# Attitude Representation - Other

- Gibbs Vector / Rodrigues Parameter Representation
- Modified Rodrigues Parameters
- Cayley-Klein
- References:

## Gibbs Vector / Rodrigues Parameter Representation

The Gibbs vector also known as Rodrigues Parameter is a set of three parameters denoted by $ g $ (or $ P $) and can be directly derived from axis-angle $ (e, \theta) $ or quaternion representation as follows:

where $ \mathbf{q}_{v} $ is the vector part of the quaternion, $ e $ is the unit vector of the axis of rotation, and $ \theta $ is the angle of rotation.

The Gibbs vector is a unit vector that represents the axis of rotation and the magnitude of the vector represents the angle of rotation. The Gibbs vector can be used to represent the rotation matrix $C_{\psi\theta\phi}$ as:

where $ g_1 $, $ g_2 $, and $ g_3 $ are the components of the Gibbs vector.

The Gibbs vector components expressed in DCM can be calculated using the following:

The Rodrigues Parameter preferred as an attitude error representation because it is a unit vector and it is easy to calculate the attitude error between two quaternions. The attitude error between two quaternions can be calculated using the following:

## Modified Rodrigues Parameters

In attitude filter design Modified Rodrigues Parameters (MRP) is preferred for attitude error representation. The MRP is a set of three parameters denoted by $ \mathbf{m} $ and can be directly derived from axis-angle $ (e, \theta) $ or quaternion representation as follows:

Due to above equation the maximum equivalent rotation to describe is $ \pm 360^{\circ}$ (the singularity occurs in $ \pm 360^{\circ}$).

## Cayley-Klein

The Cayley-Klein parameters are consisting of 4 parameters which are closely related to the quaternions and denoted by matrix $ \mathbf{K}_{2\times 2} $.

The corresponding quaternions are defined_as:

## References:

[1] Markley, F. Landis, and John L. Crassidis. Fundamentals of spacecraft attitude determination and control. Vol. 1286. New York, NY, USA:: Springer New York, 2014.

[2] Junkins, John L., and Hanspeter Schaub. Analytical mechanics of space systems. American Institute of Aeronautics and Astronautics, 2009.

[3] De Ruiter, Anton H., Christopher Damaren, and James R. Forbes. Spacecraft dynamics and control: an introduction. John Wiley & Sons, 2012.

[4] Wertz, James R., ed. Spacecraft attitude determination and control. Vol. 73. Springer Science & Business Media, 2012.

[5] Vepa, Ranjan. Dynamics and Control of Autonomous Space Vehicles and Robotics. Cambridge University Press, 2019.

[6] Shuster, Malcolm D. “A survey of attitude representations.” Navigation 8.9 (1993): 439-517.

[7] Markley, F. Landis. “Attitude error representations for Kalman filtering.” Journal of guidance, control, and dynamics 26.2 (2003): 311-317.

[8] Markley, F. Landis, and Frank H. Bauer. Attitude representations for Kalman filtering. No. AAS-01-309. 2001.