Independent Glide Slope Gain Programming System

The subject invention relates to a landing approach system for aircraft using ILS beam guidance information. The system disclosed converts angular glide slope information into linear altitude deviation relative to the glide slope beam center, using computed range information.

The aircraft range from the glide slope transmitter is computed in a closed loop circuit using rate of change of altitude, an assumed glide slope angle and derived glide slope rate of deviation. The initial range is computed from the rate of change of beam error, velocity and rate of change of altitude.

The resulting linear glide slope deviation signal is free from arbitrary sensitivity changes.

Systems of the prior art, employing radio altitude or time based glide slope gain programming, are adversely affected by terrain variations or timer set and reset characteristics.

Early autopilot designs employed a time based gain programmer to convert the angular glide slope error signal into a near-linear deviation signal suitable for autopilot closed loop control. This technique assumes a fixed altitude (1500 ft.) for starting the timer, a constant predetermined groundspeed and a predetermined glide slope angle. With such a system capture gains are selected for 1500 ft. altitude and are less than optimum for other altitudes. Groundspeed variations as well as glide slope angles different than those assumed also cause less than optimum gain, particularly at the end of the timer run. To improve gain control at the lower portion of the final approach the timer is reset at the middle marker.

Such system designs not only can produce a 2:1 gain variation at identical locations on the glide slope depending on glide slope angle and approach speed, but also allow for a 2:1 gain jump at the middle marker, potentially causing a control surface transient, as may be further understood from FIGS. 1, 2 and 3, showing the effect of two different airport approach conditions on a time base programmer output as a function of altitude.

Second generation autopilot designs employ radio altitude as a substitute measure for aircraft range from the glide slope transmitter. Here the assumption is that the approach terrain is relatively even and at the same elevation as the runway. Where this assumption is made, altitude above the terrain is taken as a representative measure for distance. In reality there are a number of airports with quite uneven approach terrain, particularly sharp terrain dips, and also approach terrain with a significant elevation difference relative to the runway. Such conditions produce less than optimum glide slope control gains.

Further, less than optimum glide slope control gains are produced as the result of the glide slope deviation itself, particularly at low altitudes, even for a perfectly smooth approach terrain. For example, a 20-ft. deviation under the glide slope at 100 ft. altitude produces a 20% gain deviation. A 20-ft. deviation at 100 feet may not be critical for making a successful landing, however, a more critical deviation at that altitude may render the glide slope control gains too low for recovery.

Accordingly, it is the object of this invention to provide an improved glide slope gain programming system which avoids the aforementioned problems associated with time based gain programming and radio altitude gain programming.

A further object of the invention is to provide a gain programmer which utilizes only the basic glide slope control and damping signals, readily available in a normal autopilot design, thereby avoiding the use and reliance on an additional costly information source such as a radio altimeter.

A third object of this invention is to provide a low cost high performance glide slope gain control device, making it attractive and economically feasible to design automatic glide slope control autopilots for general aviation type aircraft.

FIG. 1 is a graph illustrative of transient conditions present in prior art systems;

FIG. 2 in conjunction with FIG. 1 is illustrative of a first and second airport condition on the prior art time base programmer output as a function of altitude;

FIG. 3 is a plot of the radio altitude gain program of the present system;

FIG. 4 is depictive of glide slope geometry deemed helpful in understanding the equations hereinafter developed relating to the present system;

FIG. 5 is a diagram depicting a general glide slope intercept condition of the aircraft; and

FIG. 6 is a system schematic embodiment of the present invention.

The relationship between ILS glide slope beam deviation of an aircraft and the actual linear displacement of the aircraft from the beam center is given by

wherein

.DELTA.h.sub.GS = linear displacement of aircraft from the beam center (feet)

.epsilon. = angular glide slope deviation of aircraft from the beam center (radians)

R = aircraft range to the glide slope transmitter (feet)

The glide slope geometry depicting this relationship is given in FIG. 4. Using relationship (1) the rate of deviation from the beam center is obtained by differentiating: ##EQU1## It should be noted that in equation (2) (dR/dt) is approximately equal to the ground speed V.sub.G. For typical glide slope capture conditions the term .epsilon. (dR/dt) is small compared to R (d.epsilon./dt), as may be illustrated by the following example: Assume a level approach at altitude h = 1500 ft. and a vertical speed h = -10 ft/sec on glide slope with .gamma..sub.GS = -2.75.degree. after capture. The corresponding ground speed is V.sub.G = 208.4 ft/sec. Before capturing the glide slope, while flying level, the aircraft's rate of closure on the beam is h.sub.GS = -V.sub.G sin.gamma..sub.GS = +10 ft/sec. The aircraft's range from the glide slope transmitter at h = 1500 ft. on the glide slope is R = 31,264 ft. At glide slope capture the altitude deviation from the beam center is typically 100 ft. Therefore .epsilon. = .DELTA.h.sub.GS /R = -100/31,264 = -0.00320 radians or 0.183.degree.. Then it follows .epsilon..dR/dt .apprxeq. .epsilon..V.sub.G = -0.66 and this is much smaller than h.sub.GS. Therefore

or ##EQU2## Formula (4) shows that during the glide slope intercept phase a close estimate of the range R may be obtained from beam rate d.epsilon./dt and the rate of closure on the glide slope h.sub.GS. The quantity d.epsilon./dt may be obtained from .epsilon. by a rate-taking network having a transfer function of the form S/(.tau.S+1). The rate of closure on the glide slope h.sub.GS would be h.sub.GS = -V.sub.G sin.gamma..sub.GS for level capture of the beam. However, in case of descending captures of the beam this formula is invalid and its use to estimate the Range would lead to significant inaccuracies.

A more accurate measure for the rate of closure on the slide slope h.sub.GS is obtained in the following way. In FIG. 5 a general glide slope intercept condition is depicted with the aircraft's inertial velocity V.sub.I and the aircraft's descent angle .gamma..sub.O. The aircraft's descent rate relative to the earth is h = V.sub.I sin.gamma..sub.O and V.sub.I .apprxeq. V.sub.G.

Therefore

substitution of (5) into (6)

Substitution of (7) in (4) now yields ##EQU3## A close estimate of R can now be made using formula (8) regardless of intercept angle with the glide slope beam. In formula (8) .gamma..sub.GS may be selected to be 2.75 degrees or 0.048 radians, causing a maximum range error of 10% for .gamma..sub.GS = 2.5.degree. or .gamma..sub.GS = 3.0.degree.. Ground speed V.sub.G is not always available. In such a case true airspeed V.sub.T may have to be used, resulting in potential errors in R due to wind in the order of 10%.

The signals h and d.epsilon./dt may be noisy and require filtering. Computation of R according to (8) may become impossible after the glide slope tracking mode has been established since d.epsilon./dt goes to zero and also (V.sub.G .gamma..sub.GS + h) goes to zero.

Formula (8) is therefore used only to establish an initial range estimate R.sub.O. ##EQU4##

The implementation of the initial range calculation according to (9) is shown in FIG. 6. Herein is the beam error signal output from the glide slope receiver 10 processed in rate circuit 20 to produce a signal representative of d.epsilon./dt. The ground speed from ground speed sensing means 30 is amplified by a gain factor .gamma..sub.GS and added in adding means 40 to a signal representative of the rate of change of altitude of the aircraft from the rate of change of altitude sensing means 50. The output of adding means 40, representative of the signal (V.sub.G..gamma..sub.GS + h) is divided in divider means 60 by the output signal from rate circuit means 20 representative of d.epsilon./dt to produce at the output of divider means 60 the signal ##EQU5## representative of the range R.sub.O, during the glide slope capture maneuver. This initial range R.sub.O is memorized in a memory circuit consisting of adder means 70 and integrator means 80. The output of integrator 80 is coupled back to itself with negative gain through adder circuit 70 and switching means 90. With switch 90 closing the circuit loop around the integrator 80, the output in integrator 80 will track the output of the divider circuit 60 representative of the range estimate R.sub.O.

As soon as a stable and reliable range estimate R.sub.O is obtained the input to the integrator may be switched over to a signal representative of the rate of change of range: the computed ground speed "V.sub.G." From this point on, the range is updated by integrating the computed ground speed "V.sub.G " in integrator 80 producing the computed range R = R.sub.O - "V.sub.G " x t. A convenient switch-over from the initial range R.sub.O computation to the continuously updated range R computation is "glide slope capture," which occurs at a predetermined magnitude of the beam error.

The output of integrator 80 representative of the range R of the aircraft from the glide slope transmitter is further used in processing circuits 100 and 110 to produce the desired linearization of the glide slope beam error signal.

The angular beam error signal .epsilon. is unsuitable for direct use in an automatic glide slope control system because the true deviation of an aircraft from the glide slope beam .DELTA.h.sub.GS in feet is not only a function of the beam error .epsilon. but also a function of the range R:

with the range R continually computed .DELTA.h.sub.GS can now also be computed by multiplying .epsilon. and R as done in multiplier 110 of FIG. 6.

Processing means 100 may be employed to alter R in such a way as to program the beam deviation signal .DELTA.h.sub.GS from the output of multiplier 110 to zero at a range R where the airplane starts the flare maneuver.

The output of multiplier 110 .DELTA.h.sub.GS representative of the linear deviation of the aircraft from the beam center may now be used to stabilize the range computation, using a signal feedback technique.

For this purpose the rate of change of altitude signal h from rate of change of altitude sensing means 50 is correlated to the rate of change of deviation from the glide slope in correlation circuit 120. Circuit 120 produces at its output the difference between the lagged rate of change of altitude h/(.tau.S + 1) and the lagged rate of change deviation from the glide slope beam (.DELTA.h.sub.GS S)/(.tau.S + 1). This output, representative of ##EQU6## will be a steady state signal h.sub.SS representative of the rate of change of altitude which is required to track the glide slope. Any variation in the rate of change of altitude indicates a perturbation of the glide slope tracking which will be reflected equally in the signal representative of the rate of change of glide slope deviation and the signal representative of the rate of change of altitude. The non-steady state components of these signals will therefore cancel out in the correlation circuit 120.

This output of correlation circuit 120, h.sub.SS, representative of the rate of change of altitude required for glide tracking, is representative of the ground speed since

therefore a measure for ground speed can be obtained from formula (10) using small angle approximation:

since the h signal from the rate of change of altitude sensing means 50 may be noisy, it may also be expected that the signal h.sub.SS from the output of correlation circuit 120 is noisy. Before using it to produce V.sub.GC according to formula (11), this signal is therefore filtered in filtering circuit 130 producing at the output thereof the filtered signal h.sub.SS.

Formula (11) for computing ground speed requires knowledge of the glide slope angle .gamma..sub.GS. It is conceivable that this information could be entered for every specific ILS glide slope installation the system would be used on. However, present glide slope angles vary between -2.5.degree. and -3.degree. and a fixed selecting of .gamma..sub.GS = -2.75.degree. would therefore produce no more than a 10% error in the computation of the ground speed according to (11).

The signal output h.sub.SS from filter circuit 130 is therefore amplified in amplifying circuit means 140 by a factor -1/.gamma..sub.GS whereby .gamma..sub.GS = -2.75.degree. may be selected to produce at the output thereof a signal representative of the ground speed of the aircraft.

To the output signal from amplifying circuit 140, representative of the ground speed of the aircraft, is added a signal K.sub.3 (h.sub.SS).sub.EF, from the range error feedback circuit 200, in adder circuit 150 to produce the compensated ground speed signal -V.sub.GC.

This signal -V.sub.GC from the output of adding circuit 150 is now used to couple to the integrator 80 through switch 90 to update the range calculation after the glide slope beam has been captured. Accordingly, ##EQU7## This range computation as provided in the embodiment of FIG. 6 and described above utilizing the glide slope deviation signal .DELTA.h.sub.GS and range error feedback for computation of the signal V.sub.GC representative of ground speed is self-correcting and converges to the correct range even if the initial range R.sub.O was in error. This feature can be shown as follows.

In the range error feedback circuit 200 the signal h.sub.SS and the signal .DELTA.h.sub.GS S/(.tau..sub.1 S+1) from the correlation circuit 120 are processed to determine the error in the range R. During the initialization of the range error computation (h.sub.SS).sub.EF is synchronized to zero using a small washout time constant 1/K.sub.1. After initialization, the signal (h.sub.SS).sub.EF is washed out with a long time constant 1/K.sub.2, only to ensure that bias errors on h.sub.SS cannot adversely effect the long term range computation. If the range R is correct, the h.sub.SS is constant, therefore (h.sub.SS).sub.EF is zero. The sign of the range error is determined by the sign of the signal .DELTA.h.sub.GS S/(.tau..sub.1 S+1) from the correlation circuit 120 and the sign of the signal (h.sub.SS).sub.EF from the error feedback circuit 200. For example, if the signal (h.sub.SS).sub.EF from the error feedback circuit 200 has the same sign as the signal .DELTA.h.sub. GS S/(.tau..sub.1 S+1) from the correlation circuit 120, the range R is too small and the error feedback circuit 200 will put out a positive signal proportional (h.sub.SS).sub.EF, thereby reducing V.sub.GC and thus eliminating the range error. This comes about by multiplying the signal (h.sub.SS).sub.EF with the sign of the signal .DELTA.h.sub.GS S/(.tau..sub.1 S+1). Vice versa, if the signal (h.sub.SS).sub.EF from the range error feedback circuit 200 is positive and the signal .DELTA.h.sub.GS S/(.tau..sub.1 S+1) is negative, the range is too large and error feedback circuit 200 will put out a negative signal proportional to (h.sub.SS).sub.EF, thereby increasing the signal V.sub.GC and thus eliminating the range error. The gain K.sub.3 of the range error feedback circuit 200 is selected to allow sufficiently fast range correction to prevent initial range errors to cause unacceptable large errors toward the lower portion of the glide slope tracking, but small enough to prevent range perturbations due to glide slope multi-path signal distortion.

A further use of the signal h.sub.SS representative of the filtered steady state rate of change of altitude of the aircraft during glide slope tracking is shown in adding circuit 170. Adding circuit 170 takes the signal h.sub.SS from the output of the rate limited lag circuit 130 with a negative sign and adds it to the rate of change of altitude sensing means 50 to produce at the output thereof a signal h representative of rate of change of deviation from the glide slope. This signal becomes available after glide slope capture and may be used to damp the glide slope control mode.

Switches 90 and 160 are controlled by the capture logic circuit 180 which establishes a "glide slope capture logic discrete" based on the linear glide slope deviation signal output .DELTA.h.sub.GS from multiplying circuit 110.