  This paper investigates the problem of continuous attitude estimation on
$SO(3)$ using continuous angular velocity and linear acceleration measurements
as well as intermittent linear velocity and inertial vector measurements.
First, we propose a nonlinear observer for the case where all the measurements
are continuous and almost global asymptotic stability (AGAS) is shown using the
notion of almost global input-to-state stability (ISS) on manifolds.
Thereafter, a hybrid attitude observer, with AGAS guarantees, is proposed in
terms of intermittent linear velocity and vector measurements. Numerical
simulation results are presented to illustrate the performance of the proposed
hybrid observer.
