  This paper considers the problem of simultaneous estimation of the attitude,
position and linear velocity for vehicles navigating in a three-dimensional
space. We propose two types of hybrid nonlinear observers using continuous
angular velocity and linear acceleration measurements as well as intermittent
landmark position measurements. The first type relies on a fixed-gain design
approach based on an infinite-dimensional optimization, while the second one
relies on a variable-gain design approach based on a continuous-discrete
Riccati equation. For each case, we provide two different observers with and
without the estimation of the gravity vector. The proposed observers are shown
to be exponentially stable with a large domain of attraction. Simulation and
experimental results are presented to illustrate the performance of the
proposed observers.
