacon may have other uses such an RF cross-links or laser cross-links of the two spacecraft.  
         [0007]     In one aspect of the invention, a method of determining structural thermal deformation is provided. The method includes the acts of: method of determining structural thermal deformation comprising: determining ephemeris of a first spacecraft and a second spacecraft; receiving a beacon from the second spacecraft at a beacon sensor co-located with a payload of the first spacecraft; determining the direction of the received beacon with respect to the attitude of the first spacecraft; determining the attitude of the first spacecraft using at least one attitude sensor; and calculating a structural thermal deformation between the at least one attitude sensor and the payload using the ephemeris of the first and second spacecraft, the direction of the received beacon, and the attitude of the first spacecraft.  
         [0008]     Beneficially, the present invention enables precision attitude control system design that does not rely on any ground-based beacons. This feature is advantageous because a ground-based beacon is not always available for various reasons. In addition, certain spacecraft payloads such as RF and laser cross-link antennas may not be Earth pointed and require space based beacons for pointing references.  
         [0009]     By accurately determining structural thermal deformation, the present invention enables the reduction of spacecraft payload pointing error by two to three times.  
         [0010]     The scope of the invention is defined by the claims, which are incorporated into this section by reference. A more complete understanding of embodiments of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more embodiments. Reference will be made to the appended sheets of drawings that will first be described briefly. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  illustrates a beacon transmitting spacecraft and a beacon receiving spacecraft in orbit around the Earth.  
         [0012]      FIG. 2  is a simplified illustration of the beacon receiving spacecraft in accordance with the present invention.  
         [0013]      FIG. 3  is a computational diagram for a calculation of thermal deformation for the beacon receiving spacecraft in accordance with an embodiment of the present invention.  
         [0014]      FIG. 4  is a flowchart for a calculation of thermal deformation for the beacon receiving spacecraft in accordance with an embodiment of the present invention. 
     
    
       [0015]     Embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures.  
       DETAILED DESCRIPTION  
       [0016]     This invention provides a solution for spacecraft structural thermal deformation determination using a space-based beacon to improve spacecraft pointing error. Turning now to the Figures, a first spacecraft  100  and a second spacecraft  102  are shown in orbit in  FIG. 1 . First spacecraft  100  and second spacecraft  102  can be any type of spacecraft that are capable of being placed in Earth orbiting orbits. As will be described further herein, second spacecraft  102  may correct for thermal deformation that affects pointing of a payload in a specific direction by processing a beacon signal  104  received from first spacecraft  100 . First spacecraft  100  and second spacecraft  102  may be, for example, communications satellites with Earth pointing antennas or cross-link antennas, which need to be pointed accurately. The first spacecraft  100  may be a spacecraft that carries only beacon sources, such as a GPS spacecraft.  
         [0017]      FIG. 2  is a simplified view of an exemplary spacecraft  200 , which represents an embodiment of either first spacecraft  100  or second spacecraft  102 . Should spacecraft  200  represent an embodiment of first spacecraft  100 , it will include a beacon source  202 . Beacon source  202  transmits beacon signal  104 , which can be received by second spacecraft  102  (see  FIG. 1 ). Beacon source  202  in spacecraft  200  can be a dedicated signal source specifically used to assist the pointing of the spacecraft. Alternatively, beacon source  202  can be an RF or laser crosslink that is used to establish a communication link between spacecrafts.  
         [0018]     Alternatively, should spacecraft  200  represent an embodiment of second spacecraft  102 , it will include a beacon sensor  204 , a spacecraft payload  206 , a processor disposed in spacecraft bus  208 , and at least one attitude sensor  210 , such as a star tracker. Beacon sensor  204  may be co-located with spacecraft payload  206 , or alternatively, beacon sensor  204  may not be required if, for example, spacecraft payload  206  provides the same functionality as beacon sensor  204 .  
         [0019]     Spacecraft payload  206  may be any payload that requires precise pointing, such as a phased array antenna, a laser communication terminal, and the like. Spacecraft payload  206  is mechanically coupled to spacecraft  200  and is therefore subject to perturbations caused by the thermal deformation of the structural components of spacecraft  200 , even though star tracker  210  mounted on spacecraft bus  208  can provide accurate pointing of spacecraft bus  208 . It is the structural perturbations between star tracker  210  and spacecraft payload  206  that may cause the pointing accuracy of spacecraft payload  206  to be degraded. The pointing (orientation) of spacecraft payload  206  may be steered by mechanical gimbals. Alternatively, payload  206  may be fixed with respect to spacecraft bus  208 .  
         [0020]     As will be explained further herein, the accounting for the structural perturbations causing pointing accuracy degradation for payload  206  uses ephemeris of first spacecraft  100  and second spacecraft  102 . Ephemeris of first spacecraft  100  and second spacecraft  102  can be uploaded from ground to second spacecraft  102 . Alternatively, ephemeris of first spacecraft  100  can be computed by a processor located on-board second spacecraft  102 . In addition to the ephemeris just described, the pointing attitude of the second spacecraft  102  as measured by the one to a plurality of attitude sensors  210 , such as star trackers and the like, and also the pointing angle of the received beacon  104  with respect to payload  106  are also used to determine the structural thermal deformation. Given the ephemeris of first spacecraft  100  and second spacecraft  102 , the pointing angle of received beacon signal  104  with respect to payload  106 , and the estimated attitude of second spacecraft  102 , the processor on second spacecraft  102  computes direction vectors of beacon signal  104 .  
         [0021]     As described in detail below, the direction vectors are computed in two representative coordinate frames. The first frame is defined prior to structural thermal deformation. The second frame is defined by including the deformation. In accordance with the present invention, the two direction vectors are used to determine the thermal deformation between star tracker  210  and spacecraft payload  206 . Quantifying the thermal deformation in this manner makes it possible to account for and compensate for pointing errors caused by the thermal deformation. The determination of the direction vectors will now be further explained as follows.  
         [0000]     A) Direction Vector Determination  
         [0022]     The calculation of the direction vectors for received beacon  104  may be better understood with reference to a computational diagram shown in  FIG. 3 . A direction vector determination  312  for the received beacon  104  depends, in part, upon spacecraft attitude estimates from a spacecraft attitude determination  310  based upon measurements made by the at least one attitude sensor  210 . Direction vector determination also depends upon spacecraft ephemeris  306  that includes the ephemeris for the source of beacon  104  at spacecraft  100  and the ephemeris for spacecraft  102 . In addition, direction vector determination also depends upon the pointing angle for received beacon  104 . In embodiments wherein payload  206  (and hence beacon sensor  204 ) mounts to spacecraft  102  through payload gimbals  302 , the pointing angle for the received beacon  104  will depend upon payload gimbal angles. In addition, the pointing angle will also depend upon beacon sensor measurements  304 .  
         [0023]     Upon processing this information, it may be shown that the unit beacon vector in inertial reference frame is given as follows:  
                 i     ⁢       u   ^     beacon     =                 i     ⁢       v   ^     SC_beacon     ⁢     -   i     ⁢       v   ^     SC_sensor              i     ⁢         v   ^     SC_beacon     ⁢     -   i     ⁢       v   ^     SC_sensor                  
 
 where  i {circumflex over (V)} SC     —     sensor  is the position vector of beacon sensor  204  in the inertial frame of reference as computed from spacecraft ephemeris for spacecraft  102 , and  i {circumflex over (V)} SC     —     beacon  is the position vector of first spacecraft  100  (and hence the position vector for the source of beacon  104 ) in the inertial frame of reference as computed from spacecraft ephemeris for first spacecraft  100 . 
 
         [0024]     It will be appreciated by those of ordinary skill that what is ultimately desired to be determined is the impact of thermal structural deformation on the orientation of payload  206 . This impact may be defined with respect to roll (θ x ), pitch (θ y ) and yaw (θ z ) angles with respect to the spacecraft frame of reference as shown in  FIG. 2 . These angles form a vector {circumflex over (θ)} DEF . The gimbal angles for payload  206  may then be adjusted accordingly to correct for this thermal deformation effect as represented by vector {circumflex over (θ)} DEF . Alternatively, the orientation of spacecraft  102  may be adjusted should payload  106  be rigidly mounted to spacecraft  102  without the use of gimbals.  
         [0025]     To derive {circumflex over (θ)} DEF  from  i û beacon , it may be shown that these two variables are related as follows: 
 
 i   û   beacon   =C   SC     —     i ( q   SC ) T   C   SCDEF     —     SC ({circumflex over (θ)} DEF ) T   C   sensorGIM     —     SCDEF ({circumflex over (θ)} ALIGN ) T   C   sensorLOS     —     sensorGIM ({circumflex over (θ)} sensorGIM ) T sensorLOS   û   beacon  
 
 where C SC     —     i (q SC ) is the direction cosine matrix from inertial frame to spacecraft frame, q SC  is the quoterion of spacecraft attitude in inertial frame that is determined by spacecraft attitude determination, C SCDEF     —     SC ({circumflex over (θ)} DEF ) is the direction cosine matrix from spacecraft frame to the frame after structural deformation,  
           θ   ^     DEF     =     [           θ   x               θ   y               θ   z           ]         
 
 is the angular vector as just described, C sensorGIM     —     SCDEF ({circumflex over (θ)} ALIGN ) is the alignment direction cosine matrix of beacon sensor gimbals, {circumflex over (θ)} ALIGN  is the known alignment angular vector, C sensorLOS     —     sensorGIM ({circumflex over (θ)} sensorGIM ) is the direction cosine matrix from sensor gimbals base frame to sensor LOS frame, {circumflex over (θ)} sensorGIM  is the gimbals axes rotation vector that is known or is measured by gimbals control subsystem, and  sensorLOS û beacon  is the unit beacon vector in sensor LOS frame that is measured by beacon sensor  204 . 
 
         [0026]     To solve for {circumflex over (θ)} DEF , the preceding expression for  i û beacon  may be rewritten as follows:  
                 [         1         θ   z           -     θ   y                 -     θ   z           1         θ   x               θ   y           -     θ   x           1         ]     ⁢       u   ^     beacon               SC       =       u   ^     beacon               SCDEF             Equation   ⁢           ⁢     (   1   )               
 
 where  SC û beacon  is the unit direction vector for the beacon in the spacecraft  102  frame of reference without thermal deformation, and  SCDEF û beacon  is the unit direction vector for the beacon in the spacecraft  102  frame of reference with thermal deformation. The vector  SC û beacon  is related to the  i û beacon  vector by the following relationship:  
           u   ^     beacon               SC     =           C   SC_i     ⁡     (     q   SC     )       ⁢       u   ^     beacon               i       =     [           a   1               a   2               a   3           ]           
 
 whereas the vector  SCDEF û beacon  may be determined from the following expression:  
                 u   ^     beacon               SCDEF     =       ⁢         C   sensorGIM_SCDEF     ⁡     (       θ   ^     ALIGN     )       T                     ⁢           C   sensorLOS_sensorGIM     ⁡     (       θ   ^       sensor   ⁢           ⁢   GIM       )         T   ⁢           ⁢   sensor   ⁢           ⁢   LOS       ⁢       u   ^     beacon                   =       ⁢     [           b   1               b   2               b   3           ]               
 
 where the variables are defined as discussed above. Using these expressions for vectors  SCDEF û beacon  and  SC û beacon , equation (1) may be rewritten as:  
                 [         0         -     a   3             a   2               a   3         0         -     a   1                 -     a   2             a   1         0         ]     ⁡     [           θ   x               θ   y               θ   z           ]       =       [             b   1     -     a   1                   b   2     -     a   2                   b   3     -     a   3             ]     =     [           m   1               m   2               m   3           ]               Equation   ⁢           ⁢     (   2   )               
 
         [0027]     Equation (2) is the measurement equation of structural thermal deformation. As set forth above, the parameters of this equation are computed from gimbals alignment angles and gimbals axes angles ( 302 ), beacon sensor measurements ( 304 ), spacecraft positions of first spacecraft  100  and second spacecraft  102  from their respective ephemeris ( 306 ), and spacecraft attitude estimates ( 310 ). Equation (2) may be simplified to:  
                 H   ⁢           ⁢     θ   DEF       =   M     ⁢     
     ⁢   where   ⁢     
     ⁢       H   =     [         0         -     a   3             a   2               a   3         0         -     a   1                 -     a   2             a   1         0         ]       ,     M   =       [           m   1               m   2               m   3           ]     .                 Equation   ⁢           ⁢     (   3   )               
 
 By solving Equation (3) for the variable θ DEF , the effect of structural thermal deformation of second spacecraft  102  between attitude sensor  210  and beacon sensor  204  (and therefore the spacecraft payload  206 ) can be determined. 
 
 B) Thermal Deformation Update Equation 
 
         [0028]     It will be appreciated that to continuously receive beacon signal  104  may be impossible due to the fact that the payload pointing angle is not fixed and may be steered away from the beacon source from time to time. However, due to regular heating of a geosynchronous satellite by the sun, the thermal deformation is predictable and repeatable over a 24-hour period. Accordingly, the structural thermal deformation of the second spacecraft  102  over time may be modeled and predicted, using, for example, harmonic analysis. The model may be updated based upon periodic updates. Alternatively, the model may be updated using non-periodic updates.  
         [0029]     For example, due to the periodic nature of the spacecraft thermal environment in Earth orbit, structural thermal deformation over time can be decomposed into harmonic components in a harmonic equation as follows:  
         [           ⁢             x     1   ⁢   _s       ⁡     (     k   +   1     )                   x     1   ⁢   _c       ⁡     (     k   +   1     )               ⋮               x   N_s     ⁡     (     k   +   1     )                   x   N_c     ⁡     (     k   +   1     )                   y     1   ⁢   _s       ⁡     (     k   +   1     )                   y     1   ⁢   _c       ⁡     (     k   +   1     )               ⋮               y   N_s     ⁡     (     k   +   1     )                   y   N_c     ⁡     (     k   +   1     )                   z     1   ⁢   _s       ⁡     (     k   +   1     )                   z     1   ⁢   _c       ⁡     (     k   +   1     )               ⋮               z   N_s     ⁡     (     k   +   1     )                   z   N_c     ⁡     (     k   +   1     )             ⁢           ]     =       [           ⁢           c     ω   1             s     ω   1                                                                                                                                                                             -     s     ω   1               c     ω   1                                                                                                                                                                                                   ⋰                                                                                                                                                                                                 c     ω   N             s     ω   N                                                                                                                                                                             -     s     ω   N               c     ω   N                                                                                                                                                                                                     c     ω   1             s     ω   1                                                                                                                                                                             -     s     ω   1               c     ω   1                                                                                                                                                                                                   ⋰                                                                                                                                                                                                 c     ω   N             s     ω   N                                                                                                                                                                             -     s     ω   N               c     ω   N                                                                                                                                                                                                     c     ω   1             s     ω   1                                                                                                                                                                             -     s     ω   1               c     ω   1                                                                                                                                                                                                   ⋰                                                                                                                                                                                                 c     ω   N             s     ω   N                                                                                                                                                                             -     s     ω   N               c     ω   N             ⁢           ]     [           ⁢             x     1   ⁢           ⁢   _s       (   k   ⁢           )                 x     1   ⁢           ⁢   _c       (   k   ⁢           )             ⋮               x   N_s     ⁡     (   k   )                   x   N_c     ⁡     (   k   )                   y     1   ⁢           ⁢   _s       ⁡     (   k   )                   y     1   ⁢           ⁢   _c       ⁡     (   k   )               ⋮               y   N_s     ⁡     (   k   )                   y   N_c     ⁡     (   k   )                   z     1   ⁢           ⁢   _s       ⁡     (   k   )                   z     1   ⁢           ⁢   _c       ⁡     (   k   )               ⋮               z   N_s     ⁡     (   k   )                   z   N_c     ⁡     (   k   )             ⁢           ]         
 
 where c ω     i   =cos ω i Δt, s ω     i   =sin ω i Δt are the sine and cosine functions of the i th  harmonic frequency of the spacecraft orbit period, and x i     —     s , x i     —     c , y i     —     s , y i     —     c , z i     —     s , z i     —     c  are the i th  harmonics components of thermal deformation in x-, y-, z- axes as shown in  FIG. 2 . In this example, the indices k and k+1 represent arbitrary periodic sampling times separated by the sampling period. It will be appreciated, however, that this harmonic analysis may be generalized for non-periodic updates. 
 
         [0030]     Therefore, the structural thermal deformation determination update equation can be represented by:  
               [             θ   x     ⁡     (   k   )                   θ   y     ⁡     (   k   )                   θ   z     ⁡     (   k   )             ]     =       ⁢     [           ⁢         1       1       …       1       1                                                                                                                                                                                               1       1       …       1       1                                                                                                                                                                                               1       1       …       1       1         ⁢           ]                     ⁢     [           ⁢             x     1   ⁢           ⁢   _s       (   k   ⁢           )                 x     1   ⁢           ⁢   _c       (   k   ⁢           )             ⋮               x   N_s     ⁡     (   k   )                   x   N_c     ⁡     (   k   )                   y     1   ⁢           ⁢   _s       ⁡     (   k   )                   y     1   ⁢           ⁢   _c       ⁡     (   k   )               ⋮               y   N_s     ⁡     (   k   )                   y   N_c     ⁡     (   k   )                   z     1   ⁢           ⁢   _s       ⁡     (   k   )                   z     1   ⁢           ⁢   _c       ⁡     (   k   )               ⋮               z   N_s     ⁡     (   k   )                   z   N_c     ⁡     (   k   )             ⁢           ]               
 
 The harmonic equation and thermal deformation update equation can be written in the following simplified matrix and vector forms: 
 
 V   k+1   =AV   k  
 
θ DEF     —     k =CV k  
 
 C) Thermal Deformation Estimation Equations 
 
         [0031]     Combining Equation (3) with the preceding harmonic and thermal update equations yields the following estimation equations to determine structural thermal deformation (s 406 ): 
 
 {circumflex over (V)}   k+1   =A{circumflex over (V)}   k   +G ( M   k   −H   k   C{circumflex over (V)}   k ) 
 
{circumflex over (θ)} DEF     —     k =C{circumflex over (V)} k  
 
 where G is the estimation gain matrix. The thermal deformation calculation and model update process may be summarized with respect to a flowchart  400  shown in  FIG. 4 . 
 
         [0032]     Embodiments described above illustrate but do not limit the invention. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present invention. Accordingly, the scope of the invention is defined only by the following claims.