Abstract:
For use in an aircraft warning system which measures the descent below a radio glideslope and compares it with the aircraft&#39;s altitude above ground to generate a warning when the descent below glideslope exceeds predefined limits for a particular altitude, circuitry is provided to generate an advisory warning above a predefined altitude and a hard warning, or command to take immediate action, below the predetermined altitude.

Description:
CROSS REFERENCE 
     This application is a modification of my copending application Ser. No. 480,727 filed June 19, 1974, entitled &#34;Aircraft Ground Proximity Warning Instruments&#34; and assigned to the same assignee as the instant application. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to aircraft ground proximity warning systems and in particular to warning systems utilizing a glide slope radio beam wherein a warning is generated when the aircraft&#39;s descent below glide slope exceeds a predetermined value for a particular altitude. 
     In the prior art, specifically the U.S. Pat. application of Bateman Ser. No. 480,727, entitled &#34;Aircraft Ground Proximity Warning Instrument,&#34; filed on June 19, 1974 and assigned to the assignee of this application, a warning is generated whenever the aircraft is not in a safe zone of operation below a predefined altitude such as 650 feet above ground level. This safe zone of operation, or the distance below the glide slope, will vary depending upon the aircraft&#39;s altitude above ground. For example, between 650 feet and 150 feet above ground the aircraft must stay within a predefined angular distance, measured in dots, of the glide slope or a warning signal will be triggered. This feature allows the pilot to adjust the absolute altitude of the aircraft with somewhat greater latitude at greater altitudes where such maneuvers are inherently safer. 
     However, there are certain conditions under which the crew of an aircraft may wish to descend below the glide slope sufficiently to trigger the warning where such a descent would be in conformity with safe operating procedures. For example, there are a number of runways where the glide slope beam is substantially displaced from the threshold toward the middle of the runway. Thus, under visual operating conditions, it is not unusual for the crew of an aircraft to make use of this additional runway which in turn necessitates descending sufficiently below the glide slope to trigger a warning. A warning triggered under these conditions, especially at higher altitudes such as 650 feet, can be considered advisory at best and as a practical matter most crew members will consider it a nuisance warning with a resultant loss in credibility of the system as a whole. 
     However, in a majority of cases it is still considered desirable to have the warning envelope of the same general type provided by the system in Bateman 480,727 so that the aircraft&#39;s crew may be apprised of a potentially dangerous descent under the glide slope even at these greater altitudes. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a glide slope warning system with an advisory only warning above a predetermined altitude and a positive or hard warning below that predetermined altitude. 
     It is another object of the invention to provide a system for generating an advisory warning envelope above a predetermined altitude and a hard warning envelope below the predetermined altitude. 
     An additional object of the invention is to provide a glide slope warning system that generates a first type of warning when the aircraft descends below a first angular limit from a glide slope above a predefined altitude and generates a second type of warning when it descends below a second angular limit below the predefined altitude. 
     The glide slope warning system utilizes the combination of a glide slope signal from an instrument landing system (ILS) which indicates the aircraft&#39;s angular position with respect to a glide slope radio beam and an altitude above ground signal which is most commonly derived from a radio altimeter. The polarity and amplitude of the glide slope signal indicate the aircraft&#39;s relative position with respect to the glide slope radio beam. For example, if the aircraft is below the beam the glide slope signal will be positive, indicating &#34;fly-up&#34; condition. 
     The advisory warning is generated when the combination of the glide slope signal and the altitude signal exceeds a predetermined value above a predefined altitude. Normally the advisory portion of the warning system is activated between a maximum altitude, for example, 1000 feet and the predetermined lower altitude of 300 feet. The glide slope signal and the altitude signal are scaled to represent the number of dots, representing the angular distance that the aircraft is below the glide slope beam. When the air craft is in the advisory region, between 1000 feet and 300 feet, and the aircraft is below the predefined number of dots, an advisory warning signal is triggered. 
     By the same token, below the advisory level, 300 feet, a command warning, for example, a command to the crew to &#34;pull up&#34; will be generated when the combination of the glide slope signal and the altitude signal exceeds a value that represents a predefined number of dots. The number of dots of deviation from the glide slope is increased as compared to the number of dots allowed in the advisory area. 
     Once the aircraft descends below a certain altitude, such as 150 feet, it is assumed that the aircraft is fairly close to the end of the runway and, hence, the source of the glide slope beam. As the aircraft approaches the source of the glide slope beam, even a minor change of actual altitude above ground will result in a fairly substantial angular change. So, in order to reduce the sensitivity of the warning system, in terms of the number of dots required to trigger a warning, as the aircraft approaches the end of the runway, the sensitivity of the system is reduced on a linear basis from 150 feet to 50 feet. Below 50 feet the system is inhibited entirely in order to prevent nuisance warnings as the aircraft approaches touchdown. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graphical illustration of the advisory and command warning envelopes with respect to altitude and glide slope deviation. 
     FIG. 2 is a functional block diagram of the glide slope warning system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1 the advisory and command warning envelopes are set forth in terms of glide slope deviation and the aircraft&#39;s altitude above ground. The horizontal axis of FIG. 1 represents the aircraft&#39;s deviation below the glide slope as represented in dots. The vertical axis of FIG. 1 represents the aircraft&#39;s altitude above ground, as preferably measured by a radio altimeter. The shaded area 10 representing the advisory portion of the envelope extends from a maximum atitude of 1000 feet to a cut off altitude of 50 feet. The command warning area represented by the shaded portion 12 of the warning envelope extends from a maximum altitude of 300 feet above ground to the cut off altitude of 50 feet. The graph in FIG. 1 illustrates that an advisory warning will be generated when the aircraft is one dot or more below the glide slope at an altitude of 150 to 1000 feet and similarly will generate the command warning when the aircraft is two dots or greater below the glide slope, anywhere from 150 to 300 feet above the ground. 
     The preferred embodiment of the circuitry for carrying out the invention and generating warnings in conformity with the graph in FIG. 1 is set forth in FIG. 2. Both the altitude above ground signal h R  and the glide slope signal G/S are generated in a manner similar to that described in the application of Bateman Ser. No. 480,727. The altitude signal h R  is a direct current signal with the amplitude representing the aircraft&#39;s altitude above the ground. The altitude signal h R , having a negative polarity, is an input to the negative terminal of the summing junction 20. A bias signal representing a minus 150 feet is also an input to the summing junction 20 at its positive terminal. The resulting signal is then applied to the limiter 22, which limits the output of the summing junction 20 to a maximum of zero for altitudes of 150 feet or greater. 
     The limited altitude signal is then utilized as an input to the scaling amplifier 24, which produces a signal proportional to 1.17 dots per 100 feet of altitude. The scaled altitude signal is utilized as an input to the summing junction 26. The other input to the summing junction 26 is the glide slope deviation signal G/S, which also has its amplitude scaled in terms of dots. The scaling of the G/S signal in terms of dots, corresponds with the typical glide slope display utilized in conjunction with standard instrument landing systems in which a pointer is associated with a field of dots. Normally, a three dot indication is provided for indicating maximum &#34;fly up&#34; or &#34;fly down&#34; conditions. This C/S signal representing the number of dots the aircraft is below the glide slope is combined with the scaled altitude signal in the summing junction 26. The detector 28 will generate a positive signal, or a logic 1, on the line 30 when the sum of these two signals is greater than 1.0 dots. When the aircraft is between 1000 feet and 50  feet above ground and a valid G/S signal is received, the positive signal on line 30 will be transmitted through the AND gates 32 and 34 to an advisory alarm system. Typically, the advisory alarm will be a flashing light indicating to the crew that the aircraft has entered the advisory warning portion of the glide slope area. 
     The determination as to whether the glide slope signal is valid or not is provided by the glide slope signal filter 36, which is disclosed in detail in Batemen Ser. No. 480,727. The AND gate 32 also has an input 38 which serves to inhibit the gate when the aircraft is not utilizing an ILS system. The AND gate 34 is inhibited by a signal on line 40 indicating that the aircraft&#39;s altitude is less than 50 feet above the ground and is also inhibited by a signal on line 42 indicating the aircraft&#39;s flight configuration. For example, if the landing gear of the aircraft is up, a low signal will be produced on line 42 thereby inhibiting the glide slope advisory warning. In addition, the AND gate 34 receives a positive signal on line 44 whenever the aircraft is below 1,000 feet, which has the effect of activating the advisory warning system. 
     The function of the flip-flop 46 is to permit both the testing and the inhibiting of the warning system. Since it is occasionally desirable to be able to inhibit the system during flight, for example, if the aircraft&#39;s crew knows that it will be making a descent sufficiently below the glide slope to trigger a warning, the inhibit circuitry of FIG. 2 has been provided. The inhibiting of the warning system in the preferred embodiment of the invention can only occur when the aircraft is below 1,000 feet above ground and is not within the command warning portion 12 of the warning envelope. Under these conditions a positive inhibit signal will be transmitted from the OR gate 48 through the AND gate 50 having the effect of setting the flip-flop 46. The resulting positive signal from the Q terminal of the flip-flop 46 will inhibit the AND gate 32 thereby preventing either an advisory warning or a command warning signal from being generated. 
     The command warning signal 12 of FIG. 1 is generated only when the aircraft is below 300 feet. The altitude signal h R , as limited by the limiter 22, is scaled by the scaling amplifier 52 by a ratio equal to 1.33 dots per 100 feet of altitude. It should be noted here that the function of the scaling amplifier 52 can be accomplished with the scaling amplifier 24, thereby eliminating the amplifier 52. As with the advisory warning signal 10 of FIG. 1, the scaled altitude signal is combined with the G/S signal at the summing junction 54. The detector 56 generates a positive signal, or logic 1, on line 58 whenever the output of the summing junction 54 is equivalent to 2.0 dots or more. The AND gate 60 inhibits the command warning whenever the aircraft&#39;s altitude above ground is greated than 300 feet or whenever the aircraft is outside advisory warning area 10. It can therefore be appreciated that, as in conformity with the graph in FIG. 1, there will always be an advisory warning before the command warning signal is activated. The command warning alarm itself can be generated in a manner similar to the type alarm disclosed in Bateman Ser. No. 480,727. 
     It will be understood that the various altitudes and deviations from the glide slope, represented in dots, were provided to illustrate the preferred embodiment of the invention and are not in any way intended to limit the concepts disclosed or claimed in any manner.