Patent GB-1167837-A (GB)
Title: Integrity Monitor for ILS Navigation Receiver.
IPC: G01S19/11
Filed: 1968-04-24
Abstract: 1,167,837. Radio navigation. BENDIX CORP. 24 April, 1968 [4 May, 1967], No. 19278/68. Heading H4D. A monitoring system for an aircraft navigation receiver of the kind which produces a guidance signal by the comparison of the magnitudes of modulating tone signals, comprises a duplicated guidance signal channel, comparing means for the two signals thereby made available, and a suppressed carrier-type modulator which is connected to the comparing means, and which gives a carrier leak if there is output from said means, when an alarm is given by means including a logic circuit. As described, a localizer or glide slope receiver 10 provides audio tones of 90 c/s. and 150 c/s. at its output, which are applied both to a standard ILS system comprising filters 12, detectors 13 and an indicator 14, and to a duplicated channel comprising filters 15 and detectors 16. The D.C. voltages from the detectors are compared in a difference network 17, and any output is fed to one input terminal of a summing network 18, which also receives an input from the 90 c/s. section of the filters 15. In normal conditions there is no output from the network 17, and the output from the network 18, which is fed to one input of a balanced modulator 19, is of 90 c/s. tone only. A carrier of 400 c/s. is also fed to the modulator 19 and, in normal circumstances, its output is of sideband frequencies 310 c/s. and 490 c/s. and not of carrier frequency. After amplification, the output of the modulator 19 is presented to two channels, one containing a peak limiter 22 and the other an offset limiter 26, the latter only passing signals which exceed a predetermined amplitude. Normally, the signals (of sideband frequencies) pass the limiter 22 and to not pass the limiter 26. They are demodulated in a synchronous demodulator 23 and, after amplification and rectification, are applied to one input of a summing network 29, output from which enables an AND gate 33. A 2800 c/s. signal is fed through an AND NOT gate 32, the AND gate 33, and an AND gate 34, to a flag alarm circuit. The gate 32 is not inhibited in normal conditions, since there is no signal via the limiter 26, and the gate 34 is enabled in the presence of 90 c/s. and 150 c/s. tones in the standard ILS system, as is determined by an arrangement connected to the filters 12 comprising rectifiers 36, 37 and a network 38 with output to the gate 34. The 2800 c/s. signal holds off an alarm flag. If the D.C. voltages from the detectors 13, 16 are not substantially the same, an output is given by the network 17, which results in the output from the modulator 19 containing a component of frequency 400 c/s., which predominates where there is a significant degree of imbalance. The output of the demodulator 23 is then D.C., and is blocked by the amplifier which follows it, so that there is no consequent input to the summing network 29. The amplitude of the output from the modulator 19 is sufficient for the threshold of the limiter 26 to be overcome, so that it gives an output which, after amplification and rectification, is applied through a summing network 31 to the INHIBIT input of the AND NOT gate 32, so that the 2800 c/s. signal is cut off, and the alarm flag is shown. A desensitizer circuit is provided to prevent the appearance of false alarms when the aircraft is some distance off course. Output from the detectors 16 is applied to a balanced modulator 41, which gives a carrier frequency output signal when a large guidance error is present. The said output signal is amplified and is applied through a positively-poled rectifier 43 to an input of the summing circuit 29 and through a negatively-poled rectifier 44 to an input of the summing circuit 31, so that it tends to keep the gates 32 and 33 open for the passage of the 2800 c/s. signal which holds off the alarm flag.