The GRANIT experiment is a project build at ILL high neutron flux reactor, expected to start data taking in 2015. It aims at measuring the transition energies between gravitational quantum level of the neutron in the Earth gravity field. Using ultra cold neutrons bouncing over a flat mirror, one can access to both the energy levels and the wavefunction of the neutron, having thus sensitivity to new interaction at the micrometer scale. We will report the current status of the commissioning of the GRANIT spectrometer, in particular the performance of the UCN superthermal source as well as new simulation tools devised for the characterization of the source