Proper construction of an unscented Kalman filter (UKF) for unit quaternionic systems is not straightforward due to the incompatibility between the algebraic properties of the unit quaternions and the common real vector space operations (additions and scalar multiplications) needed in the steps of a filter algorithm. This work studies, in detail, all UKFs and square-root UKFs for quaternionic systems proposed in the literature. First, we classify the algorithms according to the preservation of the unity norm of the quaternion variables. Second, we propose two new algorithms: the quaternionic additive unscented Kalman filter (QuAdUKF) and a square-root variant of it. The QuAdUKF encompasses all known UKFs for quaternionic systems of the literature preserving, in all steps, the norm of the unit quaternion variables. Besides, it can also yield new UKFs with this norm preservation property. The QuAdUKF's square-root variant has better properties in comparison with all the square-root UKFs for quaternionic systems of the literature. Numerical experiments for a spacecraft attitude estimation problem illustrate the theoretical results. KEYWORDS attitude estimation, square-root unscented Kalman filter (SRUKF), unit quaternion, unscented Kalman filter (UKF)In the control literature, the most common state space models are those where the underlying state, input, and output variables are real vectors, that is, elements that lie in a Euclidean vector space ℝ n . For bodies under two-dimensional or three-dimensional (3D) motions, these Euclidean models fit for dimensionless material points, that is, linear displacements and velocities. However, for large rigid bodies, besides these linear displacement characteristics, the body pointing direction and angular (rotational) movements become important. [1][2][3] It is well known that rotations of rigid bodies in a 3D space are mathematically represented by state trajectories, restricted to a compact manifold called the special orthogonal group, SO(3). The high nonlinearity of this manifold, due to the restriction of variables in specific regions, leads to difficulties in dealing with these variables. In estimation algorithms, performing calculations with variables in SO(3) is often computationally expensive, and more computationally-efficient rotation parameterizations were proposed in the literature. Among the alternative rotation parameterizations, such as Euler angles, rotation vectors (RoVs), and unit quaternions, the unit quaternions are often chosen because, unlike other 4500