Laser Tomographic systems, such as ATLAS, will rely on natural guide stars to sense low order aberrations. LIFT is a novel focal plane wavefront sensor (WFS), performing a maximum likelihood phase retrieval on a single image, with better sensitivity than a 2x2 Hartmann-Shack WFS. We first present a characterization of LIFT's noise propagation performance and working domain by means of simulations. We then show the results of experiments on ONERA's test bench for LIFT. These experiments validate the estimation of tip/tilt and focus, in monochromatic light and in large bandwidth (for a spectral resolution of 3), as well as the expected noise propagation. They also confirm the validity of the imaging models used for simulations. Finally, we focus on the preparation of an on-sky validation based on Gemini Multi-conjugate adaptive optics System (GeMS) calibration data at Gemini Southern Observatory.