Abstract:
A terrain avoidance system, method and computer program product for reducing nuisance alarms. The system includes a geometric altitude component, first and second vertical safety margin generators, and an alert component.

Description:
PRIORITY CLAIM 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/263,862, filed in the name of Conner et al. on Jan. 23, 2001, the complete disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     An enhanced ground proximity warning system (EGPWS) monitors a threat in front of an aircraft. In cases where the aircraft experiences an altitude error, the aircraft could crash into terrain without any EGPWS generated alerts. This problem arises especially when the EGPWS is installed in aircraft that operates very close to the ground, such as helicopters. Therefore, there exists a need for a system that would provide consistent and reliable EGPWS alerts in such an environment, thereby enhancing aircraft safety. 
     SUMMARY OF THE INVENTION 
     An improved aircraft terrain avoidance system, method and computer program product that reduces nuisance alarms is provided. The system includes a geometric altitude component, first and second vertical safety margin generators, and an alert component. The geometric altitude component generates a geometric altitude vertical error value based on barometric altitude and a positioning system value, such as a global positioning system generated value. The first vertical safety margin generator generates a first vertical safety margin value based on the generated vertical error value and safety margin limits. The second vertical safety margin generator generates a second vertical safety margin value based on the generated first vertical safety margin value, aircraft groundspeed, and aircraft distance to a selected runway. The alert component outputs an alert signal to the flight crew if it determines an alert condition exists based on the generated vertical safety margin. 
     In accordance with further aspects of the invention, the second vertical safety margin generator includes a groundspeed-based generator, a distance-from-runway-based generator, and a selector. The groundspeed-based generator generates a groundspeed safety margin value based on the first safety margin value, aircraft speed, and a predefined hover and approach speed. The distance-from-runway-based generator generates a distance-from-runway safety margin value based on the first safety margin value, a predefined runway distance bias, and aircraft distance to a selected runway. The selector makes the lesser of the groundspeed safety margin value and distance-from-runway safety margin value the second vertical safety margin value. 
     In accordance with other aspects of the invention, the system further includes a terrain floor generator that generates a terrain floor height value based on aircraft groundspeed, a predefined hover speed and approach speed, and a runway distance based terrain floor height value. The alert component further outputs an alert signal to the flight crew if an alert condition exists based on the generated terrain floor height value. 
     As will be readily appreciated from the foregoing summary, the invention provides a ground avoidance system that takes into consideration an altitude error value, and an aircraft&#39;s speed and position relative to an airport when determining how to evaluate threats. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings. 
     FIG. 1 is a black diagram illustrating components of the present invention; 
     FIGS. 2-5 are flow diagrams of an embodiment of the present invention showing a process for generating an improved delta height bias component; 
     FIG. 6 is a flow diagram for determining a terrain floor delta height boundary; and 
     FIG. 7 is a graph of terrain floor delta height as a function of distance from a runway. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention is an Enhanced Ground Proximity Warning System (EGPWS) that provides a vertical safety margin (delta height (DH) bias) that takes into consideration geometric altitude error (vertical figure of merit (VFOM)), see FIGS. 1-5. Geometric altitude is an aircraft altitude value that has taken into consideration barometric altitude and a global positioning system (GPS) component. VFOM is a vertical error component of geometric altitude. Geometric altitude and VFOM are described by example in U.S. Pat. No. 6,216,064, which is hereby incorporated by reference. The result of this embodiment is an EGPWS that more accurately estimates aircraft position. With a more accurate measurement, the terrain floor is reduced in order to reduce the occurrence of nuisance warnings, see FIGS. 1 and 6. 
     FIG. 1 illustrates an example EGPWS  18  that includes a DH component  20  for providing a DH bias based on VFOM, aircraft groundspeed (GS) and aircraft distance to selected runway, a terrain floor delta height (TFDH) component  22  for providing a TFDH value based on groundspeed and a previously determined TFDH boundary, and an alerting component  23 . The DH component  20  includes a nominal DH bias processing component  24 , a GS-based DH bias processing component  28 , an aircraft-distance-to-selected-runway-based (distance to runway) DH bias processing component  30 , and a selector  34 . The nominal DH bias processing component  24  generates a nominal DH bias based on an EGPWS generated DH bias upper and lower limits and VFOM. The GS-based DH bias processing component  28  generates a DH bias based on the GS and the generated nominal DH bias. The distance to runway processing component  30  generates a DH bias based on the corrected aircraft distance to selected runway end and the nominal DH bias. The selector  34  selects the lesser of the DH biases generated from the components  28  and  30 . The selected DH bias is sent to the alerting component  23 . These processes are described in more detail below in FIGS. 3-6. 
     Also shown in FIG. 1 are the contents of the TFDH component  22 . The TFDH component  22  generates a TFDH value that takes into consideration aircraft groundspeed and a previously determined TFDH boundary. The TFDH component  22  includes a GS-based TFDH processing component  42 , a runway distance-based TFDH processing component  44 , and a selector  48 . The GS-based TFDH processing component  42  generates a TFDH value based on a TFDH vs. GS curve up to a predetermined TFDH limit value. The runway distance-based TFDH processing component  44  generates a TFDH value based on a TFDH vs. distance to runway curve (i.e. TFDH boundary). The TFDH vs. distance to runway curve is previously stored EGPWS memory and is shown in FIG. 7 below. The selector  48  chooses the lesser of the TFDH values outputted from the components  42  and  44 . 
     The component  20  and  22  send the chosen DH bias value and TFDH value, respectively, to the alert component  23  for analyzing the present flight parameters and determining if an alerting condition exists. 
     FIG. 2 illustrates a preferred process performed by the component  20  of the EGPWS  18 . First, at block  70 , the process generates a nominal DH bias based on geometric altitude and VFOM, see FIG. 3 for more detail. Next, at block  72 , the process determines a first DH bias using GS and the generated nominal DH bias, see FIG. 4 for more detail. At block  74 , the process generates a second DH bias based on a corrected distance to runway end value and the generated nominal DH bias, see FIG. 5 for more detail. The process selects the lesser of the first and second DH bias and sends it to the alerting component  23 , see block  76 . 
     FIG. 3 illustrates the process from block  70  of FIG.  2 . First, at decision block  90 , the process determines whether VFOM is greater than or equal to a DH bias lower limit. The DH bias lower limit is preferably zero. If VFOM is not greater than or equal to the DH bias lower limit, a nominal DH bias is made equal to zero, see block  92 . If VFOM is greater than or equal to the DH bias lower limit, a nominal DH bias is set equal to VFOM minus the DH bias lower limit, see block  96 , and a decision shown in decision block  98  is performed. At decision block  98 , the process determines whether the nominal DH bias from block  96  is greater than a DH bias upper limit. If the nominal DH bias is greater than the DH bias upper limit, the nominal DH bias is made equal to the DH bias upper limit, see block  100 . If the nominal DH bias is not greater than the DH bias upper limit, the nominal DH bias does not change and the process, at block  102 , outputs the nominal DH bias to the alerting component  23  and then returns to decision block  90  as long as the EGPWS  18  and the component  20  remain activated. The DH bias upper limit is preferably very large for allowing the nominal DH bias to be the default DH bias more often than not. After blocks  92  and  100 , the process also proceeds to block  102  where the nominal DH bias is outputted. From block  102  the process returns to decision block  90 . The DH bias upper and lower limits are preferably predetermined values stored in the EGPWS  18 . 
     FIG. 4 illustrates the process form block  72  of FIG.  2 . First, at decision block  110 , the process determines whether the aircraft&#39;s GS is less than or equal to a predefined hover speed. The hover speed is a value previously determined based on the flight parameters of the associated aircraft, such as an aircraft that performs vertical or near vertical take-off and landings (VTOL) (e.g., helicopters, Harriers, Ospreys). If the GS is less than or equal to the hover speed, the DH bias is made equal to zero, see block  112 . If the GS is not less than or equal to the hover speed, the process determines whether the GS is less than or equal to an approach speed, see decision block  114 . If the condition in decision block  114  is true, DH bias is solved as follows in Equation (1).                DH                 bias     =         nominal                 DH                 bias         V   App     -     V   Hov              (       V   g     -     V   Hov       )               (   1   )                                
     V g =groundspeed 
     V App =approach speed 
     V Hov =hover speed 
     Otherwise, DH bias is made equal to the nominal DH bias. The approach speed, like the hover speed, is previously determined according to associated aircraft flight parameters. 
     FIG. 5 illustrates the process from block  74  of FIG.  2 . First, at decision block  130 , the process determines whether the aircrafts distance from a selected runway is less than or equal to a runway bias. FIG. 8 shows that the runway bias is the distance from the runway end where a previously determined TFDH limit is reached. In this example runway bias equals 2.5 nm (offset(1 nm)+1.5 nm (i.e., distance to reach the TFDH limit of 150 ft. at a DH slope of 100 ft/nm). If the check at decision block  130  is true, DH bias is set equal to zero, at block  132 , If the check at decision block  130  is false, the process checks whether the condition in Equation (2) is true.              Drwy              ≤                nmRwyBias              +                  nominal                 DH                 bias     DHslp1               (   2   )                                
     Drwy=aircraft&#39;s corrected distance from runway 
     nmRwy Bias=runway bias 
     DHslp1=DH slope 
     If equation (2) is false, the DH bias is made equal to the nominal DH bias, otherwise, equation (3) is applied. 
     
       
           DH bias=DH slp 1( Drwy−nmRwyBias )  (3)  
       
     
     FIG. 6 illustrates a preferred process performed by the TFDH processing component  22  of the EGPWS  18 . First, at block  150 , the process generates a GS-based TFDH value. At block  152 , the generated process selects the lesser of the GS-based TFDH and an EGPWS generated TFDH. Next, at block  156 , the selected TFDH is sent to the alerting component  23  for processing. 
     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.