Version 1.3 (JASP) 26-Jul-2009
For purposes of these tools, a quaternion, q, is just a four element vector where q(1:3) is the "imaginary" or "vector" portion of the hypercomplex number, and q(4) is the "real" or "scalar" portion. Consequently, if q represents a rotation, then:
q(1) = v1*sin(phi/2)
q(2) = v2*sin(phi/2)
q(3) = v3*sin(phi/2)
q(4) = cos(phi/2)
where phi is the amount of rotation about the unit vector [v1 v2 v3].
All tools are vectorized, so "vectors" of quaternions (4xN or Nx4 matrices) can be handled as well. Since it is most common to work with normalized quaternions (also referred to as "unit quaternions" and "versors"), if a set of 4 quaternions, i.e., a 4x4 matrix, is input, the tools will attempt to determine the shape of the component quaternions (4x1 or 1x4) based on whether the rows or columns are normalized.
Of course, some of the tools, like QDECOMP, only make sense for normalized quaternions, and thus those tools enforce normality via QNORM.
isq - determines whether or not input is a quaternion
isnormq - determines whether or not input is a normalized quaternion
qconj - quaternion conjugate
qnorm - normalize quaternion
qmult - multiply quaternions
qdecomp - decompose quaternion into unit vector and rotation angle
qcvq - operation on vector: qconj(q) v q
qvqc - operation on vector: q v qconj(q)
Because the author uses the convention described in "Spacecraft Attitude Determination and Control" (Wertz, 1978), the following aliases exist:
qvxform - quaternion/vector transform (alias for qcvq)
qvrot - quaternion/vector rotation (alias for qvqc)
Likewise, the following operations assume the relationship between the DCM and the quaternion is: R*v = qvxform(q, v) = qcvq(q, v). That is, the q that performs the equivalent operation on v is the "right hand quaterion".
q2dcm - quaternion to direction cosine matrix
dcm2q - direction cosine matrix to quaternion
Note that many more recent applications, particularly computer graphics libraries, choose the opposite convention. That is, the equivalent "q" is the "left hand quaternion" and consequently the qvxform/qvrot aliases and q2dcm and dcm2q functions would be "backward".
See also qlib, the Quaternion block library for simulink.