Abstract:
The present invention is a method of deriving a ground speed of an aircraft on a descent along a flight path. A vertical speed signal is produced as a function of an altitude signal and a vertical acceleration signal. The vertical speed signal is transformed to a nominal ground speed signal based upon a glide slope defined by a glide slope beam. A correction is produced based on a glide slope deviation rate representative of deviation of the aircraft from the glide slope. The nominal ground speed signal is corrected with the correction to produce a corrected ground speed signal. The corrected ground speed signal is filtered with a horizontal acceleration signal and a runway heading signal to produce a smoothed ground speed signal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the field of aircraft guidance systems. More specifically, the present invention relates to a system and method for monitoring the ground speed of a descending aircraft. 
     The interest in developing a global positioning satellite (GPS) sensor as a low-cost alternative to high-grade inertial reference systems (IRS) for various avionics applications has been germinating for some time now. Recently, there has been increased interest in the application of such an alternative to support head-up guidance systems (HGS). 
     Head-up guidance systems (HGS) are used primarily to enhance a pilot&#39;s situation awareness and provide guidance to pilots during low visibility conditions. With HGS, pilots can focus on external view and read critical flight data from the HGS instead of from the instrument panel. Although HGS can be used during all phases of flight, they are most renowned for providing guidance during approach and landing. Current HGS use instrument landing systems (ILS) for position guidance during approach and landing, and are currently coupled to high-grade inertial reference systems (IRS) to provide display orientation parameters. Attitude heading reference systems (AHRS) can provide some of these parameters with adequate accuracy, but they do not provide ground speed and track angle. An integrated GPS/AHRS function can provide improved attitude accuracy along with ground speed and track angle, but, by itself, the integrity of the ground speed and track angle are tied to the well-known shortcoming of standalone GPS integrity. This shortcoming needs to be rectified with enhancements from information sources external to the integrated GPS/AHRS function itself. 
     Accordingly, there is a need for a system and method for accurately monitoring the ground speed of a descending aircraft that is completely independent from the GPS function. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a system and method for deriving a ground speed of an aircraft on a descent along a flight path. A vertical speed signal is produced as a function of an altitude signal and a vertical acceleration signal. The vertical speed signal is transformed to a nominal ground speed signal based upon a glide slope defined by a glide slope beam. A correction is produced based on a glide slope deviation rate representative of deviation of the aircraft from the glide slope. Based on the nominal ground speed signal and the deviation correction, a corrected ground speed signal is produced. The corrected ground speed signal is filtered with a horizontal acceleration signal and a runway heading signal to produce a smoothed ground speed signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram of an aircraft guidance system incorporating the present invention. 
     FIG. 2 shows a block diagram of a GPS/AHRS processor of the aircraft guidance system of FIG.  1 . 
     FIG. 3 shows a block diagram of a complementary filter of the GPS/AHRS processor of FIG.  2 . 
     FIG. 4 shows a block diagram of a glideslope-based ground speed monitor of the GPS/AHRS processor of FIG.  2 . 
     FIG. 5 shows a block diagram of a complementary filter of the glideslope-based ground speed monitor of FIG.  4 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a block diagram of an aircraft guidance system  10  incorporating the present invention. Aircraft guidance system  10  is a multiple component system which includes magnetic flux sensor  12 , GPS sensor  14 , radar altimeter  16 , air data computer  18 , ILS receiver  20 , GPS/AHRS processor  22 , and head-up guidance system (HGS)  24 . 
     GPS/AHRS processor  22  accepts data from magnetic flux sensor  12 , GPS sensor  14 , radar altimeter  16 , air data computer (ADC)  18 , and ILS receiver  20 . Magnetic flux sensor  12  provides magnetic heading data; GPS sensor  14  provides GPS data; radar altimeter  16  provides altitude data; ADC  18  includes a baro-altimeter which provides baro-altitude data; and ILS receiver  20  provides glideslope deviation data. Glidepath angle and runway heading data are also provided to GPS/AHRS processor  22  from approach plate charts by the pilot. The data output of GPS/AHRS processor  22 , in turn, supports HGS  24 . GPS/AHRS processor  22  provides attitude, vertical speed, and ground speed data. HGS  24  also accepts data from radar altimeter  16 , ADC  18 , and ILS receiver  20 . 
     FIG. 2 shows a functional block diagram  30  of the processing that occurs within GPS/AHRS processor  22  of aircraft guidance system  10  of FIG.  1 . The GPS data from GPS sensor  14  of aircraft guidance system  10  is initially pre-processed at step  32 . The GPS data is then checked for soft failures for GPS deltaranges at step  34 . Soft failures can be caused by a slow drifting or a sudden shift of the satellite clock frequency, errors in broadcast ephemeris data for satellite clock corrections, etc. Inertial measurements are taken in step  36 , and compensated for temperature in step  38 . The GPS data from step  34  and the inertial data from step  38  are each used in step  40  for Integrated Navigation processing. The output data of step  40  include attitude, vertical acceleration, horizontal acceleration, and ground speed (each compensated for accelerometer bias errors). The inertial data from step  38  is also used in step  42  for standard AHRS processing. In addition, the baro-altitude signal from ADC  18  and the magnetic heading signal from magnetic flux sensor  12  are processed in step  42 . The output data from step  42  include attitude and vertical acceleration. Step  44  compares the Integrated Navigation attitude signal from step  40  to the standard AHRS attitude signal from step  42 . Any out-of-bound comparison is flagged as an alert. The Integrated Navigation attitude signal will be the output attitude signal of GPS/AHRS processor  22  unless an alert is flagged, in which case the standard AHRS attitude signal will be the output attitude signal. The vertical acceleration signal from step  40  and the baro-altitude signal from ADC  18  are complementary filtered in step  46  to produce a first complementary-filtered vertical speed signal. The standard AHRS vertical acceleration signal from step  42  and the baro-altitude signal from ADC  18  are complementary filtered in step  48  to produce a second complementary-filtered vertical speed signal. Step  50  compares the first and second complementary-filtered vertical speed signals from steps  46  and  48  respectively, and any out-of-bound comparison is flagged as an alert. The first complementary-filtered vertical speed signal from step  46  will be the output vertical speed signal of GPS/AHRS processor  22  unless an alert is flagged, in which case the second complementary-filtered vertical speed signal from step  48  will be the output vertical speed signal. The Integrated Navigation horizontal acceleration signal from step  40 , the complementary-filtered vertical speed signal from step  48 , the glideslope deviation signal from ILS receiver  20 , and the glidepath angle and runway heading signals from approach plate charts are all used in step  52  to generate a glideslope-based ground speed signal. Step  54  compares the Integrated Navigation ground speed signal from step  40  to the glideslope-based ground speed signal from step  52 , and any out-of-bound comparison is flagged as an alert. The Integrated Navigation ground speed signal from step  40  will be the output ground speed signal of GPS/AHRS processor  22  unless an alert is flagged, in which case the glideslope-based ground speed signal from step  52  will be the output ground speed signal. 
     FIG. 3 shows a transfer function block diagram  60  of the processing that occurs at step  48  of functional block diagram  30  of FIG.  2 . The AHRS vertical acceleration signal from step  42  of functional block diagram  30  is corrected for gravity, Coriolis acceleration, and estimated sensor errors in step  62 . The corrected vertical acceleration signal from step  62  is mathematically integrated in step  66  to produce a vertical speed signal. This vertical speed signal is integrated in step  70  to produce an altitude signal that is used in steps  72  and  76 . Step  72  takes the altitude signal and provides a correction signal (based on gravity and the Earth&#39;s radius) that is added in step  64  to the corrected vertical acceleration signal from step  62 . The vertical speed signal from step  66  is multiplied by a lag constant Kl in step  74  to compensate the lag in the baro-altitude measurement. In step  76 , the lag signal from step  74  and the baro-altitude signal from ADC  18  are subtracted from the altitude signal from step  70  to produce an altitude error signal that is used in steps  78 ,  80 , and  82 . Step  78  multiplies the altitude error signal by a position constant Kp and provides a correction signal that is subtracted in step  68  from the vertical speed signal from step  66 . Step  80  multiplies the altitude error signal by a velocity constant Kv and provides a correction signal that is used in step  86 . Step  82  multiplies the altitude error signal by an acceleration constant Ka and provides a correction signal that is integrated in step  84 . Constants Kl, Kp, Kv, and Ka are each design parameters chosen for the filter response desired. In step  86 , the correction signal from step  84  is added to the correction signal from step  80 , and the resulting signal is subtracted in step  64  from the corrected vertical acceleration signal from step  62 . The output signals of functional block diagram  60  are the smoothed vertical speed signal from step  66  and the altitude signal from step  70 . 
     FIG. 4 shows a functional block diagram  90  of the processing that occurs at step  52  of functional block diagram  30  of FIG.  2 . The glidepath angle data from approach plate charts is used in step  92  to convert the smoothed (complementary-filtered) vertical speed signal from step  48  of functional block diagram  30  to a nominal ground speed signal. The glide slope deviation signal from ILS receiver  20  is used in step  94  to produce a ground speed correction signal. In step  96 , the ground speed correction signal from step  94  is subtracted from the nominal ground speed signal from step  92  to produce a corrected ground speed signal. The horizontal acceleration signal from step  40  of functional block diagram  30  and the runway heading data from approach plate charts are used in step  98  to produce an along track acceleration signal. In step  98 , the horizontal acceleration signal is projected in the direction of the runway heading to compensate for the crab angle effect. The corrected ground speed signal from step  96  and the along track acceleration signal from step  98  are complementary filtered in step  100  to produce a smoothed ground speed signal. 
     FIG. 5 shows a transfer function block diagram  110  of the processing that occurs at step  100  of functional block diagram  90  of FIG.  4 . The along track acceleration signal from step  98  of functional block diagram  90  is mathematically integrated in step  112  to produce an along track ground speed signal. In step  114 , the corrected ground speed signal from step  96  of functional block diagram  90  is subtracted from the along track ground speed signal from step  112  to produce a ground speed error signal. This ground speed error signal is filtered by a transfer function block in step  116  to reduce the high frequency noise contribution of the corrected ground speed signal (step  114 ), where K and τ are design parameters. In step  118 , the output signal from step  116  is subtracted from the along track ground speed signal from step  112  to produce a smoothed ground speed signal. 
     In summary, the present invention introduces a system and method for accurately monitoring the ground speed of a descending aircraft that is completely independent from a GPS function. By incorporating the present invention, a head-up guidance system can utilize a relatively low-cost integrated GPS/AHRS function to provide very accurate ground speed without the shortcoming of standalone GPS integrity. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.