¹⁹As you may have guessed, the remaining light — white, and labelled “A” — indicates passage of the inner marker. Our approach doesn’t have one, but San Fransisco’s runway 28R does. While passing over it, you should hear a rapid, high-pitched series of dots. Why is it labelled “A” and not “I”? Because in ancient times, it was also used to identify passage over “airway” markers along radio range tracks.