How Pilots Land When They Can’t See the Runway

The Instrument Landing System (ILS) uses two radio beams. The localizer, from an antenna array beyond the far end of the runway, marks the centerline. The glide slope, from an antenna beside the touchdown zone, marks a descent path of about three degrees. Each beam is made of two overlapping signals; the receiver compares them, and when they're equal, the airplane is on the line. In the lowest-visibility conditions (Category III), approved airliners and crews can land with autoland, with runway visual range as low as 150 feet under FAA Category IIIb.

Notes: The beams are drawn in a stylized way (real antenna patterns are more complex), and the sideways drift is exaggerated so it's visible. Low-visibility authorization depends on the aircraft, the airport, and the crew.

Sources:
– FAA AC 120-118 (CAT I/II/III operations):
– SKYbrary, Precision Approach:
– ILS overview:
– Glide path:
– FAA AIM, runway markings: