Monitor for instrument landing system
BACKGROUND OF THE INVENTION 
     The following references relate to Instrument Landing System Localizers: 
     1) U.S. Pat. No. 4,415,902; 
     2) U.S. Pat. No. 4,907,005; and 
     3) U.S. Pat. No. 4,605,930. 
     Instrument Landing System (ILS) Localizers provide azimuth guidance to aircraft on final approach to landing. Localizers commonly radiate two superimposed beams. One of these, called &#34;Course&#34; signal, is confined to a relatively narrow angular region on either side of a vertical plane through runway centerline. The other, called &#34;Clearance&#34; signal, typically extends from about ±4° to ±35° from the same vertical plane. Course and Clearance are radiated on different carrier frequencies in the 108-112 megahertz band. Typically, the two carries differ by about 8 khz. Further details may be found in the references. 
     Course and Clearance are radiated by a single antenna array in many cases, and transmissions from the array are monitored by an &#34;Integral Monitor&#34; which consists of a recombiner (Ref. 3, e.g.) that sums signals from pickups installed on each antenna of the array to form an analog of the composite Course and Clearance signal radiated at a specific azimuth angle. Zero degrees azimuth and a &#34;width angle&#34; of approximately 2° azimuth are always monitored. In the United States, FAA specifications require separation of the composite signal into its constitutent Course and Clearance components, so that Course and Clearance are separately monitored as if each existed alone. The present invention is a means of precisely achieving the required separation, using a single recombiner, conventional monitor detectors, and a simple, non-critical audio filter. It is simpler, cheaper, and more precise than prior art, which achieves separation by means of networks that form weighted sums of the signals from individual integral monitor pickups, these sums being weighted so that the network output is either all Course signal or all Clearance signal. 
     SUMMARY OF THE INVENTION 
     The physical basis for the present invention is the fact that, at zero degrees azimuth and at the width angle, Course exceeds Clearance by at least 10 db., in order that airborne receivers will be &#34;captured&#34; by Course signal, that is, respond as if Clearance was absent. Capture occurs because the weak Clearance signal beats against Course to generate amplitude modulation of Course at a frequency Δf equal to the difference between Course carrier frequency and Clearance carrier frequency, and Δf is outside the passband of subsequent circuitry which measures the difference in depth of modulation (DDM) of the 90 and 150 hz. modulations that carry azimuth information. Deviation if the pilot&#39;s ILS indicator is proportional to DDM. 
     The invention recovers Clearance DDM from the normally unused amplitude modulation (AM) at Δf. It recognizes and uses the fact that superposition of a weak Clearance signal on a strong Course signal is a heterodyning process in which Course signal plays the role of a local oscillator. If the composite signal is subjected to AM detections, as it is in the invention, an intermediate frequency Δf will exist which is modulated by the same percentages of 90 and 150 hz. as were originally impressed on the Clearance carrier of the composite signal. In the invention the intermediate frequency is separated from the demodulated 90 and 150 hz. Course signals by a simple bandpass filter working at audio frequencies. Clearance DDM is then recovered from the intermediate frequency by a second AM detector followed by conventional DDM computing circuitry. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the invention, showing its subcircuits as blocks and indicating signal flow by arrows. 
     FIG. 2 illustrates the voltage waveform at point &#34;A&#34; of FIG. 1. 
     FIG. 3 shows the transmission characteristic of the separation filter of FIG. 1. 
     FIG. 4 illustrates the voltage waveform at point &#34;B&#34; of FIG. 1. 
     FIG. 5 illustrates the voltage waveform at point &#34;C&#34; of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, composite Course and Clearance signal form an Integral Monitor recombinder is the input to AM detector #1, whose output voltage waveform is shown in FIG. 2 and consists of a superposition of: 
     a) A D.C. voltage proportional to Course carrier amplitude. 
     b) 90 hz. and 150 hz. voltages proportional to the respective amplitude modulations on the course carrier. 
     c) an amplitude modulated voltage of frequency Δf, where Δf is the difference between Course and Clearance carrier frequencies. 
     The output of AM Detector #1 is input to: 
     1) A conventional DDM computing circuit which rejects voltage c), above, and computes Course DDm from voltages a) and b), above. This circuit is not part of the invention but is included for illustrative purposes because it would be present in any application of the invention. 
     1) A separation filter having the transmission characteristic shown in FIG. 3. In FIG. 3, f 1  is equal to the lowest anticipated value of Δf, minus 150 hz., and f 2  is equal to the highest anticipated value of Δf, plus 150 hz.. f 1  and f 2  will be the lowest and highest spectral components of voltage c), above, that will be encountered in practice, where Δf is subject to drift and may vary from one installation to another. 
     Output of the separation filter is illustrated by FIG. 4, and consists of a carrier at frequency Δf, amplitude modulated with percentages of 90 z. and 150 hz. equal to those on the Clearance carrier of the composite signal. Separation filter output is input to AM detector #2, whose output is illustrated in FIG. 5 and which consists of a D.C. voltage proportional to the clearance carrier of the composite signal input, and superimposed 90 and 150 hz. voltages proportional to the amplitude modulations impressed on the Clearance carrier of the composite signal input. The output of AM detector #2 is supplied to a conventional DDM computing circuit whose output is Clearance DDM.