Integrated barometric altitude and satellite altitude-based vertical navigation system

A flight management system (FMS) for an aircraft, that includes an integrated barometric altitude and satellite altitude-based vertical navigation (VNAV) system. The integrated barometric altitude and satellite altitude-based vertical navigation system, includes an altitude blending component for providing a smooth transition from a barometric altimetry source to a satellite altimetry source; and, a satellite altitude containment component operatively connected to the altitude blending component for limiting the difference of a barometric altitude path deviation to within a desired margin of a satellite altitude path deviation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to vertical navigation (VNAV) systems for aircraft and more particularly to an integrated barometric altitude and satellite altitude-based system for enhancing aircraft flight management system (FMS) vertical navigation systems.

2. Description of the Related Art

Present day FMS VNAV designs lack the capability to support LPV approaches which make use of the FAA's new Wide-Area Augmentation System (WAAS) and/or Satellite Based Augmentation System (SBAS) technologies. The new capability requires use of barometric altitude in the beginning of an aircraft descent with a transition to a satellite altitude-based system for the final approach. However, presently a seamless methodology is lacking for providing this transition. The present invention provides a means to smoothly integrate current baro altitude-based VNAV with GNSS altitude in the intermediate and final approach path vertical deviation computations to support this new capability.

SUMMARY OF THE INVENTION

In a broad aspect, the present invention is a flight management system (FMS) for an aircraft, that includes an integrated barometric altitude and satellite altitude-based vertical navigation (VNAV) system. The integrated barometric altitude and satellite altitude-based vertical navigation system, includes an altitude blending component for providing a smooth transition from a barometric altimetry source to a satellite altimetry source; and, a satellite altitude containment component operatively connected to the altitude blending component for limiting the difference of a barometric altitude path deviation to within a desired margin of a satellite altitude path deviation.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings and the characters of reference marked thereon,FIG. 1illustrates an integrated barometric altitude and satellite altitude-based vertical navigation (VNAV) system for a flight management system in accordance with the principles of the present invention, designated generally as10. The integrated barometric altitude and satellite altitude-based VNAV system10includes an altitude blending component, designated generally as12, for providing a smooth transition from a barometric altimetry source to a satellite altimetry source. A satellite altitude containment corridor component14is operatively connected to the altitude blending component for limiting the difference of a barometric altitude path deviation to within a desired margin of a satellite altitude path deviation. These two features as described below in detail enhance the existing FMS VNAV function. The current barometric altitude-based VNAV can be expanded to integrate use of global navigational satellite system (GNSS) altitude in the path deviation computations.

FIG. 2shows the components used by the altitude blending component12for blending the barometric altitude and GNSS altitude path deviations together. The blending technique allows for a smooth transition from using one altimetry source to the other in the VNAV solution for approach.FIG. 3depicts the components of the satellite altitude containment corridor component14. The corridor component14is used to limit the difference of the baro VNAV solution to within some margin of the GNSS altitude when baro VNAV is still used in the VNAV solution just prior to when the blending occurs.

VNAV Path Deviation Referenced to Baro Altitude

As can be seen by reference toFIG. 1, a first step15of the altitude blending component12is to calculate a barometric altitude referenced path deviation (ΔPATHbaro). The baro altitude referenced path deviation (ΔPATHbaro) represents the difference between the VNAV descent path referenced to mean sea level (MSL) and current aircraft altitude as measured by the Air Data Computer (ADC). Sensed baro-corrected altitude is used to measure this difference as shown in the following equation:
ΔPATHbaro=ALTa/c—baro−PATHalt—msl, where:

ΔPATHbarois defined as the deviation from a VNAV descent path using barometric altitude. A positive value represents an above path situation and a negative value represents a below path situation.

ALTa/c—barois defined as the sensed pressure altitude corrected for local atmospheric pressure (i.e., baro altitude) measured by the ADC.

PATHalt—mslis defined as the vertical point (i.e., altitude) on the FMS VNAV descent path, which corresponds to aircraft lateral present position, referenced to MSL.

VNAV Path Deviation Referenced to GNSS Altitude

A second step16of the altitude blending component12is to calculate the GNSS altitude referenced path deviation (ΔPATHgnss). The GNSS altitude referenced path deviation (ΔPATHgnss) represents the difference between the VNAV descent path referenced to MSL and current aircraft altitude as measured by the GPS receiver. Sensed GNSS altitude is used to measure this difference as shown in the following equation:
ΔPATHgnss=ALTa/c—gnss−PATHalt—msl, where:

ALTa/c—gnssis defined as the sensed GNSS altitude measured by a GPS receiver; and,

PATHalt—mslis the vertical point on the FMS VNAV descent path referenced to the MSL.

A positive value for ΔPATHgnssrepresents an above path situation and a negative value represents a below path situation.

MSL Path Height

In a next step, represented by numeral designation18, the mean sea level (MSL) path height (Hpath) is calculated. (See alsoFIG. 2.) The path height Hpathrepresents height above the flight plan final approach fix (FAF) referenced to the vertical point on the FMS VNAV descent path corresponding to the aircraft lateral present position and is calculated using the following equation:
Hpath=PATHalt—msl−PATHalt—msl—FAF, where:

PATHalt—mslis the vertical point on the FMS VNAV descent path referenced to said MSL; and

PATHalt—msl—FAFis defined as the vertical point (i.e., altitude) on the VNAV descent path at the FAF.

Glide Path Gradient

Next, a glide path gradient (SLOPEGPA) is calculated (see process block20) where SLOPEGPAis defined as the glide path gradient (typically in ft per nautical mile (ft/NM)) representing the vertical gradient on final approach. The glide path gradient is a function of the final approach glide path angle and is represented by the following equation:
SLOPEGPA=tan(θ)*6076.115, where:

θ is defined as the final approach glide path angle referenced to the horizontal plane in degrees.

tan(θ) is the tangent function and is unitless.

6076.115 is a conversion factor which represents ft/NM.

The glide path angle θ is assumed to be a positive value.

Blending Parameters

The path deviation blending algorithms use two parameters to smoothly transition the vertical guidance cue (i.e., path deviation information) from the baro altitude-referenced path to the GNSS altitude-referenced path. The parameters are two path heights above the FAF where: 1) the path deviation blending starts (Hblend—hi); and, 2) the path deviation blending ends (Hblend—lo). (See process block22.)

SLOPEGPAis utilized to determine Hblend—hias a function of Hblend—lo, where Hblend—lois defined as the path height above the FAF where the limited barometric altitude to GNSS altitude blending ends and is a constant (typically in ft), and Hblend—hiis defined as the path height above the FAF where the barometric altitude to GNSS altitude blending starts (typically in ft). The values for the two path heights are determined as follows:

GACCΔh—minis the allowable difference between baro altitude and GNSS altitude based path deviations for the minimum width of the GACC and is a constant (typically in ft); and,

SLOPEblendis defined as the slope of the baro altitude to GNSS altitude blending and is a constant in (typically in ft/NM).

The typical value for the Hblend—loconstant is 0 ft. The typical value for the GACCΔh—minconstant is 200 ft. The typical value for the SLOPEblendconstant is 100 ft/NM.

In the final step (process block24) of the altitude blending component ΔPATHbaro, ΔPATHgnss, Hpath, Hblend—hi, Hblend—loare used to determine ΔPATH, as will be discussed below.

Containment Corridor Parameters

The path deviation containment corridor algorithms use two parameters to smoothly limit the vertical guidance cue (i.e., path deviation information) of the baro altitude-referenced path from a wide margin of the GNSS altitude-referenced path to a narrow margin. See process block26inFIG. 1. Additionally, the components of the GNSS Altitude Containment Corridor (GACC) are shown inFIG. 2. The parameters are two path heights above the FAF where: 1) the path deviation limit ramping starts (Hgacc—max); and, 2) the path deviation limit ramping ends (Hgacc—min). The values for the two path heights are determined as follows:
Hgacc—min=Hblend—hi

Hgacc—minis defined as the path height above the FAF where the barometric altitude to GNSS altitude path deviation limit ramping ends (typically in ft); and Hgacc—maxis the path height above the FAF where the baro altitude to GNSS altitude path deviation limit ramping starts (typically in ft).

Hblend—hiis the path height above the FAF where the baro altitude to GNSS altitude blending starts in ft;

ΔHgaccis defined as the allowable difference between barometric altitude and GNSS altitude based path deviations of the GACC at the aircraft lateral present position;

GACCΔh—maxis the maximum allowable difference between barometric altitude and GNSS altitude based path deviations of the GACC;

GACCΔh—minis the minimum allowable difference between barometric altitude and GNSS altitude based path deviations of the GACC;

SLOPEgaccis defined as the slope of the barometric altitude to GNSS altitude path deviation limit ramping and is a constant (typically in ft/NM); and,

SLOPEGPAis the glide path gradient (typically in ft/NM).

A typical value for the GACCΔh—maxconstant is 1000 ft. A typical value for the SLOPEgaccconstant is 200 ft/NM.

GNSS Altitude Containment Corridor

The GACC is used to bound the baro altitude path deviation to a specified margin of the GNSS altitude path deviation depending on the lateral location of the aircraft within the corridor. In cases where the baro altitude path deviation is significantly different from the corresponding GNSS altitude deviation, the baro altitude path deviation is limited. A linear taper is used to smoothly transition the containment limit from a wide margin to a narrow margin as the aircraft descends. The GACC path deviation margin is a function of path height above the FAF and automatically compensates for the glide path angle intended to be flown on final approach. The satellite altitude containment corridor component14includes means for calculating a GNSS containment corridor (ΔHgacc) from the Hgacc—minand Hgacc—max(see block28). The path deviation containment operation is represented by the following algorithms, where:

Hpathis the height above the FAF referenced to the vertical point on the FMS VNAV descent path corresponding to aircraft lateral present position;

ΔHgaccis defined as the allowable difference between barometric altitude and GNSS altitude based path deviations of the GACC at the aircraft lateral present position;

GACCΔh—maxis the maximum allowable difference between barometric altitude and GNSS altitude based path deviations of the GACC; and,

GACCΔh—minis the minimum allowable difference between barometric altitude and GNSS altitude based path deviations of the GACC.

Limited Baro Altitude Path Deviation

In cases where the baro altitude path deviation exceeds the GACC boundary, the deviation is limited to the GACC boundary. The limited baro altitude path deviation is then used for blending with the GNSS altitude path deviation as the aircraft nears the FAF. The limited baro altitude path deviation is calculated using the following algorithms.

First the difference between the baro and GNSS altitude path deviations (ΔHa/c) is determined as follows:
ΔHa/c=ΔPATHbaro−ΔPATHgnss, where,

ΔHa/cis defined as the difference between the barometric and GNSS altitude path deviations at the aircraft lateral present position;

ΔPATHbarois the deviation from the VNAV descent path using barometric altitude; and,

ΔPATHgnssis the deviation from the VNAV descent path using GNSS altitude.

Once ΔHa/cis determined, the path deviations difference correction (ΔHcorr) is calculated using the following equation (see also, process block30):
ΔHcorr=ΔHa/c−[sign(ΔHa/c)*ΔHgacc], where:

ΔHa/cis the difference between the barometric and GNSS altitude path deviations at the aircraft lateral present position.

The limited baro altitude path deviation can now be calculated (process block32) per the following algorithms:

|x| is the absolute value function and is unitless.

Blended Path Deviation

The blended path deviation operation is used to smoothly transition the vertical guidance cue (i.e., path deviation information) from the limited baro altitude-referenced path to the GNSS altitude-referenced path yet provide stabilized vertical guidance for the entire descent. This is accomplished by emphasizing ΔPATHbaro—limfor regions of the descent path which are far above (i.e., before) the FAF and ΔPATHgnssfor regions of the descent path which are near, at and after (i.e., below) the FAF. The blending region is a function of path height above the FAF and automatically compensates for the glide path angle intended to be flown on final approach. The path deviation blending operation is represented by the following algorithms:

If Hpath≧Hblend—hi, then
ΔPATH=ΔPATHbaro—lim

All units are preferably in feet;

ΔPATHbaro—limis the limited deviation from the VNAV descent path using barometric altitude;

ΔPATHgnssis the deviation from the VNAV descent path using GNSS altitude;

Hblend—hiis the blending algorithm parameter which determines the path height above the FAF where the limited baro altitude to GNSS altitude blending starts; and,

Hblend—lois the blending algorithm constant which determines the path height above the FAF where the limited baro altitude to GNSS altitude blending ends.

Filtered Path Deviation

A low pass filter is applied to the blended path deviation value as the final step in the VNAV path descent computation. The equation for the filtered path deviation is shown below:
ΔPATHlpf=ΔPATH+[Klpf*(ΔPATHlast−ΔPATH)]

ΔPATHlpfis defined as the filtered deviation value computed for the VNAV path descent;

ΔPATH is the current deviation value that was computed for the VNAV path descent;

Klpfis defined as a low pass filter time constant and is unitless; and,

ΔPATHlastis defined as the prior deviation value that was computed for the VNAV path descent.

The typical value for Klpfis 0.5.

Other embodiments and configurations may be devised without departing from the spirit of the invention and the scope of the appended claims.