1360144 Radio navigation BENDIX CORP 8 Sept 1972 [5 Oct 1971] 41837/72 Heading H4D A monitoring system for an aircraft navigation system, which system comprises a receiver providing guidance signals on two similar channels by comparing tone modulations, comprises means for combining with each of the guidance signals an A.C. tracer signal and means for comparing the combined tracer-guidance signals with one another, the comparing means normally giving a second tracer signal of amplitude proportional to the sum of the amplitudes of the tracer signal components of the tracerguidance signals, amplifying means including a variable gain amplifier for amplifying the second tracer signal and producing a signal provided the difference between the guidance signals does not cause limiting of the amplifier, control means responsive to the guidance signal magnitude for varying the gain and for varying the tracer signal level, and means for producing an alarm when signal from the amplifier varies a predetermined amount. Receiver 10 produces guidance signals at the output of amplifiers 18 and 25. A tracer signal from generator 28 is added into the guidance signals and the combined signals are compared at the input of amplifier 26. If the system is operating correctly the D.C. signals from amplifiers 18, 24 should cancel so that the trace appears at the output of amplifier 26. If the D.C. signals are not equal and differ by a certain amount amplifier 26 is saturated and produces no tracer output. A further amplifier 31 is arranged to receive the output of amplifier 26, a reference voltage and a signal representing the magnitude of the signals from the detectors. Amplifier 31 is saturated if the last mentioned signal exceeds a threshold and hence produces no tracer output if the sensitivity of the system exceeds a threshold. Schmitt triggers 53, 54 receive the guidance signals. If the aircraft is oncourse trigger 53 switches off transistors 36 and 43. The result is that the gain of amplifier 26 is increased, tightening the tolerable difference between the outputs from amplifiers 18, 24. In addition, amplifier 31 is rendered operative to give no tracer output if the system sensitivity increases or decreases beyond preset limits. Attenuator 29 is also switched on to reduce the tracer amplitude, the gain of amplifier 26 having been increased. If the aircraft is in the clearance area or not on-course, trigger 53 is operative to switch off the attenuator and reduce the gain of amplifier 26 by turning on the transistors. The reduced gain results in a relaxation of the tolerable differences between the outputs of amplifiers 18, 24. In addition, amplifier 31 is operative to give no tracer output only if the system sensitivity drops beyond a certain limit and not if it increases. The output of amplifier 31 is fed via a.g.c. amplifier 44 to detection circuitry. The output is fed through amplifier 44 as a check on the operation of this amplifier. Thus, if the system is not operating correctly a loss of tracer signal appears at the output of amplifier 44. The remaining circuitry detects if the tracer is present, and if not, produces an alarm.