Abstract: First and second redundant command signals are formulated in parallel-by-bit digital format by two ganged wafer switch sets in a control unit. The first redundant command signal is stored in a shift register and subsequently strobed out, by clock pulses received from a remote unit, and transmitted to the remote unit in serial-by-bit format where it is stored and used to control the remote unit. The command signal is subsequently clocked out of the remote unit and returned to the control unit in serial-by-bit format where it is compared against the second redundant command signal to insure that the remote unit has responded to the proper command.A renewable timer is reset by the clock pulses. If the clock pulses are interrupted for more than a predetermined time the renewable timer times out and indicates a fault.

Abstract: This invention comprises means for monitoring the operation of an aircraft landing guidance system which functions independently of the landing guidance system. The means include a radio altimeter aboard the aircraft, a reflector positioned on the ground at a predetermined location along the prescribed approach path to provide enhanced return of the altimeter signal and means for comparing the actual radio altitude with a predetermined altitude value to provide an indication of the location of the aircraft within a tolerable distance from a prescribed point on the approach path.

Abstract: An integrity monitor includes means at the ground station for periodically transmitting an integrity test signal comprising a coded first portion which identifies the signal and a coded second portion which is equivalent to a predetermined quantity. The airborne station includes a decoder which determines the type of test signal received and decodes the second portion to obtain a number which, if the system is operating properly, will compare favorably with a fixed reference. If the comparison is unfavorable, an alarm is sounded.