# Calculate attitude quaternion from 2 vector measurements

I am using a method proposed by R. G. Reynolds to estimate attitude based on two vector measurements, taken from: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19990052720.pdf

Suppose we measure 2 unit vectors $b_{1}$ and $b_{2}$ in the body frame, and we know the components in the inertial frame are $r_{1}$ and $r_{2}$. The attitude quaternion can be calculated:

$$c = |(b_{1} - r_{1}) \times (b_{2} - r_{2})|^2 + ((b_{1} + r_{1})\cdot (b_{2} - r_{2}))^2$$

$$q = \frac{1}{\sqrt{c}} [b_{2} \cdot r_{1} - b_{1} \cdot r_{2}, (b_{1} - r_{1}) \times (b_{2} - r_{2})]$$

Where $c$ is a normalisation factor and $q$ is in the form $[w, \vec{v}]$.

I've been trying this method out and it works half the time, but occasionally all the elements of the quaternion are their negative. Is there a way around this problem?

I know there is also a singularity problem when the two vectors are parallel but I don't think this is the problem?

• They must also obey handedness or they would flip. Cross product and all that. Mar 31 '18 at 11:39

In this answer I will represent a quaternion as $q = (q_r,q_v)$, with $q_r$ the real part and $q_v$ the complex vector part of the quaternion.
Rotating a 3D vector $x$ by a rotation represented by a unit quaternion can be done as follows
$$(0,x') = (q_r,q_v)\,(0,x)\,(q_r,-q_v) =q\,(0,x)\,\bar{q}$$
with $\bar{q}$ the complex conjugate of $q$. Note that the complex conjugate of $-q$ is equal to $-\bar{q}$. So if the quaternion is negated $q^*=-q$ then the rotation with this gives
$$(0,x') = q^*\,(0,x)\bar{q^*} = (-q)\,(0,x)\,(-\bar{q}) = q\,(0,x)\,\bar{q})$$
which is the same end result. So $q$ and $-q$ represent the same rotation.