  This paper addresses the problem of estimating the position of an object
moving in $R^n$ from direction and velocity measurements. After addressing
observability issues associated with this problem, a nonlinear observer is
designed so as to encompass the case where the measured velocity is corrupted
by a constant bias. Global exponential convergence of the estimation error is
proved under a condition of persistent excitation upon the direction
measurements. Simulation results illustrate the performance of the observer.
