  This paper considers the problem of attitude, position and linear velocity
estimation for rigid body systems relying on landmark measurements. We propose
two hybrid nonlinear observers on the matrix Lie group $SE_2(3)$, leading to
global exponential stability. The first observer relies on fixed gains, while
the second one uses variable gains depending on the solution of a continuous
Riccati equation (CRE). These observers are then extended to handle biased
angular velocity measurements. Both simulation and experimental results are
presented to illustrate the performance of the proposed observers.
