Index Page qdq2av_c
A B C D E F G H I J K L M N O P Q R S T U V W X
Procedure Abstract Required_Reading Keywords Brief_I/O Detailed_Input | Detailed_Output Parameters Exceptions Files Particulars Examples | Restrictions Literature_References Author_and_Institution Version Index_Entries
#### Procedure
       void qdq2av_c ( ConstSpiceDouble    q  [4],
                       ConstSpiceDouble    dq [4],
                       SpiceDouble         av [3]  )
#### Abstract
       Derive angular velocity from a unit quaternion and its derivative
       with respect to time.
#### Required_Reading
       ROTATION
#### Keywords
       MATH
       POINTING
       ROTATION
#### Brief_I/O
       VARIABLE  I/O  DESCRIPTION
       --------  ---  --------------------------------------------------
       q          I   Unit SPICE quaternion.
       dq         I   Derivative of `q' with respect to time.
       av         O   Angular velocity defined by `q' and `dq'.
#### Detailed_Input
       q              is a unit length 4-vector representing a SPICE-style
                      quaternion. See the discussion of "Quaternion Styles"
                      in the Particulars section below.

                      Note that multiple styles of quaternions are in use.
                      This routine will not work properly if the input
                      quaternions do not conform to the SPICE convention.
                      See the Particulars section for details.

       dq             is a 4-vector representing the derivative of `q' with
                      respect to time.
#### Detailed_Output
       av             is 3-vector representing the angular velocity defined
                      by `q' and `dq', that is, the angular velocity of the
                      frame defined by the rotation matrix associated with
                      `q'.  This rotation matrix can be obtained via the
                      CSPICE routine q2m_c; see the Particulars section for
                      the explicit matrix entries.

                      `av' is the vector (imaginary) part of the quaternion
                      product

                               *
                         -2 * q  * dq

                      This angular velocity is the same vector that could
                      be obtained (much less efficiently ) by mapping `q'
                      and `dq' to the corresponding C-matrix `r' and its
                      derivative `dr', then calling the CSPICE routine
                      xf2rav_c.

                      `av' has units of

                         radians / T

                      where

                         1 / T

                      is the unit associated with `dq'.
#### Parameters
       None.
#### Exceptions
       Error free.

       1) A unitized version of input quaternion is used in the
          computation.  No attempt is made to diagnose an invalid
          input quaternion.
#### Files
       None.
#### Particulars
       Quaternion Styles
       -----------------

       There are different "styles" of quaternions used in
       science and engineering applications. Quaternion styles
       are characterized by

          - The order of quaternion elements

          - The quaternion multiplication formula

          - The convention for associating quaternions
            with rotation matrices

       Two of the commonly used styles are

          - "SPICE"

             > Invented by Sir William Rowan Hamilton
             > Frequently used in mathematics and physics textbooks

          - "Engineering"

             > Widely used in aerospace engineering applications

       CSPICE function interfaces ALWAYS use SPICE quaternions.
       Quaternions of any other style must be converted to SPICE
       quaternions before they are passed to CSPICE functions.

       Relationship between SPICE and Engineering Quaternions
       ------------------------------------------------------

       Let M be a rotation matrix such that for any vector V,

          M*V

       is the result of rotating V by theta radians in the
       counterclockwise direction about unit rotation axis vector A.
       Then the SPICE quaternions representing M are

          (+/-) (  cos(theta/2),
                   sin(theta/2) A(1),
                   sin(theta/2) A(2),
                   sin(theta/2) A(3)  )

       while the engineering quaternions representing M are

          (+/-) ( -sin(theta/2) A(1),
                  -sin(theta/2) A(2),
                  -sin(theta/2) A(3),
                   cos(theta/2)       )

       For both styles of quaternions, if a quaternion q represents
       a rotation matrix M, then -q represents M as well.

       Given an engineering quaternion

          QENG   = ( q0,  q1,  q2,  q3 )

       the equivalent SPICE quaternion is

          QSPICE = ( q3, -q0, -q1, -q2 )

       Associating SPICE Quaternions with Rotation Matrices
       ----------------------------------------------------

       Let FROM and TO be two right-handed reference frames, for
       example, an inertial frame and a spacecraft-fixed frame. Let the
       symbols

          V    ,   V
           FROM     TO

       denote, respectively, an arbitrary vector expressed relative to
       the FROM and TO frames. Let M denote the transformation matrix
       that transforms vectors from frame FROM to frame TO; then

          V   =  M * V
           TO         FROM

       where the expression on the right hand side represents left
       multiplication of the vector by the matrix.

       Then if the unit-length SPICE quaternion q represents M, where

          q = (q0, q1, q2, q3)

       the elements of M are derived from the elements of q as follows:

            +-                                                         -+
            |           2    2                                          |
            | 1 - 2*( q2 + q3 )   2*(q1*q2 - q0*q3)   2*(q1*q3 + q0*q2) |
            |                                                           |
            |                                                           |
            |                               2    2                      |
        M = | 2*(q1*q2 + q0*q3)   1 - 2*( q1 + q3 )   2*(q2*q3 - q0*q1) |
            |                                                           |
            |                                                           |
            |                                                   2    2  |
            | 2*(q1*q3 - q0*q2)   2*(q2*q3 + q0*q1)   1 - 2*( q1 + q2 ) |
            |                                                           |
            +-                                                         -+

       Note that substituting the elements of -q for those of q in the
       right hand side leaves each element of M unchanged; this shows
       that if a quaternion q represents a matrix M, then so does the
       quaternion -q.

       To map the rotation matrix M to a unit quaternion, we start by
       decomposing the rotation matrix as a sum of symmetric
       and skew-symmetric parts:

                                          2
          M = [ I  +  (1-cos(theta)) OMEGA  ] + [ sin(theta) OMEGA ]

                       symmetric                   skew-symmetric

       OMEGA is a skew-symmetric matrix of the form

                     +-             -+
                     |  0   -n3   n2 |
                     |               |
           OMEGA  =  |  n3   0   -n1 |
                     |               |
                     | -n2   n1   0  |
                     +-             -+

       The vector N of matrix entries (n1, n2, n3) is the rotation axis
       of M and theta is M's rotation angle.  Note that N and theta
       are not unique.

       Let

          C = cos(theta/2)
          S = sin(theta/2)

       Then the unit quaternions Q corresponding to M are

          Q = +/- ( C, S*n1, S*n2, S*n3 )

       The mappings between quaternions and the corresponding rotations
       are carried out by the CSPICE routines

          q2m_c {quaternion to matrix}
          m2q_c {matrix to quaternion}

       m2q_c always returns a quaternion with scalar part greater than
       or equal to zero.

       SPICE Quaternion Multiplication Formula
       ---------------------------------------

       Given a SPICE quaternion

          Q = ( q0, q1, q2, q3 )

       corresponding to rotation axis A and angle theta as above, we can
       represent Q using "scalar + vector" notation as follows:

          s =   q0           = cos(theta/2)

          v = ( q1, q2, q3 ) = sin(theta/2) * A

          Q = s + v

       Let Q1 and Q2 be SPICE quaternions with respective scalar
       and vector parts s1, s2 and v1, v2:

          Q1 = s1 + v1
          Q2 = s2 + v2

       We represent the dot product of v1 and v2 by

          <v1, v2>

       and the cross product of v1 and v2 by

          v1 x v2

       Then the SPICE quaternion product is

          Q1*Q2 = s1*s2 - <v1,v2>  + s1*v2 + s2*v1 + (v1 x v2)

       If Q1 and Q2 represent the rotation matrices M1 and M2
       respectively, then the quaternion product

          Q1*Q2

       represents the matrix product

          M1*M2

       About this routine
       ==================

       Given a time-dependent SPICE quaternion representing the
       attitude of an object, we can obtain the object's angular
       velocity AV in terms of the quaternion Q and its derivative
       with respect to time DQ:

                            *
          AV  =  Im ( -2 * Q  * DQ )                                  (1)

       That is, AV is the vector (imaginary) part of the product
       on the right hand side (RHS) of equation (1).  The scalar part
       of the RHS is zero.

       We'll now provide an explanation of formula (1). For any
       time-dependent rotation, the associated angular velocity at a
       given time is a function of the rotation and its derivative at
       that time. This fact enables us to extend a proof for a limited
       subset of rotations to *all* rotations:  if we find a formula
       that, for any rotation in our subset, gives us the angular
       velocity as a function of the rotation and its derivative, then
       that formula must be true for all rotations.

       We start out by considering the set of rotation matrices

          R(t) = M(t)C                                                (2)

       where C is a constant rotation matrix and M(t) represents a
       matrix that "rotates" with constant, unit magnitude angular
       velocity and that is equal to the identity matrix at t = 0.

       For future reference, we'll consider C to represent a coordinate
       transformation from frame F1 to frame F2.  We'll call F1 the
       "base frame" of C.  We'll let AVF2 be the angular velocity of
       M(t) relative to F2 and AVF1 be the same angular velocity
       relative to F1.

       Referring to the axis-and-angle decomposition of M(t)

                                                  2
          M(t) = I + sin(t)OMEGA + (1-cos(t))OMEGA                    (3)

       (see the Rotation Required Reading for a derivation) we
       have

          d(M(t))|
          -------|     = OMEGA                                        (4)
            dt   |t=0

       Then the derivative of R(t) at t = 0 is given by

          d(R(t))|
          -------|     = OMEGA  * C                                   (5)
            dt   |t=0

       The rotation axis A associated with OMEGA is defined by        (6)

          A(1) =  - OMEGA(2,3)
          A(2) =    OMEGA(1,3)
          A(3) =  - OMEGA(1,2)

       Since the coordinate system rotation M(t) rotates vectors about A
       through angle t radians at time t, the angular velocity AVF2 of
       M(t) is actually given by

          AVF2  =  - A                                                (7)

       This angular velocity is represented relative to the image
       frame F2 associated with the coordinate transformation C.

       Now, let's proceed to the angular velocity formula for
       quaternions.

       To avoid some verbiage, we'll freely use 3-vectors to represent
       the corresponding pure imaginary quaternions.

       Letting QR(t), QM(t), and QC be quaternions representing the
       time-dependent matrices R(t), M(t) and C respectively, where
       QM(t) is selected to be a differentiable function of t in a
       neighborhood of t = 0, the quaternion representing R(t) is

          QR(t) = QM(t) * QC                                          (8)

       Differentiating with respect to t, then evaluating derivatives
       at t = 0, we have

          d(QR(t))|         d(QM(t))|
          --------|     =   --------|     * QC                        (9)
             dt   |t=0         dt   |t=0

       Since QM(t) represents a rotation having axis A and rotation
       angle t, then (according to the relationship between SPICE
       quaternions and rotations set out in the Rotation Required
       Reading), we see QM(t) must be the quaternion (represented as the
       sum of scalar and vector parts):

          cos(t/2)  +  sin(t/2) * A                                  (10)

       where A is the rotation axis corresponding to the matrix
       OMEGA introduced in equation (3).  By inspection

          d(QM(t))|
          --------|     =   1/2 * A                                  (11)
             dt   |t=0

       which is a quaternion with scalar part zero.  This allows us to
       rewrite the quaternion derivative

          d(QR(t))|
          --------|     =   1/2  *  A  *  QC                         (12)
             dt   |t=0

       or for short,

          DQ = 1/2 * A * QC                                          (13)

       Since from (7) we know the angular velocity AVF2 of the frame
       associated with QM(t) is the negative of the rotation axis
       defined by (3), we have

          DQ = - 1/2 * AVF2 * QC                                     (14)

       Since

          AVF2 = C * AVF1                                            (15)

       we can apply the quaternion transformation formula
       (from the Rotation Required Reading)

                                   *
          AVF2 =  QC  *  AVF1  * QC                                  (16)

       Now we re-write (15) as

                                       *
          DQ = - 1/2 * ( QC * AVF1 * QC ) * QC

             = - 1/2 *   QC * AVF1                                   (17)

       Then the angular velocity vector AVF1 is given by

                         *
          AVF1  = -2 * QC  * DQ                                      (18)

       The relation (18) has now been demonstrated for quaternions
       having constant, unit magnitude angular velocity.  But since
       all time-dependent quaternions having value QC and derivative
       DQ at a given time t have the same angular velocity at time t,
       that angular velocity must be AVF1.
#### Examples
       The following test program creates a quaternion and quaternion
       derivative from a known rotation matrix and angular velocity
       vector.  The angular velocity is recovered from the quaternion
       and quaternion derivative by calling qdq2av_c and by an
       alternate method; the results are displayed for comparison.

          #include <stdio.h>
          #include "SpiceUsr.h"
          #include "SpiceZfc.h"

          int main()
          {
             /.
             Local constants
             ./

             /.
             Local variables
             ./
             SpiceDouble             angle  [3];
             SpiceDouble             av     [3];
             SpiceDouble             avx    [3];
             SpiceDouble             dm     [3][3];
             SpiceDouble             dq     [4];
             SpiceDouble             expav  [3];
             SpiceDouble             m      [3][3];
             SpiceDouble             mout   [3][3];
             SpiceDouble             q      [4];
             SpiceDouble             qav    [4];
             SpiceDouble             xtrans [6][6];

             SpiceInt                i;

             /.
             Pick some Euler angles and form a rotation matrix.
             ./
             angle[0] =  -20.0 * rpd_c();
             angle[1] =   50.0 * rpd_c();
             angle[2] =  -60.0 * rpd_c();

             eul2m_c ( angle[2], angle[1], angle[0], 3, 1, 3, m );

             m2q_c ( m, q );

             /.
             Choose an angular velocity vector.
             ./
             expav[0] = 1.0;
             expav[1] = 2.0;
             expav[2] = 3.0;

             /.
             Form the quaternion derivative.
             ./
             qav[0]  =  0.0;
             vequ_c ( expav, qav+1 );

             qxq_c ( q, qav, dq );

             vsclg_c ( -0.5, dq, 4, dq );

             /.
             Recover angular velocity from `q' and `dq' using qdq2av_c.
             ./
             qdq2av_c ( q, dq, av );

             /.
             Now we'll obtain the angular velocity from `q' and
             `dq' by an alternate method.

             Convert `q' back to a rotation matrix.
             ./
             q2m_c ( q, m );

             /.
             Convert `q' and `dq' to a rotation derivative matrix.  This
             somewhat messy procedure is based on differentiating the
             formula for deriving a rotation from a quaternion, then
             substituting components of `q' and `dq' into the derivative
             formula.
             ./

             dm[0][0]  =  -4.0  * (   q[2]*dq[2]  +  q[3]*dq[3]  );

             dm[0][1]  =   2.0  * (   q[1]*dq[2]  +  q[2]*dq[1]
                                    - q[0]*dq[3]  -  q[3]*dq[0]  );

             dm[0][2]  =   2.0  * (   q[1]*dq[3]  +  q[3]*dq[1]
                                    + q[0]*dq[2]  +  q[2]*dq[0]  );

             dm[1][0]  =   2.0  * (   q[1]*dq[2]  +  q[2]*dq[1]
                                    + q[0]*dq[3]  +  q[3]*dq[0]  );

             dm[1][1]  =  -4.0  * (   q[1]*dq[1]  +  q[3]*dq[3]  );

             dm[1][2]  =   2.0  * (   q[2]*dq[3]  +  q[3]*dq[2]
                                    - q[0]*dq[1]  -  q[1]*dq[0]  );

             dm[2][0]  =   2.0  * (   q[3]*dq[1]  +  q[1]*dq[3]
                                    - q[0]*dq[2]  -  q[2]*dq[0]  );

             dm[2][1]  =   2.0  * (   q[2]*dq[3]  +  q[3]*dq[2]
                                    + q[0]*dq[1]  +  q[1]*dq[0]  );

             dm[2][2]  =  -4.0  * (   q[1]*dq[1]  +  q[2]*dq[2]  );

             /.
             Form the state transformation matrix corresponding to `m'
             and `dm'.
             ./

             /.
             Upper left block:
             ./
             for ( i = 0;  i < 3;  i++ )
             {
                vequ_c ( m[i], xtrans[i] );
             }

             /.
             Upper right block:
             ./
             for ( i = 0;  i < 3;  i++ )
             {
                vpack_c ( 0.0, 0.0, 0.0, xtrans[i]+3 );
             }

             /.
             Lower left block:
             ./
             for ( i = 0;  i < 3;  i++ )
             {
                vequ_c ( dm[i], xtrans[3+i] );
             }

             /.
             Lower right block:
             ./
             for ( i = 0;  i < 3;  i++ )
             {
                vequ_c ( m[i], xtrans[3+i]+3  );
             }

             /.
             Now use xf2rav_c to produce the expected angular velocity.
             ./
             xf2rav_c ( xtrans, mout, avx );

             /.
             The results should match to nearly full double precision.
             ./
             printf ( "Original angular velocity:   \n"
                      "   %24.16e, %24.16e, %24.16e \n"
                      "qdq2av_c's angular velocity: \n"
                      "   %24.16e, %24.16e, %24.16e \n"
                      "xf2rav's angular velocity:   \n"
                      "   %24.16e, %24.16e, %24.16e \n",
                      expav[0],   expav[1],  expav[2],
                      av   [0],   av   [1],  av   [2],
                      avx  [0],   avx  [1],  avx  [2]    );

             return ( 0 );
          }
#### Restrictions
       None.
#### Literature_References
       None.
#### Author_and_Institution
       N.J. Bachman    (JPL)
#### Version
       -CSPICE Version 1.0.1, 27-FEB-2008 (NJB)

          Updated header; added information about SPICE
          quaternion conventions.

       -CSPICE Version 1.0.0, 31-OCT-2005 (NJB)
#### Index_Entries
       angular velocity from  quaternion and derivative
#### Link to routine qdq2av_c source file qdq2av_c.c

    Wed Jun  9 13:05:28 2010