  The Minkowski product of unit quaternion sets is introduced and analyzed,
motivated by the desire to characterize the overall variation of compounded
spatial rotations that result from individual rotations subject to known
uncertainties in their rotation axes and angles. For a special type of unit
quaternion set, the spherical caps of the 3-sphere $S^3$ in $\mathbb{R}^4$,
closure under the Minkowski product is achieved. Products of sets characterized
by fixing either the rotation axis or rotation angle, and allowing the other to
vary over a given domain, are also analyzed. Two methods for visualizing unit
quaternion sets and their Minkowski products in $\mathbb{R}^3$ are also
discussed, based on stereographic projection and the Lie algebra formulation.
Finally, some general principles for identifying Minkowski product boundary
points are discussed in the case of full-dimension set operands.
