At the altitude of a LEO orbit the satellite is close to the Earth and would require sensors with an extremely wide field of view to sense the leading and trailing edges of the Earth. For instance at GEO the field of view would be on the order of 30 degrees where at LEO it could be more than 160 degrees.  Therefore ADACS systems typically use Earth Detectors to sense the presents of the Earth as a course indication of pointing and are located on the face of the satellite that will be Earth pointed. One exception to this is when they are used on spin stabilized satellites and are mounted so they sweep across the Earth.