Abstract:
In a ground proximity warning system for an aircraft, a signal representing clearance of the aircraft from the underlying terrain is produced from a sea level related altitude signal and a terrain database in addition to the radio altitude signal of the aircraft&#39;s radio altimeter. An indication of reasonableness of the aircraft radio altitude signal is formed jointly responsive to the aircraft radio altitude signal and the terrain clearance signal.

Description:
This application claims priority under 35 U.S.C. § 119(e) to U.S. provisional application Ser. No. 60/157,901 filed Oct. 5, 1999 that is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to aircraft warning systems and more particularly to arrangements for determining the conditions to inhibit false warnings of a ground proximity warning system. 
     BACKGROUND OF THE INVENTION 
     An aircraft generally uses a ground proximity warning system (GPWS) to alert its flight crew to conditions that could result in aircraft crashes due to the position of the aircraft with respect to the terrain along its flight path. GPWS is designed to generate warnings if an aircraft enters a flight path to the ground that could lead to a potentially dangerous situation. Such hazardous conditions could result from an excessive descent rate, an excessive terrain closure rate, loss of altitude after takeoff, insufficient terrain clearance when not in a landing configuration or descent below the instrument landing system (ILS) glide-slope. The GPWS uses inputs from systems that provide indications of radio altitude, aircraft position, airspeed/Mach number, landing gear and flap position and decision height (DH) setting. In situations where an aircraft is deemed too close to the ground, the GPWS system issues an audible and/or visual alarm. In response, the flight crew immediately changes the flight path of the aircraft to avoid a potential crash. 
     The GPWS system usually relies on a radio altimeter using radar to track the position of the aircraft with respect to the ground. The radio altimeter determines the altitude of the aircraft by reflecting radio waves from the ground. There are, however, numerous reported GPWS nuisance alarms caused by false radio altimeter tracking. The rapid upward pitching of the aircraft when the flight crew responds to a nuisance alarm results in avoidable passenger and flight attendant discomfort. 
     When the aircraft is not in a landing configuration, a warning issued for insufficient terrain clearance may be generated if there is false radio altimeter tracking. Such false tracking can occur, for example, when the aircraft is flying through severe rain and/or hail conditions or when the aircraft is overflying another aircraft. The returns from the aircraft radio altimeter system can reflect the position of the rain and/or hail condition or the underlying aircraft and falsely indicate that the aircraft is too close to the ground so that a GPWS warning is issued. 
     FIG. 1 illustrates two conditions under which a radio altimeter may return false indications of altitude. In FIG. 1, an aircraft  105  is at a safe height D 1  over the terrain  100  but is flying above an area  110  exhibiting heavy clouds, severe rain and/or hail. Radio altimeter signals are returned to the aircraft  105  that indicate a distance D 2  from a severe cloud condition  110 . The indication of the aircraft radio altimeter of the D 2  height of aircraft  105  above the condition  110  may trigger a GPWS warning. Aircraft  115  in FIG. 1 is also at a safe height D 1  over the terrain  100  but is passing over another aircraft  120 . Its radio altimeter returns signals indicating a height D 3  over the terrain  100  due to the presence of the aircraft  120  and the false indication may trigger a warning from the GPWS system in aircraft  115 . In these situations, the aircraft crew must respond by changing aircraft flight path to avoid an apparent hazard. Such false indications of an aircraft&#39;s radio altimeter present a serious problem to safe and comfortable aircraft flight. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention is directed to inhibiting false ground proximity warnings of an aircraft resulting from misleading aircraft radio altimeter indications. According to the invention, a radio altitude signal for the aircraft and a separately determined signal representing the clearance of the aircraft from the underlying terrain are generated. In response to the radio altimeter altitude signal and the terrain clearance signal, an indication of the reasonableness state of the radio altitude signal is issued. 
     According to one aspect of the invention, an indication of an unreasonable state of the radio altimeter signal is issued in response to a comparison of the terrain clearance signal and the radio altitude signal. 
     According to another aspect of the invention, an indication of an unreasonable state of the radio altimeter signal is issued in response to the terrain clearance signal less the radio altitude signal being larger than a preset value. 
     According to another aspect of the invention, issuance of a radio altitude unreasonableness state indication is inhibited when the separately determined terrain clearance signal is less than a predetermined value. 
     According to yet another aspect of the invention, the terrain clearance signal is formed by generating a signal representative of the geometric altitude of the aircraft, generating a signal representative of the height of the terrain underlying the aircraft and producing a signal corresponding to the difference between the geometric altitude and the terrain height. 
     According to yet another aspect of the invention, the terrain height signal is generated from a terrain database that includes signals representative of the maximum heights of sections of the terrain under the flight path of the aircraft. 
     According to yet another aspect of the invention, the geometric altitude signal corresponds to the altitude of the aircraft relative to sea level and is generated from Global Positioning System signals received by the aircraft. 
     According to still another aspect of the invention, a signal representative of the validity of the aircraft position and terrain data is formed. If the aircraft position or terrain data is in an invalid state, the indication of reasonableness of the radio altitude signal is inhibited. 
     In an embodiment of the invention, signals corresponding to the geometric altitude of an aircraft relative to sea level and the location of the aircraft are generated responsive to data from a Global Position System receiver. A terrain clearance signal is formed from the difference between the geometric altitude and data of the height of the underlying terrain in a terrain database. A processor checks whether flight path data, terrain data and vertical and horizontal data are valid. The aircraft&#39;s radio altitude signal is compared to the terrain clearance signal and an indication of unreasonableness of the radio altitude signal is issued when the terrain clearance less the radio altitude is larger than a first predetermined amount. The indication of unreasonableness is inhibited when the aircraft is below a preset altitude or when any one of the terrain database data, latitude or longitude data, or altitude data is determined to be invalid. 
    
    
     The invention will be better understood from the following more detailed description taken together with the accompanying drawings and the claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates altitude measurements obtained by the radio altimeter of an aircraft; 
     FIG. 2 is a block diagram depicting an arrangement for determining the reasonableness state of an aircraft radio altimeter reading illustrative of the invention; 
     FIG. 3 is a flow chart illustrating the operation of the system validity processor of FIG. 2; 
     FIG. 4A shows terrain heights in blocks of the terrain along the flight path of the aircraft; 
     FIG. 4B depicts a database entry for the terrain blocks shown in FIG. 4A; 
     FIG. 5 illustrates the operation of the arrangement of FIG. 2.; and 
     FIG. 6 is a flow chart illustrating the operation of an alternative processor arrangement for determining the reasonableness state of an aircraft radio altimeter altitude indication illustrative of the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 2 shows a block diagram of a system that determines the reasonableness of a radio altimeter value for use in a GPWS system so that nuisance alarms are avoided under prescribed flight conditions. In FIG. 2, there is shown a system data validity processor  201 , a radio altimeter signal former  205 , a geometric altimeter signal former  210 , a terrain database  215 , arithmetic circuits  220  and  225 , comparators  230  and  235 , an AND gate  240  and a pulse former  245 . The outputs of the system data validity processor  201  and the outputs of the comparator  235  and the comparator  230  are coupled to inputs of the AND gate  240  and the output of the AND gate  240  is connected to the input of the pulse former  245 . The outputs of the geometric altitude signal former  210  and the terrain database  215  are coupled to inputs of arithmetic circuit  220  and the output of the radio altimeter signal former  205  and the output of the arithmetic circuit  220  are coupled to inputs of the arithmetic circuit  225 . A preset signal A and the output of the arithmetic circuit  220  are connected to inputs of the comparator  235  and a preset signal B and the output of the arithmetic circuit  225  are connected to inputs of the comparator  230 . 
     In operation, the system data validity processor  201  receives information as to the validity of indicator signals used. These indicator signals are obtained from aircraft instruments, from a global positioning system (GPS) receiver and other receivers on the aircraft. The indicator signals indicate the validity of data in the terrain database  215 , latitude data and longitude data from the GPS, and altitude data from a source such as the GPS. The validity processor  201  operates according to the flow chart of FIG.  3  and produces an output that inhibits radio altitude reasonableness checking in response to detection of an invalid state of any of the indicator signals applied to the system data validity processor  201 . 
     Referring to FIG. 3, the repeated sequence of data validity checks is started in a decision step  301  in which it is determined whether a validity flag in the terrain database  215  is set. If the terrain database validity flag is set, the validity of the latitude and longitude signals from the GPS receiver of the aircraft are sequentially determined in decision steps  310  and  315 . Whether the altitude indicator signals from the aircraft instruments are valid is checked in the decision step  330 . Upon determining a “yes” in each of decision steps  301  through  330 , the output of the system data validity processor  201  is set to provide an enabling input to the AND gate  240  to check the reasonableness of the radio altitude signal in a step  335 . In the event a “no” is determined in any of the decision steps  301 - 330  during the validity checking sequence, a step  340  is entered in which the system data validity processor is set to a state that provides an input to the AND gate  240  for inhibiting checking the reasonableness of the radio altimeter signal. The decision step  301  is reentered from either the step  335  or the step  340  to reiterate the validity checking of the system validity processor  201 . 
     The geometric altitude signal former  210  in FIG. 2 generates a signal representative of the altitude of the aircraft relative to sea level. Such geometric altitude data may be obtained from a GPS receiver in the aircraft. The geometric altitude data is supplied to a positive input of the arithmetic unit  220 . 
     The terrain database  215  that supplies data to a negative input of the arithmetic unit  220  is illustrated in FIGS. 4A and 4B. FIG. 4A depicts a view of nine sections  401 - 1  through  401 - 9  of predetermined dimensions (e.g., ¼ mile by ¼ mile) of a terrain  400  underlying the flight path of the aircraft. The data in each section represents the maximum height of the terrain section. FIG. 4B illustrates an entry in the terrain database that includes a validity flag  410  and terrain section location data  420  and the terrain maximum height data  425  according to location. The maximum height of terrain section  401 - 5  underlying the location of the aircraft is subtracted from the geometric altitude of the aircraft in the arithmetic unit  220  and the arithmetic unit  220  forms an output signal representing the computed terrain clearance of the aircraft. The terrain clearance signal applied to arithmetic unit  225  and to the comparator  235  is independent of altitude indications of aircraft&#39;s radio or barometric altimeters. 
     The comparator  235  operates to compare the terrain clearance signal from the arithmetic unit  220  to a preset value A representing an altitude below which there should be no unreasonabless signal from the system of FIG.  2 . The terrain clearance signal corresponds to a pseudo radio altitude generated independently of the radio altimeter. In the event that the terrain clearance signal is less than the preset value A, the pseudo radio altitude of aircraft is in a range wherein the system of FIG. 2 should not be activated and the output of the comparator  220  inhibits the AND gate  240 . 
     The terrain clearance signal is also applied to the arithmetic unit  225  which compares the terrain clearance signal representing the pseudo radio altitude with the aircraft&#39;s radio altitude signal. The arithmetic unit  225  operates to provide an enabling signal to the AND gate  240  when the terrain clearance signal less the radio altitude signal obtained from the aircraft radio altimeter  205  is larger than a preset value B. Accordingly, the AND gate  240  is placed in its ON state when the output of the arithmetic unit  225  is larger than the preset value B provided that the terrain clearance signal is greater than the preset value A and the system data validity processor is in its enabling state. When enabled, the AND gate  240  triggers the pulse former  245 . In response to the ON state output of the AND gate  240 , the pulse former  245  (e.g., a one shot circuit) outputs a single pulse of predetermined duration. In this way, pseudo radio altitude value of the terrain clearance signal cross checks the aircraft radio altitude signal to inhibit a false GPWS alert. If there is an occurrence of a condition in which pseudo altitude and the aircraft&#39;s radio altitude indication are sufficiently different when the system data is valid and the aircraft is above a predetermined altitude, the GPWS is inhibited for the predetermined period. 
     The operation of the system shown in FIG. 2 is illustrated in FIG. 5 wherein an aircraft  510  flies over a terrain  500  at an altitude represented by a dotted line  512 . A line  520  represents a preset altitude A (e.g., 4000 feet) below which the operation of the system of FIG. 2 is inhibited and a line  525  represents the upper edge of a heavy rain area  530 . Assume that the pseudo radio altitude of the terrain clearance signal is between the altitude  512  and the line  525  and that the area  530  causes the aircraft radio altitude signal to indicate the distance between the lines  520  and  525  (e.g., 500 feet) as the aircraft&#39;s altitude. The output of the comparator  230  in FIG. 2 then provides an enabling input to the AND gate  240 . If the output of the system data validity processor  201  is enabling, the terrain clearance signal less the aircraft radio altitude signal from arithmetic unit  225  being greater than 2000 feet enables the AND gate  240 . The pulse former  245  then generates a single pulse for a preset GPWS inhibit period (e.g., 60 seconds). During this preset inhibit period, a radio altitude unreasonableness signal is applied to prevent a GPWS alert. Absent the heavy rain area  530 , the difference between the radio altitude signal (line  512 ) and the terrain clearance (between lines  512  and  525 ) is less than the preset value A (2000 feet). Consequently, no pulse is generated by the pulse former  245  to inhibit GPWS. 
     A signal processor unit under control of a program stored in a memory may be used instead of the arrangement of arithmetic units  220  and  225 , comparators  230  and  235 , AND gate  240  and pulse former  245  to perform the reasonableness processing function. FIG. 6 shows a flow chart illustrating the operation of a processor arrangement that may be substituted for the circuit of FIG. 2 in determining the reasonableness of a radio altimeter altitude indication for a GPWS system. In FIG. 6, it is determined in decision step  601  whether system data checked by system validity processing shown in FIG. 3 is valid. If yes, the terrain clearance signal is formed in step  605  by subtracting the terrain height signal of the terrain underlying the aircraft in the terrain database of FIG. 4A from the geometric altitude signal formed in a GPS receiver. If no in step  601 , decision box  601  is reentered. 
     Whether the terrain clearance is greater than the preset value A (e.g., 4000 feet) is checked in decision step  610 . If yes in the step  610 , whether the terrain clearance signal less the aircraft&#39;s radio altitude signal is greater than the preset value B (e.g., 2000 feet) is determined in the step  615 . If no in the step  610 , the step  601  is reentered. When the difference between the pseudo altitude of the terrain clearance and the aircraft&#39;s radio altitude is greater than B, an unreasonableness pulse is generated for a predetermined period in step  620  and the step  601  is reentered from the step  620 . If the difference between the pseudo altitude and the aircraft&#39;s radio altitude is less than B in the step  615 , the step  601  is reentered from the step  615 . 
     While the invention has been described by way of particular illustrative embodiments, it is to be understood that the invention is not limited to the above-described embodiments but that various changes and modifications may be made by those of ordinary skill in the art without departing from the scope and spirit of the invention. Accordingly, the foregoing embodiments should not be construed as limiting the scope of the invention which is encompassed instead by the following claims.