  This paper addresses the leader-following attitude consensus problem for a
group of spacecraft when at least one follower can access the leader's attitude
and velocity relative to the inertial space. A nonlinear distributed observer
is designed to estimate the leader's states for each follower. The observer
possesses one important and novel feature of keeping attitude and angular
velocity estimation errors on second-order sliding modes, and thus provides
finite-time convergent estimates for each follower. Further, quaternion-based
hybrid homogeneous controllers recently developed for single spacecraft are
extended and then applied, by establishing a separation principle with the
proposed observer, to track the leader's attitude motion. As a result, global
finite-time attitude consensus is achieved on the entire attitude manifold,
with either full-state measurements or attitude-only measurements, as long as
the network topology among the followers is undirected and connected. Numerical
simulations are presented to demonstrate the performance of the proposed
methods.
