Abstract:
An acousto-optic coupler for enhancing safety and control of glide slope  ection between approaching aircraft and carrier landing facilities. Aircraft engine noise is mechanically coupled through the airframe to modulate light beam signals generated by an aircraft landing light. An optical receiver on board the carrier receives the modulated light beam which is detected, filtered and amplified to produce an output signal from an acoustic transducer. The acoustic output signal is monitored by landing personnel for changes in engine noise responsive to carrier originated commands for glide slope control.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for government purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to aircraft glide slope control apparatus and particularly to an acousto-optic communication system between an approaching aircraft and base landing facilities such as an aircraft carrier for enhancing the control and safety of aircraft landings. 
     Successful aircraft landings, particularly those maneuvers involving a landing between an approaching aircraft and a seagoing landing facility such as an aircraft carrier, necessitate the highest degree of safety and control in order to insure maximum protection for personnel and equipment. The control and guidance of an aircraft approaching a carrier is generally assisted by a landing signal officer (LSO) placed on the carrier deck and who has both visual and radio contact with the approaching aircraft. Upon approaching the carrier, the oncoming aircraft begins to assume a predetermined glide slope pattern which defines the path along which the approaching aircraft can most safely and accurately land on the carrier deck. The pilot of the aircraft receives glide slope correction information such as engine power level, altitude and turning commands from the LSO aboard the carrier during the time the aircraft is in the glide slope path. Historically, radio communications between the pilot and the LSO have provided the basic method for communicating glide slope commands between the carrier and aircraft. One important operating parameter indicative of a proper response to a pilot initiated directive is the engine noise produced by the aircraft compressor or pistons and the deviations in levels of the noise as sensed by the pilot in response to the control change. These changes in engine noise are continuously monitored by the pilot because of the pilot&#39;s proximity to the source of the noise and thus provide an added degree of confidence to the pilot in confirming engine response to pilot command. The benefit of this in-built aircraft performance monitoring system, however, is not available to the LSO positioned on the carrier deck some distance from the approaching aircraft. The capability of monitoring engine noise as an acoustic sensation would enable the landing signal officer to sense the power level and pitch of the aircraft engine thereby to confirm if the engine is properly responding to glide slope signals communicated from the LSO to the pilot and thus add an additional measure of safety and accuracy to successful aircraft carrier landings. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an acousto-optic coupler for glide slope control systems which monitors aircraft engine noise and which communicates signals representative thereof to landing personnel. Another object of the present invention is to provide an acousto-optic communication system for the transmission of signals indicative of aircraft engine noise between an approaching aircraft and landing facilities such as an aircraft carrier. A further object of the present invention is to instantaneously provide to a landing signal officer or similar personnel an acoustic reproduction of aircraft engine noise as experienced by the pilot guiding an approaching aircraft along a glide slope pattern. A still further object of the invention is to enable the landing signal officer or similar personnel to provide corrective commands to an approaching pilot in a glide slope pattern based on information received from the acousto-optic communication system. 
     Briefly, these and other objects are accomplished by an acousto-optic coupler in a glide slope control system involving the transmission of at least one modulated light beam generated by approaching aircraft landing lights and which beam is modulated according to perturbations generated by engine noise which is mechanically coupled through the airframe to the filaments of the respective landing lights. An optical receiver system positioned on board the landing facilities such as an aircraft carrier is directed to optically receive the modulated light beam from the approaching aircraft, which light beam is detected by a photodiode to produce an analog output signal which is filtered to provide an output response in the frequency band of interest. The filtered output signal is amplified and coupled to an acoustic transducer such as a loudspeaker or headphone which provides monitoring of engine noise by landing facilities personnel on an aircraft carrier. 
     For a better understanding of these and other aspects of the invention, reference may be made to the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the use of the present invention between an aircraft carrier and an approaching aircraft along a glide slope path; and 
     FIG. 2 is a block diagram of the present invention as utilized within the illustration of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, there is shown a landing facility such as an aircraft carrier 10 positioned on a water body 12. An aircraft shown generally as 14 is approaching the carrier 10 along a glide slope S. The aircraft 14 illuminates an area forward of the aircraft and along the glide slope S by means of at least one landing light 18 attached to the airframe. An optical receiver system 16, which will be described in greater detail hereinafter, is positioned on board the carrier 10 and adapted to receive the oncoming light beam generated by the light 18 of approaching aircraft 14 within a portion of an illuminated area A denoted in the figure. 
     Referring now to FIG. 2, there is shown a block diagram of the present invention illustrating an aircraft landing light 18. The light 18 generates an output beam which is optically transmitted to the receiver 16 on board the carrier 10. An optical element denoted by a lens 20 is focused to receive the beam from the landing light 18. A photojunction detector 22 having a fast rise time response receives the light radiation from the lens 20 and produces an analog output signal indicative of the changes in light intensity. The analog output signal from the detector 22 is amplified by a preamp 24 and filtered by a filter 26 having a predetermined band pass. The filtered signal is connected to the input of audio amplifier 28 whose output is coupled to an acoustic transducer such as headphones 30. 
     Referring again to FIG. 1 in conjunction with FIG. 2, the operation of the invention will now be explained. Noise generated within the engine of the aircraft 14 is communicated along the airframe of the aircraft 14 to the landing light 18. The filament 19 within the light 18 is sensitive to the motion and vibration of the aircraft 14 and thus modulates the light beam in accordance with the engine noise produced, for example, by the rotating compressor. Typical engine noise develops over a region of approximately 800 to 1250 Hz. with a center frequency of 1000 Hz. The light 18 illuminates an area A forward of the approaching aircraft 14 while in the glide slope pattern and which illuminated area or portion thereof is sensed by the optical receiver 16 placed on board the carrier 10. One method of assisting the receiver 16 in focusing upon the light beam generated by an approaching aircraft is to mount the receiver 16 on the Fresnel lens platform of the carrier. The Fresnel lens provides, in a well known manner, a changing color pattern viewed by the approaching pilot to indicate a high, normal or low position in the glide slope path. Thus the gimbaled Fresnel lens platform is directed to continually focus on approaching aircraft and is not susceptible to changing pitch or roll in carrier position. The modulated information indicative of engine noise is transmitted by the light beam of the landing light 18 and is collected by the lens 20 in the receiver 16. The lens 20 may, for example, comprise a mirrored optical element of approximately 300 mm. focal length and an aperture speed of F1.8 to F2.3. With such a lens, the optically communicable distance between the carrier 10 and the aircraft 14 may be extended to as much as a mile. The lens 20 passes the collected light radiation to the photojunction detector 22 which, due to its fast rise time operating characteristics and while operating in a photovoltaic mode, generates as analog output signal indicative of the incoming optically modulated signal. The detector 22 will preferably comprise a planar diffused silicon photojunction diode. When operating in the photovoltaic mode as shown in FIG. 1, the only significant contribution from undesirable signal sources is thermal noise generated in the detector as contrasted with operating the detector as a biased PN junction which would have the undesirable effect of introducing dark current cathode noise as well. The detector 22 detects the changes in received light intensity and produces an analog output signal, for example, over the range of 1 to 5 mv. This output signal is then amplified by the preamp 24 which amplifies the incoming signal to an approximate one volt level. The preamp also serves to isolate the filter 26 from the detector 22, otherwise the signal bandwidth of the detector 22 may be restricted by direct connection to the filter 26. The amplified analog signal is then processed through the filter 24 having a band pass for example, of 800 Hz to 1250 Hz. indicative of the normal operating range of engine noise. The filter, of course, rejects all other undesirable signals not relevant to normally operating engine parameters and the band pass may be selectively changed so as to accommodate various aircraft engine noise frequencies. The filtered output signal is then amplified by audio amp 28 so as to provide sufficient energy to drive the headphones 30 which may alternately be a loudspeaker or similar acoustic transducer as the situation may require. The landing signal officer or similar personnel on board the carrier may then easily monitor the output of the headphones 30 so as to hear the engine noise generated by the approaching aircraft after having given a particular radio transmitted command to the pilot to correct or even abort a glide slope approach. This insures that the former communicated command has been properly interpreted and transmitted to the engine. The engine response is confirmed by an appropriate change in the level of compressor or other engine noise indicative of changing power levels. It will be appreciated that the incorporation of the acoustic signals generated by the present invention with other receiving apparatus used by the LSO may be easily accomplished by audio mixing devices well known in the art. 
     Thus it may be seen that there has been provided a novel acousto-optic coupler which provides a safety backup to insure the successful and accurate landing of approaching aircraft when in a glide slope pattern. 
     Obviously, many modifications and variations of the invention are possible in light of the above teachings. For example, the acousto-optic communications system need not be utilized exclusively for carrier operations, but may also be used in land based facilities wherein it is intended to enhance the safety of landing operations. Moreover, the preamp in the receiver may be provided with an AGC function to accommodate changing signal levels from approaching aircraft. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.