Blind Approach Beacon System

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Blind Approach Beacon System (BABS) was an early automatic radar landing aid from the 1940s. A small transponder in a van at the end of the runway answered signals from the aircraft. The system sent dots on one side of the runway and dashes on the other, and the signal strength told the plane how far off the centerline it was. Using this information, the aircraft could determine its position and steer toward the centerline to land. BABS was an early forerunner of the modern instrument landing system (ILS). Modern ILS uses signals from the runway end to guide pilots left or right and to control descent, via a localizer and glide path.