In this paper, the design of a scheduled observer which allows to estimate the state of an affine LPV system is investigated. The state and the gain matrices of the observer are scheduled by using an interpolation method which is linear according to each parameter but which is nonlinear according to the parameter vector. The stability of the estimation error is based on the existence of an affine parameter-dependent Lyapunov function. The problem of the observer design and the existence of a such Lyapunov function is interpreted as an LMI feasibility problem with a rank constraint. An example is presented.