Complex sentence: Because the satellites are not all geostationary (they are rotating once every 24 hours around the earth but they change position in the sky), they appear and disappear over the horizon, and terrestrial signals are present only in major cities to augment the satellite signals.
Split into simpler sentences: Because the satellites are not all geostationary they appear and disappear over the horizon. Terrestrial signals are present only in major cities to augment the satellite signals.