  We consider the problem of distributed attitude estimation of multi-agent
systems, evolving on $SO(3)$, relying on individual angular velocity and
relative attitude measurements. The interaction graph topology is assumed to be
an undirected tree. First, we propose a continuous nonlinear distributed
attitude estimation scheme with almost global asymptotic stability guarantees.
Thereafter, we proceed with the \textit{hybridization} of the proposed
estimation scheme to derive a new hybrid nonlinear distributed attitude
estimation scheme enjoying global asymptotic stabilization of the attitude
estimation errors to a common constant orientation. In addition, the proposed
hybrid attitude estimation scheme is used to solve the problem of pose
estimation of $N$-vehicles navigating in a three-dimensional space, with global
asymptotic stability guarantees, where the only available measurements are the
local relative bearings and the individual linear velocities. Simulation
results are provided to illustrate the effectiveness of the proposed estimation
schemes.
