The singularity of the solutions to the Schrödinger equation on a scattering manifold

We consider the microlocal singularity of the solutions to the Schrödinger equation on a manifold with so-called "scattering metric". We prove a characterization of the wave front set using the classical mechanical wave operator. The result is similar to a recent result by Hassell and Wunsch, but the formulation and the proof are quite different. Also the assumptions are weaker.