The Fresnel Lens Optical Landing System (OLS) was developed to replace the Mirror Landing System. The light system is designed to provide a "glide slope" for aviators approaching a carrier, the lights projected through the Fresnel lenses in different colors telling the aviator when the aircraft is at the desired altitude in the approach at any distance from the ship. If the aviator sees a red light (at the bottom), it means that the aircraft is dangerously low, the subsequent flashing red light activated by the landing signal officer (LSO) indicating a wave-off requiring the pilot to go around for another attempt.