  In this paper, we address the problem of relative localization of two mobile
agents. Specifically, we consider the Dual-IMU system, where each agent is
equipped with one IMU, and employs relative pose observations between them.
Previous works, however, typically assumed known ego motion and ignored biases
of the IMUs. Instead, we study the most general case of unknown biases for both
IMUs. Besides the derivation of dynamic model equations of the proposed system,
we focus on the observability analysis, for the observability under general
motion and the unobservable directions arising from various special motions.
Through numerical simulations, we validate our key observability findings and
examine their impact on the estimation accuracy and consistency. Finally, the
system is implemented to achieve effective relative localization of an HMD with
respect to a vehicle moving in the real world.
