Patent US 3,946,358

An aircraft instrument for warning the pilot that the aircraft is in an unsafe condition in proximity to the ground. The instrument has inputs representing aircraft configuration, flight condition with respect to the ground, and glide slope. Detector circuits process the inputs and give a warning of unsafe proximity to the ground. The boundary conditions for the warning are selected to avoid false warnings. A glide slope detector distinguishes between a valid glide slope signal and spurious signals, without need for pilot input.