Patent Publication Number: US-2022236078-A1

Title: Single star-based orientation method using dual-axis level sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority from Chinese Patent Application No. 202111348637.6, filed on Nov. 15, 2021. The content of the aforementioned applications, including any intervening amendments thereto, is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     This application relates to absolute attitude measurement, and more particularly to a single star-based orientation method using a dual-axis level sensor. 
     BACKGROUND 
     Servo star sensors are designed to enable the all-day observation during the measurement of astronomical position and attitude on the ground. By means of the small view field of the servo star sensors, the celestial objects are observable in the daytime, and the observation of astronomical objects is changed from passive capture to active star-seeking. The active star-seeking process includes astronomical prediction, motion compensation and image stabilization-based tracking, and is difficult to implement. According to the principle of attitude solution, it is necessary to observe at least two non-parallel space vectors to obtain the attitude information. In view of this, with regard to the servo star sensors, the observation target needs to be continuously changed to achieve the attitude measurement. 
     Regarding the current servo star sensor-based orientation, there is still a lack of a single star observation-based orientation strategy, and the dual-axis level sensor information-based fusion method is also absent. 
     For the existing servo star sensors, the active star-seeking is usually adopted for the attitude measurement, in which a priori information is introduced to predict the orientation of 3 to 5 celestial objects, and a servo is guided by the outline attitude information for observation. The active star-seeking has acceptable reliability under quasi-static conditions; under dynamic conditions, affected by the carrier motion, the later observation targets are prone to fall out of the observation field of view. Though some star sensors use inertial components such as gyroscopes to compensate for attitude changes of the carrier during the observation, the small observation view field still fails to enable the reliable observation. 
     A miss distance is used in a control strategy of the single star tracking for control feedback, which greatly enhances the control robustness. This method is often used in some geodetic astronomical surveys for orientation, but the fine adjustment of the measuring instrument is strictly required, which is difficult to achieve for dynamic carriers. 
     In summary, there is still a lack of a convenient single star-based orientation method that can reach the stable and reliable orientation with high update rate under dynamic conditions. A multi-star simultaneous solution is not only complicated in control strategy, but also difficult in the conversion of the calculation reference system, failing to obtain high-precision orientation information. 
     SUMMARY 
     In order to solve the above-mentioned problems, the present disclosure provides a single star-based orientation method using a dual-axis level sensor, which enables the orientation survey with a single astronomical object and improves a data update rate and measurement accuracy of astronomical orientation by measuring a redundant non-level information of the dual-axis level sensor. 
     Technical solutions of this application are described as follows. 
     This application provides a single star-based orientation method using a dual-axis level sensor, comprising: 
     (S1) denoting any two orthogonal side surfaces of a hexahedron on a star sensor as a first side surface and a second side surface, respectively; denoting a component of a vector ν right  on an X n -axis of a quasi-horizontal reference system as t 4 ; and denoting a component of the vector ν right  on a Z n -axis of the quasi-horizontal reference system as t 6 ; 
     wherein the vector ν right  is obtained by rotating a normal vector of the second side surface around the Z n -axis of the quasi-horizontal reference system by a preset azimuth angle θ dir   right ; 
     (S2) calculating the t 4  and t 6  through the following equations: 
     
       
         
           
             { 
             
               
                 
                   
                     
                       
                         t 
                         4 
                       
                       = 
                       
                         - 
                         
                           ( 
                           
                             
                               sin 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 αsin 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       Δ 
                                       ⁢ 
                                       
                                         θ 
                                         x 
                                         pitch 
                                       
                                     
                                     - 
                                     
                                       θ 
                                       x 
                                       pitch 
                                     
                                   
                                   ) 
                                 
                               
                             
                             + 
                             
                               cos 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 αsin 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       Δ 
                                       ⁢ 
                                       
                                         θ 
                                         y 
                                         pitch 
                                       
                                     
                                     - 
                                     
                                       θ 
                                       y 
                                       pitch 
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                 
                 
                   
                     
                       
                         t 
                         6 
                       
                       = 
                       
                         ( 
                         
                           
                             cos 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               αsin 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     Δ 
                                     ⁢ 
                                     
                                       θ 
                                       x 
                                       pitch 
                                     
                                   
                                   - 
                                   
                                     θ 
                                     x 
                                     pitch 
                                   
                                 
                                 ) 
                               
                             
                           
                           - 
                           
                               
                           
                           ⁢ 
                           
                             sin 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               αsin 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     Δ 
                                     ⁢ 
                                     
                                       θ 
                                       y 
                                       pitch 
                                     
                                   
                                   - 
                                   
                                     θ 
                                     y 
                                     pitch 
                                   
                                 
                                 ) 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               ; 
             
           
         
       
     
     wherein α is an angle between an X s -axis of a dual-axis level sensor reference system and an X-axis of a hexahedron reference system on the star sensor; θ x   pitch  is a measurement reading of the dual-axis level sensor on the X s -axis; O y   pitch  is a measurement reading of the dual-axis level sensor on a Y s -axis of the dual-axis level sensor reference system; Δθ x   pitch  is a bias of the dual-axis level sensor on the X s -axis; and Δθ y   pitch  is a bias of the dual-axis level sensor on the Y s -axis; 
     (S3) denoting an observation vector of a single astronomical object in the hexahedron reference system as ν PRI  and denoting a reference vector of the single astronomical object in an inertial reference system as ν GND , wherein the reference vector of the single astronomical object is obtained from a star catalog, and the ν GND  and ν PRI  satisfy the following equation: 
         R   z (θ z )· R   x (θ x )· R   y (θ y )·ν GND =ν PRI ;
 
     wherein R y (θ y ) denotes that the reference vector ν GND  rotates around a Y-axis of the hexahedron reference system by an azimuth angle θ y ; R x (θ x ) denotes that the reference vector ν GND  rotates around the X-axis of the hexahedron reference system by a pitch angle θ x ; and R z (θ 2 ) denotes that the reference vector ν GND  rotates around a Z-axis of the hexahedron reference system by a roll angle θ z ; 
     (S4) calculating the pitch angle θ x  and the roll angle θ z  according to the t 4  and t 6  through the following equations: 
     
       
         
           
             { 
             
               
                 
                   
                     
                       
                         θ 
                         x 
                       
                       = 
                       
                         
                           - 
                           arcsin 
                         
                         ⁢ 
                         
                           t 
                           6 
                         
                       
                     
                   
                 
                 
                   
                     
                       
                         θ 
                         z 
                       
                       = 
                       
                         - 
                         
                           arcsin 
                           ⁡ 
                           
                             ( 
                             
                               
                                 
                                   t 
                                   4 
                                 
                                 / 
                                 cos 
                               
                               ⁢ 
                               
                                 θ 
                                 x 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               ; 
             
           
         
       
     
     and 
     (S5) letting ν GND0 =R x (−θ x )·R 2 (−θ 2 )·θ PRI  and obtaining ν GND =R y (−θ y )·ν GND0 ; and calculating the azimuth angle θ y  through the following equation to complete a single star-based orientation: 
     
       
         
           
             
               
                 θ 
                 y 
               
               = 
               
                 arctan 
                 ⁢ 
                 2 
                 ⁢ 
                 
                   ( 
                   
                     
                       
                         
                           v 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             1 
                             · 
                             v 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           6 
                         
                         - 
                         
                           v 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             3 
                             · 
                             v 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           4 
                         
                       
                       
                         
                           v 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             4 
                             2 
                           
                         
                         + 
                         
                           v 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             6 
                             2 
                           
                         
                       
                     
                     , 
                     
                         
                     
                     ⁢ 
                     
                       
                         
                           v 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             1 
                             · 
                             v 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           4 
                         
                         + 
                         
                           v 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             3 
                             · 
                             v 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           6 
                         
                       
                       
                         
                           v 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             4 
                             2 
                           
                         
                         + 
                         
                           v 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             6 
                             2 
                           
                         
                       
                     
                   
                   ) 
                 
               
             
             ; 
           
         
       
     
     wherein [ν1 ν2 ν3]=ν GND ; ν1-ν 3  are components of the ν GND  on three axes of the inertial reference system, respectively; [ν4 ν5 ν6] T =ν GND0 ; and ν4-ν6 are components of the ν GND0  on the X n -axis, the Y n -axis and the Z n -axis, respectively. 
     In some embodiments, in the step (S1), the preset azimuth angle θ dir   right  is set through steps of: 
     defining the X n -axis of the quasi-horizontal reference system as a projection of the X-axis of the hexahedron reference system on a horizontal plane; defining a Y n -axis of the quasi-horizontal reference system such that the Y n -axis is orthogonal to the X n -axis on the horizontal plane and a Z n -axis of the quasi-horizontal reference system satisfies a right-hand rule, and has a general upward pointing direction; 
     measuring, by a first theodolite and a second theodolite, a normal pitch angle θ pitch   front  of the first side surface and a normal pitch angle θ pitch   right  of the second side surface, respectively; wherein in the quasi-horizontal reference system, a normal vector ν front  of the first side surface is expressed as: 
     
       
         
           
             
               
                 v 
                 front 
               
               = 
               
                 
                   
                     
                       R 
                       y 
                     
                     ⁡ 
                     
                       ( 
                       
                         θ 
                         pitch 
                         
                           f 
                           ⁢ 
                           r 
                           ⁢ 
                           o 
                           ⁢ 
                           n 
                           ⁢ 
                           t 
                         
                       
                       ) 
                     
                   
                   · 
                   
                     [ 
                     
                       
                         
                           0 
                         
                       
                       
                         
                           0 
                         
                       
                       
                         
                           1 
                         
                       
                     
                     ] 
                   
                 
                 = 
                 
                   [ 
                   
                     
                       
                         
                           vf 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                       
                     
                     
                       
                         0 
                       
                     
                     
                       
                         
                           vf 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           3 
                         
                       
                     
                   
                   ] 
                 
               
             
             ; 
           
         
       
     
     wherein R y (θ pitch   front ) denotes that a vector [0 0 1] rotates around a Y n -axis of the quasi-horizontal reference system by the normal pitch angle θ pitch   front ; and νƒ1,0 and νƒ3 are components of the normal vector ν front  on the X n -axis, Y n -axis and Z n -axis, respectively; 
     a normal vector  ν   right  of the second side surface without rotating by the preset azimuth angle θ dir   right  is expressed as: 
     
       
         
           
             
               
                 
                   v 
                   _ 
                 
                 right 
               
               = 
               
                 
                   
                     
                       R 
                       y 
                     
                     ⁡ 
                     
                       ( 
                       
                         θ 
                         pitch 
                         right 
                       
                       ) 
                     
                   
                   · 
                   
                     [ 
                     
                       
                         
                           0 
                         
                       
                       
                         
                           0 
                         
                       
                       
                         
                           1 
                         
                       
                     
                     ] 
                   
                 
                 = 
                 
                   [ 
                   
                     
                       
                         
                           vf 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                       
                     
                     
                       
                         0 
                       
                     
                     
                       
                         
                           vf 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           3 
                         
                       
                     
                   
                   ] 
                 
               
             
             ; 
           
         
       
     
     wherein R y (θ pitch   right ) denotes that the vector [00 1] rotates around the Y n -axis of the quasi-horizontal reference system by the normal pitch angle θ pitch   right ; and νr1,0 and νr3 are components of the normal vector  ν   right  on the X n -axis, Y n -axis and Z n -axis, respectively;
         supposing that the vector ν right  is obtained by rotating the vector  ν   right  by the preset azimuth angle θ dir   right  and expressing the vector ν right  as follows:       

     
       
         
           
             
               
                 v 
                 right 
               
               = 
               
                 
                   
                     
                       R 
                       z 
                     
                     ⁡ 
                     
                       ( 
                       
                         θ 
                         dir 
                         right 
                       
                       ) 
                     
                   
                   · 
                   
                     [ 
                     
                       
                         
                           
                             vr 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
                       
                       
                         
                           0 
                         
                       
                       
                         
                           
                             vr 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             3 
                           
                         
                       
                     
                     ] 
                   
                 
                 = 
                 
                   [ 
                   
                     
                       
                         
                           vr 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             1 
                             · 
                             cos 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             θ 
                             dir 
                             right 
                           
                         
                       
                     
                     
                       
                         
                           
                             - 
                             vr 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             1 
                             · 
                             sin 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             θ 
                             dir 
                             right 
                           
                         
                       
                     
                     
                       
                         
                           vr 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           3 
                         
                       
                     
                   
                   ] 
                 
               
             
             ; 
           
         
       
     
     wherein R z (θ dir   right ) denotes that a vector [νr 0 νr3] rotates around the Z n -axis by the preset azimuth angle θ dir   right ; and 
     according to an orthogonal relationship between the normal vector ν front  and the normal vector ν right , obtaining the following equation: 
     
       
         
           
             
               ( 
               
                 
                   θ 
                   dir 
                   right 
                 
                 = 
                 
                   a 
                   ⁢ 
                   
                     cos 
                     ⁡ 
                     
                       ( 
                       
                         
                           
                             - 
                             v 
                           
                           ⁢ 
                           f 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             3 
                             · 
                             vr 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           3 
                         
                         
                           vf 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             1 
                             · 
                             vr 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                       
                       ) 
                     
                   
                 
               
               ) 
             
             . 
           
         
       
     
     In some embodiments, an axis of a measuring lens barrel of the first theodolite coincides with a normal of the first side surface of the hexahedron, and an axis of a measuring lens barrel of the second theodolite coincides with a normal of the second side surface of the hexahedron. 
     In some embodiments, in the step (S2), the angle α, the bias Δθ x   pitch  and the bias Δθ y   pitch  are obtained through steps of: 
     defining the X n -axis of the quasi-horizontal reference system as a projection of the X-axis of the hexahedron reference system on a horizontal plane; defining a Y n -axis of the quasi-horizontal reference system such that the Y n -axis is orthogonal to the X n -axis on the horizontal plane and the Z n -axis of the quasi-horizontal reference system satisfies a vertically-upward right-hand rule; allowing the dual-axis level sensor reference system to be coplanar with the hexahedron reference system; and expressing a direction vector of the X s -axis in the hexahedron reference system as [cos α 0−sin α] and a direction vector of the Y s -axis in the hexahedron reference system as [−sin α 0−cos α]; 
     denoting a normal vector of the first side surface as ν front ; and obtaining a normal vector ν up  of a third side surface orthogonal to both the first side surface and the second side surface by cross product, expressed as: 
       ν up =ν right ×ν front ;
 
     obtaining a transformation matrix R GND2PRI  from the quasi-horizontal reference system to the hexahedron reference system, expressed as: 
         R   GND2PRI =[ν front ν right ν up ];
 
     obtaining a transpose matrix of the transformation matrix R GND2PRI , expressed as: 
     
       
         
           
             
               
                 R 
                 
                   PRI 
                   ⁢ 
                   2 
                   ⁢ 
                   GND 
                 
               
               = 
               
                 
                   R 
                   
                     GND 
                     ⁢ 
                     2 
                     ⁢ 
                     PRI 
                   
                   T 
                 
                 = 
                 
                   [ 
                   
                     
                       
                         
                           t 
                           1 
                         
                       
                       
                         
                           t 
                           2 
                         
                       
                       
                         
                           t 
                           3 
                         
                       
                     
                     
                       
                         
                           t 
                           4 
                         
                       
                       
                         
                           t 
                           5 
                         
                       
                       
                         
                           t 
                           6 
                         
                       
                     
                     
                       
                         
                           t 
                           7 
                         
                       
                       
                         
                           t 
                           8 
                         
                       
                       
                         
                           t 
                           9 
                         
                       
                     
                   
                   ] 
                 
               
             
             ; 
           
         
       
     
     wherein T represents a transposition operation; and t 1 -t 9  are elements of the transformation matrix R GND2PRI ; 
     acquiring a direction vector ν x , of the X S -axis in the quasi-horizontal reference system, expressed as: 
     
       
         
           
             
               
                 v 
                 
                   X 
                   s 
                 
               
               = 
               
                 [ 
                 
                   
                     
                       
                         
                           
                             t 
                             1 
                           
                           ⁢ 
                           cos 
                           ⁢ 
                              
                           α 
                         
                         - 
                         
                           
                             t 
                             3 
                           
                           ⁢ 
                           sin 
                           ⁢ 
                              
                           α 
                         
                       
                     
                   
                   
                     
                       
                         
                           
                             t 
                             4 
                           
                           ⁢ 
                           cos 
                           ⁢ 
                              
                           α 
                         
                         - 
                         
                           
                             t 
                             6 
                           
                           ⁢ 
                           sin 
                           ⁢ 
                              
                           α 
                         
                       
                     
                   
                   
                     
                       
                         
                           
                             t 
                             7 
                           
                           ⁢ 
                           cos 
                           ⁢ 
                              
                           α 
                         
                         - 
                         
                           
                             t 
                             9 
                           
                           ⁢ 
                           sin 
                           ⁢ 
                              
                           α 
                         
                       
                     
                   
                 
                 ] 
               
             
             ; 
           
         
       
     
     acquiring a direction vector ν y , of the Y s -axis in the quasi-horizontal reference system, expressed as: 
     
       
         
           
             
               
                 v 
                 
                   Y 
                   s 
                 
               
               = 
               
                 [ 
                 
                   
                     
                       
                         
                           
                             - 
                             
                               t 
                               1 
                             
                           
                           ⁢ 
                           sin 
                           ⁢ 
                              
                           α 
                         
                         - 
                         
                           
                             t 
                             
                               3 
                                 
                             
                           
                           ⁢ 
                           cos 
                           ⁢ 
                              
                           α 
                         
                       
                     
                   
                   
                     
                       
                         
                           
                             - 
                             
                               t 
                               4 
                             
                           
                           ⁢ 
                           sin 
                           ⁢ 
                              
                           α 
                         
                         - 
                         
                           
                             t 
                             6 
                           
                           ⁢ 
                           cos 
                           ⁢ 
                              
                           α 
                         
                       
                     
                   
                   
                     
                       
                         
                           
                             - 
                             
                               t 
                               7 
                             
                           
                           ⁢ 
                           sin 
                           ⁢ 
                              
                           α 
                         
                         - 
                         
                           
                             t 
                             
                               9 
                                 
                             
                           
                           ⁢ 
                           cos 
                           ⁢ 
                              
                           α 
                         
                       
                     
                   
                 
                 ] 
               
             
             ; 
           
         
       
     
     building relations between t 4  and t 6  according to the direction vector ν x , and the direction vector ν y , expressed as: 
     
       
         
           
             { 
             
               
                 
                   
                     
                       
                         sin 
                         ⁢ 
                            
                         
                           ( 
                           
                             
                               Δ 
                               ⁢ 
                               
                                 θ 
                                 x 
                                 pitch 
                               
                             
                             - 
                             
                               θ 
                               x 
                               pitch 
                             
                           
                           ) 
                         
                       
                       = 
                       
                         
                           
                             t 
                             
                               4 
                                 
                             
                           
                           ⁢ 
                           cos 
                           ⁢ 
                              
                           α 
                         
                         - 
                         
                           
                             t 
                             6 
                           
                           ⁢ 
                           sin 
                           ⁢ 
                              
                           α 
                         
                       
                     
                   
                 
                 
                   
                     
                       
                         sin 
                         ⁢ 
                            
                         
                           ( 
                           
                             
                               Δ 
                               ⁢ 
                               
                                 θ 
                                 y 
                                 pitch 
                               
                             
                             - 
                             
                               θ 
                               y 
                               pitch 
                             
                           
                           ) 
                         
                       
                       = 
                       
                         
                           
                             - 
                             
                               t 
                               
                                 4 
                                   
                               
                             
                           
                           ⁢ 
                           sin 
                           ⁢ 
                              
                           α 
                         
                         - 
                         
                           
                             t 
                             6 
                           
                           ⁢ 
                           cos 
                           ⁢ 
                              
                           α 
                         
                       
                     
                   
                 
               
               ; 
             
           
         
       
     
     according to the relations between t 4  and t 6 , obtaining a relation between the measurement reading θ x   pitch  and the bias Δθ x   pitch  and a relation between the measurement reading θ y   pitch  and the bias Δθ y   pitch , expressed as: 
     
       
         
           
             { 
             
               
                 
                   
                     
                       
                         θ 
                         x 
                         pitch 
                       
                       = 
                       
                         
                           Δ 
                           ⁢ 
                           
                             θ 
                             x 
                             pitch 
                           
                         
                         - 
                         
                           arcsin 
                           ⁢ 
                           
                             ( 
                             
                               
                                 
                                   t 
                                   
                                     4 
                                       
                                   
                                 
                                 ⁢ 
                                 cos 
                                 ⁢ 
                                    
                                 α 
                               
                               - 
                               
                                 
                                   t 
                                   6 
                                 
                                 ⁢ 
                                 sin 
                                 ⁢ 
                                    
                                 α 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
                 
                   
                     
                       
                         θ 
                         y 
                         pitch 
                       
                       = 
                       
                         
                           Δ 
                           ⁢ 
                           
                             θ 
                             y 
                             pitch 
                           
                         
                         - 
                         
                           arcsin 
                           ⁢ 
                           
                             ( 
                             
                               
                                 
                                   - 
                                   
                                     t 
                                     
                                       4 
                                         
                                     
                                   
                                 
                                 ⁢ 
                                 sin 
                                 ⁢ 
                                    
                                 α 
                               
                               - 
                               
                                 
                                   t 
                                   6 
                                 
                                 ⁢ 
                                 cos 
                                 ⁢ 
                                    
                                 α 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               ; 
             
           
         
       
     
     changing a two-axis level degree of the dual-axis level sensor to obtain (θ x   pitch , θ y   pitch ) and (t 4 , t 6 ) under different two-axis level degrees; and plugging the (θ x   pitch , θ y   pitch ) and (t 4 , t 6 ) under different two-axis level degrees into the relation between the measurement reading θ x   pitch  and the bias Δν x   pitch  and the relation between the measurement reading, θ y   pitch  and the bias Δθ y   pitch  to obtain the angle α, the bias Δθ x   pitch  and the bias Δθ y   pitch ; 
     In some embodiments, the first side surface is a front surface of the hexahedron, and the second side surface is a right surface of the hexahedron. 
     Compared to the prior art, this application has the following beneficial effects. 
     1. By means of the two-axis level degree measured by the dual-axis level sensor, the method provided herein enables a continuous orientation survey of a single astronomical object and overcomes problems caused by astronomical predictions and attitude changes when the servo star sensor switches between multiple astronomical objects, so as to allow a long-term and continuous orientation survey with high update rate based on miss distance after successfully capturing one astronomical object, eliminating the need for simultaneous tracking of multiple astronomical objects and reducing the number of observed astronomical objects. The orientation method provided herein is particularly suitable for the observation and tracking of a single target in the day time, and can also use obvious targets such as the sun for attitude measurement, simplifying the photoelectric detection and enhancing the data update rate and measurement accuracy for the astronomical orientation. 
     2. The orientation method provided herein proposes a simple and effective mathematical modeling strategy of the reference system, which greatly reduces the complexity of calibration parameters. Specifically, when constructing the quasi-horizontal reference system, one axis is allowed to coincide with a projection of a normal vector of one side surface of the hexahedron reference system to omit an orientation variable. When constructing a relationship between the dual-axis level sensor reference system and the hexahedron reference system, the relationship is simplified to an orientation error, and the two-axis non-levelness error is incorporated into the bias of a level reading to reduce the data collection required for calibration. 
     3. In the orientation method provided herein, the hexahedron reference system is measured by means of the autocollimation of the theodolite, and the quasi-horizontal reference system is measured by the dual-axis level sensor. Then structural parameters of the quasi-horizontal reference system and the hexahedron reference system can be calibrated according to the pitch angle measured by the theodolite and the reading of the dual-axis level sensor. Accordingly, with the help of the theodolite, the collection of all calibrated data can be completed, simplifying the calibration process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow chart of a single star-based orientation method using a dual-axis level sensor; 
         FIG. 2  schematically depicts a relationship between a dual-axis level sensor reference system, a hexahedron reference system and a quasi-horizontal reference system; and 
         FIG. 3  schematically depicts a calibration of the dual-axis level sensor reference system and the quasi-horizontal reference system. 
     
    
    
     In the drawings: 1, servo star sensor; 2, hexahedron reference; 3, first theodolite; 4, second theodolite; and 5, data recording computer. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The disclosure will be clearly and completely described below with reference to the accompanying drawings and embodiments. 
     As shown in the flow chart in  FIG. 1 , a single star-based orientation method using a dual-axis level sensor includes a first calibrating process, a second calibrating process and an actual calculating process. A key of the single star-based orientation is to project an observed single astronomical object onto a “virtual” horizontal plane (quasi-horizontal plane) via a dual-axis level sensor to obtain an azimuth angle of the projection, which is then compared with that in an inertial reference system. An azimuth angle difference is a current measured azimuth angle. 
     Specifically, the actual calculating process of the single star-based orientation method is provided, which is described as follows. 
     (S1) Any two orthogonal side surfaces of a hexahedron on a star sensor are denoted as a first side surface and a second side surface, respectively. A component of a vector ν right  on an X n -axis of a quasi-horizontal reference system is denoted as t 4 . A component of the vector ν right  on a Z n -axis of the quasi-horizontal reference system is denoted as t 6 . The vector ν right  is obtained by rotating a normal vector of the second side surface  ν   right  around the Z n -axis of the quasi-horizontal reference system by a preset azimuth angle θ dir   right . 
     (S2) The t 4  and t 6  are calculated through the following equations: 
     
       
         
           
             { 
             
               
                 
                   
                     
                       t 
                       4 
                     
                     = 
                     
                       - 
                       
                         ( 
                         
                           
                             sin 
                             ⁢ 
                                
                             α 
                             ⁢ 
                             
                               sin 
                               ⁡ 
                               ( 
                               
                                 
                                   Δ 
                                   ⁢ 
                                   
                                     θ 
                                     x 
                                     pitch 
                                   
                                 
                                 - 
                                 
                                   θ 
                                   x 
                                   pitch 
                                 
                               
                               ) 
                             
                           
                           + 
                           
                             cos 
                             ⁢ 
                                
                             α 
                             ⁢ 
                             
                               sin 
                               ⁡ 
                               ( 
                               
                                 
                                   Δ 
                                   ⁢ 
                                   
                                     θ 
                                     y 
                                     pitch 
                                   
                                 
                                 - 
                                 
                                   θ 
                                   y 
                                   pitch 
                                 
                               
                               ) 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   
                     
                       t 
                       6 
                     
                     = 
                     
                       ( 
                       
                         
                           cos 
                           ⁢ 
                              
                           αsin 
                           ⁢ 
                              
                           
                             ( 
                             
                               
                                 Δ 
                                 ⁢ 
                                 
                                   θ 
                                   x 
                                   pitch 
                                 
                               
                               - 
                               
                                 θ 
                                 x 
                                 pitch 
                               
                             
                             ) 
                           
                         
                         - 
                         
                           sin 
                           ⁢ 
                              
                           α 
                           ⁢ 
                           
                             sin 
                             ⁡ 
                             ( 
                             
                               
                                 Δ 
                                 ⁢ 
                                 
                                   θ 
                                   y 
                                   pitch 
                                 
                               
                               - 
                               
                                 θ 
                                 y 
                                 pitch 
                               
                             
                             ) 
                           
                         
                       
                       ) 
                     
                   
                 
               
             
           
         
       
     
     wherein α is an angle between an X s -axis of a dual-axis level sensor reference system and an X-axis of a hexahedron reference system on the star sensor. θ x   pitch  is a measurement reading of the dual-axis level sensor on the X s -axis; θ y   pitch  is a measurement reading of the dual-axis level sensor on a Y s -axis of the dual-axis level sensor reference system. Δθ x   pitch  is a bias of the dual-axis level sensor on the X s -axis. Δθ y   pitch  is a bias of the dual-axis level sensor on the Y s -axis. 
     (S3) An observed vector of a single astronomical object in the hexahedron reference system is denoted as ν PRI . A reference vector of the single astronomical object in an inertial reference system is denoted as ν GND , where the reference vector of the single astronomical object is obtained from a star catalog, and the ν GND  and ν PRI  satisfy the following equation: 
         R   z (θ z )· R   x (θ x )· R   y (θ y )·ν GND =ν PRI  
 
     where R y (θ y ) denotes that the reference vector ν GND  rotates around a Y-axis of the hexahedron reference system by an azimuth angle θ y . R y (θ x ) denotes that the reference vector ν GND  rotates around the X-axis of the hexahedron reference system by a pitch angle θ x . R z (θ z ) denotes that the reference vector ν GND  rotates around a Z-axis of the hexahedron reference system by a roll angle θ z . 
     (S4) The pitch angle θ x  and the roll angle θ z  are calculated according to the t 4  and t 6  through the following equations: 
     
       
         
           
             { 
             
               
                 
                   
                     
                       
                         θ 
                         x 
                       
                       = 
                       
                         
                           - 
                           arcsin 
                         
                         ⁢ 
                            
                         
                           t 
                           6 
                         
                       
                     
                   
                 
                 
                   
                     
                       
                         θ 
                         z 
                       
                       = 
                       
                         
                           - 
                           arcsin 
                         
                         ⁢ 
                            
                         
                           ( 
                           
                             
                               t 
                               4 
                             
                             / 
                             cos 
                             ⁢ 
                                
                             
                               θ 
                               x 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               . 
             
           
         
       
     
     It should be noted that a transformation matrix R GND2PRI  from the quasi-horizontal reference system to the hexahedron reference system is described according to a rotation sequence of rotating around the Y-axis by the θ y , rotating around the X-axis by the θ x  and rotating around the Z-axis by the θ z  (2-1-3 rotation) to obtain relations as: 
     
       
         
           
             { 
             
               
                 
                   
                     
                       
                         t 
                         4 
                       
                       = 
                       
                         cos 
                         ⁢ 
                         
                           θ 
                           x 
                         
                         ⁢ 
                         sin 
                         ⁢ 
                         
                           θ 
                           z 
                         
                       
                     
                   
                 
                 
                   
                     
                       
                         t 
                         6 
                       
                       = 
                       
                         sin 
                         ⁢ 
                         
                           θ 
                           x 
                         
                       
                     
                   
                 
               
               . 
             
           
         
       
     
     The equations for computing the pitch angle θ x  and the roll angle θ z  are obtained through an arcsin function in accordance with the above relations. 
     (S5) ν GND0 =R x (−θ x )·R z  (−θ z )·ν PRI  is made. According to a rotation sequence of rotating around the Z-axis by the θ z , rotating around the X-axis by the θ x  and rotating around the Y-axis by the θ z  (3-1-2 rotation), ν GND  R y  (−θ y )·ν GND0  is obtained. The azimuth angle θ y  is calculated through the following equation to complete a single star-based orientation: 
     
       
         
           
             
               θ 
               y 
             
             = 
             
               arctan 
               ⁢ 
                  
               2 
               ⁢ 
               
                 ( 
                 
                   
                     
                       
                         v 
                         ⁢ 
                         
                           1 
                           · 
                           v 
                         
                         ⁢ 
                         6 
                       
                       - 
                       
                         v 
                         ⁢ 
                         
                           3 
                           · 
                           v 
                         
                         ⁢ 
                         4 
                       
                     
                     
                       
                         v 
                         ⁢ 
                         
                           4 
                           2 
                         
                       
                       + 
                       
                         v 
                         ⁢ 
                         
                           6 
                           2 
                         
                       
                     
                   
                   , 
                   
                     
                       
                         v 
                         ⁢ 
                         
                           1 
                           · 
                           v 
                         
                         ⁢ 
                         4 
                       
                       - 
                       
                         v 
                         ⁢ 
                         
                           3 
                           · 
                           v 
                         
                         ⁢ 
                         6 
                       
                     
                     
                       
                         v 
                         ⁢ 
                         
                           4 
                           2 
                         
                       
                       + 
                       
                         v 
                         ⁢ 
                         
                           6 
                           2 
                         
                       
                     
                   
                 
                 ) 
               
             
           
         
       
     
     where [ν1 ν2 ν3] T =ν GND ·ν 1 -ν3 are components of the ν GND  on the X-axis, the Y-axis and the Z-axis, respectively. [ν4 ν5 ν6] T =ν GND0 . ν4-ν6 are components of the ν GND0  on the X n -axis, the Y n -axis and the Z n -axis, respectively. 
     A first calibrating process shown in  FIG. 1  is a calibration of a transformational relationship between the quasi-horizontal reference system and the hexahedron reference system. 
     The quasi-horizontal reference system is constructed according to an angle information of a normal vector of the first side surface ν front  and the normal vector of the second side surface (considering that the first side surface is a front side surface and the second side surface is a right side surface). Referring to  FIG. 2 , the X n -axis of the quasi-horizontal reference system is defined as a projection of the X-axis of the hexahedron reference system on a horizontal plane. A Y n -axis of the quasi-horizontal reference system is defined such that the Y n -axis of the quasi-horizontal reference system is constrained by orthogonality and lies in the horizontal plane, namely, the Y n -axis is orthogonal to the X n -axis on the horizontal plane and a Z n -axis of the quasi-horizontal reference system satisfies a right-hand rule, and has a general upward pointing direction. 
     A normal pitch angle θ pitch   front  of the first side surface and a normal pitch angle θ pitch   front  of the second side surface are measured by a first theodolite and a second theodolite, respectively. An axis of a measuring lens barrel of the first theodolite coincides with a normal of the first side surface of the hexahedron, and an axis of a measuring lens barrel of the second theodolite coincides with a normal of the second side surface of the hexahedron. In the quasi-horizontal reference system, the normal vector ν front  is expressed as 
     
       
         
           
             
               v 
               front 
             
             = 
             
               
                 
                   
                     R 
                     y 
                   
                   ( 
                   
                     θ 
                     pitch 
                     front 
                   
                   ) 
                 
                 · 
                 
                   [ 
                   
                     
                       
                         0 
                       
                     
                     
                       
                         0 
                       
                     
                     
                       
                         1 
                       
                     
                   
                   ] 
                 
               
               = 
               
                 [ 
                 
                   
                     
                       
                           
                         
                           v 
                           ⁢ 
                           f 
                           ⁢ 
                           1 
                         
                       
                     
                   
                   
                     
                       0 
                     
                   
                   
                     
                       
                         v 
                         ⁢ 
                         f 
                         ⁢ 
                         3 
                       
                     
                   
                 
                 ] 
               
             
           
         
       
     
     where R y (θ pitch   front ) denotes that a vector [0 0 1] rotates around a Y n -axis by the normal pitch angle θ pitch   front . νƒ1, 0 and νƒ3 are components of the second normal vector ν front  on the X n -axis, the Y n -axis and the Z n -axis, respectively. The normal vector  ν   right  without rotating by the preset azimuth angle θ dir   right  is expressed as: 
     
       
         
           
             
               
                 
                   v 
                   _ 
                 
                 right 
               
               = 
               
                 
                   
                     
                       R 
                       y 
                     
                     ( 
                     
                       θ 
                       pitch 
                       right 
                     
                     ) 
                   
                   · 
                   
                     [ 
                     
                       
                         
                           0 
                         
                       
                       
                         
                           0 
                         
                       
                       
                         
                           1 
                         
                       
                     
                     ] 
                   
                 
                 = 
                 
                   [ 
                   
                     
                       
                         
                             
                           
                             v 
                             ⁢ 
                             r 
                             ⁢ 
                             1 
                           
                         
                       
                     
                     
                       
                         0 
                       
                     
                     
                       
                         
                           v 
                           ⁢ 
                           r 
                           ⁢ 
                           3 
                         
                       
                     
                   
                   ] 
                 
               
             
             , 
           
         
       
     
     where R y (θ pitch   right ) denotes that the vector [00 1] rotates around the Y n -axis of the quasi-horizontal reference system by the normal pitch angle θ pitch   right . νr1, 0 and νr3 are components of the normal vector  ν   right  on the X n -axis, Y n -axis and Z n -axis, respectively. 
     The vector ν right  is obtained by rotating the vector  ν   right  by the preset azimuth and the vector ν right  is expressed as follows: 
     
       
         
           
             
               v 
               right 
             
             = 
             
               
                 
                   
                     R 
                     z 
                   
                   ( 
                   
                     θ 
                     dir 
                     right 
                   
                   ) 
                 
                 · 
                 
                   [ 
                   
                     
                       
                         
                             
                           
                             v 
                             ⁢ 
                             r 
                             ⁢ 
                             1 
                           
                         
                       
                     
                     
                       
                         0 
                       
                     
                     
                       
                         
                           v 
                           ⁢ 
                           r 
                           ⁢ 
                           3 
                         
                       
                     
                   
                   ] 
                 
               
               = 
               
                 [ 
                 
                   
                     
                       
                         v 
                         ⁢ 
                         r 
                         ⁢ 
                         
                           1 
                           · 
                           cos 
                         
                         ⁢ 
                         
                           θ 
                           dir 
                           right 
                         
                       
                     
                   
                   
                     
                       
                         
                           - 
                           v 
                         
                         ⁢ 
                         r 
                         ⁢ 
                         
                           1 
                           · 
                           sin 
                         
                         ⁢ 
                         
                           θ 
                           dir 
                           right 
                         
                       
                     
                   
                   
                     
                       
                         v 
                         ⁢ 
                         r 
                         ⁢ 
                         3 
                       
                     
                   
                 
                 ] 
               
             
           
         
       
     
     where R z (θ dir   right ) denotes that a vector [νr1 0 νr3] rotates around the Z n -axis by the preset azimuth angle θ dir   right . 
     According to an orthogonal relationship between the normal vector ν front  and the normal vector ν right  (ν front  ⊥ ν right ), the following equation is obtained: 
     
       
         
           
             
               θ 
               dir 
               right 
             
             = 
             
               
                 acos 
                 ⁡ 
                 ( 
                 
                   
                     
                       - 
                       v 
                     
                     ⁢ 
                     f 
                     ⁢ 
                     
                       3 
                       · 
                       v 
                     
                     ⁢ 
                     r 
                     ⁢ 
                     3 
                   
                   
                     v 
                     ⁢ 
                     f 
                     ⁢ 
                     
                       1 
                       · 
                       v 
                     
                     ⁢ 
                     r 
                     ⁢ 
                     1 
                   
                 
                 ) 
               
               . 
             
           
         
       
     
     A third normal vector ν up  of a third surface is obtained according to a cross product, expressed as 
       ν up =ν right ×ν front .
 
     Therefore, the transformation matrix R GND2PRI  is expressed as: 
         R   GND2PRI =[ν front ν right ν up ].
 
     A second calibrating process shown in  FIG. 1  is a calibration of a transformational relationship between the dual-axis level sensor reference system and the quasi-horizontal reference system. 
     Referring to  FIG. 2 , an angle between the X s -axis and the X-axis is assumed as α. Since the dual-axis level sensor reference system is coplanar with the hexahedron reference system, an angle between the Y s -axis and the X-axis is α+π/2. A direction vector of the X s -axis in the hexahedron reference system is expressed as [cos α 0−sin α] and a direction vector of the Y s -axis in the hexahedron reference system is expressed as [−sin α 0−cos α]. 
     A transpose matrix of the transformation matrix R GND2PRI  is obtained, expressed as follows: 
     
       
         
           
             
               R 
               
                 PRI 
                 ⁢ 
                 2 
                 ⁢ 
                 GND 
               
             
             = 
             
               
                 R 
                 
                   GND 
                   ⁢ 
                   2 
                   ⁢ 
                   PRI 
                 
                 T 
               
               = 
               
                 [ 
                 
                   
                     
                       
                         t 
                         1 
                       
                     
                     
                       
                         t 
                         2 
                       
                     
                     
                       
                         t 
                         3 
                       
                     
                   
                   
                     
                       
                         t 
                         4 
                       
                     
                     
                       
                         t 
                         5 
                       
                     
                     
                       
                         t 
                         6 
                       
                     
                   
                   
                     
                       
                         t 
                         7 
                       
                     
                     
                       
                         t 
                         8 
                       
                     
                     
                       
                         t 
                         9 
                       
                     
                   
                 
                 ] 
               
             
           
         
       
     
     where T represents a transposition operation. t 1 -t 9  are elements of the transformation matrix R GND2PRI . 
     A direction vector ν x , of the X s -axis in the quasi-horizontal reference system is acquired, expressed as follows: 
     
       
         
           
             
               v 
               
                 X 
                 s 
               
             
             = 
             
               
                 [ 
                 
                   
                     
                       
                         
                           
                             t 
                             1 
                           
                           ⁢ 
                           cos 
                           ⁢ 
                              
                           α 
                         
                         - 
                         
                           
                             t 
                             3 
                           
                           ⁢ 
                           sin 
                           ⁢ 
                              
                           α 
                         
                       
                     
                   
                   
                     
                       
                         
                           
                             t 
                             4 
                           
                           ⁢ 
                           cos 
                           ⁢ 
                              
                           α 
                         
                         - 
                         
                           
                             t 
                             6 
                           
                           ⁢ 
                           sin 
                           ⁢ 
                              
                           α 
                         
                       
                     
                   
                   
                     
                       
                         
                           
                             t 
                             7 
                           
                           ⁢ 
                           cos 
                           ⁢ 
                              
                           α 
                         
                         - 
                         
                           
                             t 
                             9 
                           
                           ⁢ 
                           sin 
                           ⁢ 
                              
                           α 
                         
                       
                     
                   
                 
                 ] 
               
               . 
             
           
         
       
     
     A direction vector ν y , of the Y s -axis in the quasi-horizontal reference system is acquired, expressed as follows: 
     
       
         
           
             
               v 
               
                 Y 
                 s 
               
             
             = 
             
               
                 [ 
                 
                   
                     
                       
                         
                           
                             - 
                             
                               t 
                               1 
                             
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           sin 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           α 
                         
                         - 
                         
                           
                             t 
                             3 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           cos 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           α 
                         
                       
                     
                   
                   
                     
                       
                         
                           
                             - 
                             
                               t 
                               4 
                             
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           sin 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           α 
                         
                         - 
                         
                           
                             t 
                             6 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           cos 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           α 
                         
                       
                     
                   
                   
                     
                       
                         
                           
                             - 
                             
                               t 
                               7 
                             
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           sin 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           α 
                         
                         - 
                         
                           
                             t 
                             9 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           cos 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           α 
                         
                       
                     
                   
                 
                 ] 
               
               . 
             
           
         
       
     
     The measurement reading of the direction vector ν x , on the dual-axis level sensor is θ x   pitch . The measurement reading of the direction vector ν y , on the dual-axis level sensor is θ y   pitch . The bias of the dual-axis level sensor on the Xs-axis is Δθ x   pitch . The bias of the dual-axis level sensor on the Ys-axis is Δθ y   pitch    
     Relations between t 4  and t 6  according to the direction vector ν x , and the direction vector ν y , is built, expressed as follows: 
     
       
         
           
             { 
             
               
                 
                   
                     
                       
                         sin 
                         ⁡ 
                         
                           ( 
                           
                             
                               Δ 
                               ⁢ 
                               
                                 θ 
                                 x 
                                 pitch 
                               
                             
                             - 
                             
                               θ 
                               x 
                               pitch 
                             
                           
                           ) 
                         
                       
                       = 
                       
                         
                           
                             t 
                             4 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           cos 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           α 
                         
                         - 
                         
                           
                             t 
                             6 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           sin 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           α 
                         
                       
                     
                   
                 
                 
                   
                     
                       
                         sin 
                         ⁡ 
                         
                           ( 
                           
                             
                               Δ 
                               ⁢ 
                               
                                 θ 
                                 y 
                                 pitch 
                               
                             
                             - 
                             
                               θ 
                               y 
                               pitch 
                             
                           
                           ) 
                         
                       
                       = 
                       
                         
                           
                             - 
                             
                               t 
                               4 
                             
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           sin 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           α 
                         
                         - 
                         
                           
                             t 
                             6 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           cos 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           α 
                         
                       
                     
                   
                 
               
               . 
             
           
         
       
     
     According to the relations between t 4  and t 6 , a relation between the measurement reading θ x   pitch  and the bias Δθ x   pitch  and a relation between the measurement reading θ y   pitch  and the bias Δθ y   pitch  are obtained, expressed as: 
     
       
         
           
             { 
             
               
                 
                   
                     
                       
                         θ 
                         x 
                         pitch 
                       
                       = 
                       
                         
                           Δ 
                           ⁢ 
                           
                             θ 
                             x 
                             pitch 
                           
                         
                         - 
                         
                           arcsin 
                           ⁡ 
                           
                             ( 
                             
                               
                                 
                                   t 
                                   4 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 cos 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 α 
                               
                               - 
                               
                                 
                                   t 
                                   6 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 sin 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 α 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
                 
                   
                     
                       
                         θ 
                         y 
                         pitch 
                       
                       = 
                       
                         
                           Δ 
                           ⁢ 
                           
                             θ 
                             y 
                             pitch 
                           
                         
                         - 
                         
                           arcsin 
                           ⁡ 
                           
                             ( 
                             
                               
                                 
                                   - 
                                   
                                     t 
                                     4 
                                   
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 sin 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 α 
                               
                               - 
                               
                                 
                                   t 
                                   6 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 cos 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 α 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               . 
             
           
         
       
     
     The star sensor is placed under a calibration device, as shown in  FIG. 3 . A two-axis level degree of the dual-axis level sensor is changed to obtain (θ x   pitch , θ y   pitch ) and (t 4 , t 6 ) under different two-axis level degrees and then multiple mapping relations of (θ x   pitch , θ x   pitch )→(t 4 , t 6 ) are obtained. The (θ x   pitch , θ y   pitch ) and (t 4 , t 6 ) under different two-axis level degrees into the relation between the measurement reading θ x   pitch  and the bias Δθ x   pitch  and the relation between the measurement reading θ y   pitch  and the bias Δθ y   pitch  are plugged to obtain the angle α, the bias Δθ x   pitch  and the bias Δθ y   pitch . 
     In the orientation method provided herein, the hexahedron reference system is measured by means of the autocollimation of the theodolite, and the quasi-horizontal reference system is measured by the dual-axis level sensor. Then structural parameters of the quasi-horizontal reference system and the hexahedron reference system can be calibrated according to the pitch angle measured by the theodolite and the reading of the dual-axis level sensor. Accordingly, with the help of the theodolite, the collection of all calibrated data can be completed, simplifying the calibration process. 
     In summary, the single star-based orientation method provided herein can be described as follows. 
     The first calibrating process is performed as follows. Normal vectors of two orthogonal side surfaces on the hexahedron of the star sensor are measured to obtain the transformational relationship between the quasi-horizontal reference system and the hexahedron reference system. The X n -axis of the quasi-horizontal reference system is defined as a projection of the X-axis of the hexahedron reference system in a horizontal plane. The Y n -axis is orthogonal to the X n -axis on the horizontal plane and the Z n -axis of the quasi-horizontal reference system satisfies a vertically-upward right-hand rule. 
     The second calibrating process is performed as follows. By means of the mapping relation between the measurement reading of the dual-axis level sensor and normal measurements of the theodolites, the transformational relationship between the dual-axis level sensor reference system and the quasi-horizontal reference system is obtained. After the second calibrating process, the angle between an X s -axis of a dual-axis level sensor reference system and an X-axis of a hexahedron reference system, the bias of the dual-axis level sensor on the X s -axis, and the bias of the dual-axis level sensor on the Y s -axis are obtained. 
     The actual calculating process is performed as follows. By means of an attitude information of the X s -axis and the Y s -axis obtained in the second calibrating process, the vectors expressed in the quasi-horizontal reference system is obtained through the observation vector of the single astronomical object. An azimuth angle difference between the quasi-horizontal reference system and the inertial reference system, that is orientation information, is obtained by combining with the reference vector of the single astronomical object in the inertial reference system from the star catalog. 
     This application eliminates an uncertainty of solution when solving for an attitude with a single vector by means of the information from the dual-axis level sensor. Furthermore, a quasi-horizontal reference system is constructed through the 2-1-3 rotation. Vectors of the single astronomical object are projected onto the quasi-horizontal reference system. By comparing the azimuth angle difference between a projected azimuth angle and an azimuth angle in the inertial reference system, the single star-based orientation of is completed. 
     Mentioned above are merely some embodiments of this disclosure, which are not intended to limit the disclosure. It should be understood that any changes, replacements and modifications made by those killed in the art without departing from the spirit and scope of this disclosure should fall within the scope of the present disclosure defined by the appended claims.