# Process Noise Covariance Matrix¶

The process covariance acts as a weighting matrix for the system process. It relates the covariance between the $$i^{th}$$ and $$j^{th}$$ element of each process-noise vector. It is defined as:

$\Sigma_{ij} = cov{ \begin{pmatrix} { \vec{x}_{i},\vec{x}_{j} } \end{pmatrix} } = E{ \begin{bmatrix} { { \begin{pmatrix} { \vec{x}_{i}-\mu_{i} } \end{pmatrix} } \cdot { \begin{pmatrix} { \vec{x}_{j}-\mu_{j} } \end{pmatrix} } } \end{bmatrix} }$

A Kalman Filter can be viewed the combination of Gaussian distributions to form state estimates. $$Q$$ provides a measure of the width of the Gaussian distribution related to each noise state. The wider the distribution, the more uncertainty exists in the process model. This leads to a state-update that affects the state more than if the model had a tighter distribution, which results in an update having less influence on the particular state.

Based on the state process-noise vectors, $$\vec{w}_{k}$$ (found in previous sections), the Process Noise Covariance Matrix is:

$Q_{k} = { \begin{bmatrix} { \begin{array}{ccccc} {\Sigma_{r}} & {0_{3}} & {0_{3 \times 4}} & {0_{3}} & {0_{3}} \cr {0_{3}} & {\Sigma_{v}} & {0_{3 \times 4}} & {0_{3}} & {0_{3}} \cr {0_{4 \times 3}} & {0_{4 \times 3}} & {\Sigma_{q}} & {0_{4 \times 3}} & {0_{4 \times 3}} \cr {0_{3}} & {0_{3}} & {0_{3 \times 4}} & {\Sigma_{\omega b}} & {0_{3}} \cr {0_{3}} & {0_{3}} & {0_{3 \times 4}} & {0_{3}} & {\Sigma_{ab}} \end{array} } \end{bmatrix} }$

The individual process covariance are repeated here:

$\Sigma_{r} = {\begin{pmatrix} { \sigma_{a} \cdot {dt}^{2} } \end{pmatrix}}^{2} \cdot I_3$
$\Sigma_{v} = {\begin{pmatrix} { \sigma_{a} \cdot dt } \end{pmatrix}}^{2} \cdot I_3$
$\Sigma_{q} = { { \begin{pmatrix} { {\sigma_{\omega} \cdot dt } \over {2} } \end{pmatrix} }^{2} } \cdot { \begin{bmatrix} { \begin{array}{cccc} {1 - q_0^2} & {-{q_0 \cdot q_1}} & {-{q_0 \cdot q_2}} & {-{q_0 \cdot q_3}} \cr {-{q_0 \cdot q_1}} & {1 - q_1^2} & {-{q_1 \cdot q_2}} & {-{q_1 \cdot q_3}} \cr {-{q_0 \cdot q_2}} & {-{q_1 \cdot q_2}} & {1 - q_2^2} & {-{q_2 \cdot q_3}} \cr {-{q_0 \cdot q_3}} & {-{q_1 \cdot q_3}} & {-{q_2 \cdot q_3}} & {1 - q_3^2} \end{array} } \end{bmatrix} }$
$\Sigma_{\omega b} = {\begin{pmatrix} { \sigma_{dd,\omega} \cdot dt } \end{pmatrix}}^{2} \cdot I_3$
$\Sigma_{ab} = {\begin{pmatrix} { \sigma_{dd,a} \cdot dt } \end{pmatrix}}^{2} \cdot I_3$