Abstract:
A method and device are provided for assisting in height holding in air navigation, consisting, for any point (M) of the image of the landscape flown over by the aircraft (A), located by its angle of elevation (θ-p) with respect to the speed vector V of the aircraft, and situated in a zone between a lower elevational limit (θ 1  -p) and an upper elevational limit (θ 2  -p), in the trajectory of the aircraft, in detecting a possible deviation between the angular speed Ω of movement of this point with respect to the aircraft, depending on the height (h,H) of subsequent crossing of this point by the aircraft, for identical piloting conditions, and an angular reference speed of movement (Ω o ) corresponding to a reference height (h o ,H o ) of subsequent crossing by the aircraft of a point having the same elevation (θ-p), for identical piloting conditions, and in modifying the piloting of the aircraft, should a deviation between these angular speeds of movement be detected, so as to reduce these deviations.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method and device for assisting the holding of height in air navigation. 
     In very low altitude navigation of aircrafts, particularly military aircraft, the work load of the pilots is considerable. The pilots of fighter aircraft must thus, while flying as close as possible to ground (so as to avoid being detected by monitoring and firing means) carry out tasks of navigating on located points, acquisition of objectives, alert, counter measures, self defense, observation, etc. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is to allow a predetermined height to be held without any direct active measurement of the oblique or vertical distance, and even if the relief is not known previously. 
     For this, the invention uses the perspective image of the outside landscape in front of the aircraft, obtained either by direct vision, or by means of a sensor or otherwise (synthetic relief, for example). 
     Height holding may be achieved in accordance with the invention either by the pilot, through the use of an adequate figure superimposed in this image, or by an automatic pilot through an adequate control device. 
     The present invention provides a method of assisting height holding in air navigation, consisting essentially, for every point of the image of the landscape flown over by the aircraft, located by its angle of elevation (θ-p) with respect to the speed vector of the aircraft and situated in a zone between a lower elevation limit and an upper elevation limit, in the path of the aircraft, in detecting a possible difference between the angular speed of travel of this point with respect to the aircraft, depending on the height (h, H) at which this point is subsequently crossed by the aircraft, for identical piloting conditions, and an angular reference travel speed, corresponding to a reference height (h o , H o ) at which the aircraft subsequently crosses a point of the same elevation (θ-p), with identical piloting conditions, and in modifying the piloting of the aircraft, should a difference between these angular travel speeds be detected, so as to reduce these differences. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and features of the present invention will appear clearly from reading the following description of embodiments, made with reference to the accompanying drawings in which: 
     FIG. 1 is a diagram showing the principle of the invention and defining the parameters used in the case where the height at which the aircraft crosses the different points of the particular zone considered on the ground is measured with reference with the direction of the speed vector of the aircraft; 
     FIGS. 2a and 2b illustrate two examples of height holding through the use by the pilot of an adequate figure superimposed in the image; 
     FIGS. 3a and 3b show examples of the image of the zone considered on the ground, obtained respectively at successive times at t+Δt; 
     FIGS. 4a and 4b illustrate by means of examples the choice of the limit, lower and upper values of the elevational field defining the zone considered on the ground. 
     FIG. 5 is a diagram showing the principle of the invention and defining the parameters used in the case where the height at which the aircraft crosses the different points of the particular zone considered on the ground is measured with reference to a horizontal passing through the center of gravity of the aircraft; 
     FIG. 6 shows comparatively different examples of aircraft trajectories for a given configuration of the particular zone considered on the ground, depending on whether piloting of the aircraft is controlled by action on the slope of its speed vector or by action on the height of its center of gravity with respect to the ground; 
     FIG. 7 is a diagram of one embodiment of a piloting control device when the height holding is carried out by automatic pilot; and 
     FIG. 8 shows the image zone treated by correlation in the case of FIG. 7. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 will be considered in which an aircraft is represented by the point A and its absolute speed vector by the vector V. The apparent movement of a fixed point M on the ground is expressed by the relationship: 
     
         Ω=V/D sine (θ-p) 
    
     and, since ##EQU1## the relationship becomes: 
     
         Ω=V/h sine.sup.2 (θ-p)                         (1) 
    
     or else 
     
         h=V/Ω sine.sup.2 (θ-p)                         (2) 
    
     If V, θ, p are known, by measuring Ω the minimum distance h may be determined at which the aircraft will pass above point M of the relief (if V remains unchanged). 
     Furthermore, if h o  is the desired value h (reference value) and Ω o  (for a given angle θ-p) the associated angular value, we have: (for h and h o  ≠o) ##EQU2## 
     The invention is based on these relationships: 
     Two methods of application are envisaged: 
     assistance to piloted following of the ground 
     automatic following of the ground. 
     In the case of assistance to piloted ground tracking, there are presented to the pilot, as shown in FIG. 2a, on the one hand, the image of the ground and on the other a moving scale driven at moving speeds defined by the relationship: 
     
         Ω.sub.o =V/h.sub.o sine.sup.2 (θ-p)            (4) 
    
     This scale is presented for elevations, with respect to the speed vector V, between θ 2  -p and θ 1  -p (the angles θ 1  and θ 2  being chosen depending on the considerations which will be set forth further on). 
     By comparison, for each angle θ-p in the above defined field, between the moving speeds Ω of the points of the landscape and the moving speeds Ω o  of the reference scale, the pilot may know (C f  relationship (3)) whether he will pass too close to or too far from each of these points. 
     For example, looking again at FIG. 1, it can be seen that the points on the ground from R to P (R excepted) move more quickly than the reference (the aircraft will pass too close to them) and conversely for points R to Q (R excepted). Point R has normal travel. As for a tower of the type MM 1 , it can be seen that it moves more slowly than the reference at its foot M, more quickly at its top M 1 . 
     The moving reference scale, may, for increasing the realism, be shown as a perspective view, that is to say the gaps between the bars and possibly the lengths of the bars will be proportional to sine (θ-p). It is also possible, for example, as shown in FIG. 2b, to imagine a &#34;cascade&#34; presentation of vertical dashes, the length of each and their spacing varying as their vertical speed proportional to: 
     
         V/H sine.sup.2 (θ-p) 
    
     In FIGS. 2a and 2b, angle i designates the angle of incidence of the aircraft, that is to say the angle between the longitudinal axis of the aircraft, or horizontal fuselage reference (shortened to HFR) of the aircraft and the direction of the speed vector. 
     In the case of automatic ground tracking, by correlating homologous portions of the image obtained successively in time, the moving speed Ω is obtained for each of the points of the image, with respect to Δθ/Δt where Δθ represents the elevational shift of each point of the landscape between times t and t+Δt. 
     The value Δθ is brought out in FIGS. 3a and 3b showing respectively the position of the same element of the landscape on the images obtained at two successive times t and t+Δt. 
     Thus for eachh of the points of the image we have: 
     
         Δh=h-h.sub.o =V/Ω sine.sup.2 (θ-p)-h.sub.o (5) 
    
     This calculation is made for a device such as the one described subsequently in connection with FIG. 7. 
     The value Δh may be used directly as input to the automatic pilot for the &#34;vertical&#34; piloting of the aircraft (associated with the usual information from the inertial center of the aircraft). 
     More precisely, the smallest of the values Δh obtained for the whole of the points situated in the path of the aircraft and with elevations between θ 1  and θ 2 , called Δh min , must be equal to 0. It is then the value Δh min  which will pilot the required evolutions. 
     The reasons governing the choice of the uupper and lower limits of the elevational field: θ 1  and θ 2  will now be explained with reference to FIGS. 4a and 4b. 
     In FIG. 4a, an aircraft A has been shown approaching a hill. In this case, if the upper limit θ 1  is relatively low (as is the case in this Figure) the top B of the hill will be taken into account relatively early in determining the future trajectory of the aircraft, which will result in passing above the valley at a height greater than the reference value h o . In such a case it is therefore advantageous to choose a higher value of θ 1 . 
     More generally, the upper limit (θ-p) 1  is chosen, so as to allow the necessary pull up to be effected for crossing an obstacle with the clearance height h o , and with a normal vertical manoeuver load factor N in low altitude flight (for example ±0.5) and crossing the obstacle in a slope reduced to 0 again, which gives: 
     
         1/2h.sub.o =1/2(N-1)g(D/2V).sup.2 
    
     and total ##EQU3## where τ is the reaction time for varying the load factor, whence: ##EQU4## 
     For example with h=100 m, τ=ls, V=300 m/s, N=0.5, D=3 300 m and (θ-p) 1  =1°.7 (30 mrad). 
     Conversely, in FIG. 4b, an aircraft A has been shown moving away from a hill. In this case, if the lower limit θ 2  is relatively low, which is the case in this Figure, the top B of the hill will now be taken into account in determining the future trajectory of the aircraft, which may lead to not respecting the reference height h o  when passing above point B. In such a case a higher value θ 2  should be chosen. The lower limit (θ-p) 2  is therefore chosen: 
     (1°) So as to avoid premature return to hand control for example when approaching peaks, and 
     (2°) So as to go as far as elevations where the sensitivity of perception of the moving speed differences is sufficient. 
     In accordance with the same criterion, for example, in counter pull-up with an evolution load factor of 0.5 and a 0 reaction time, the clearance height h o  above the obstacle being reduced by h/10, we find: D=600 m and (θ-p) 2  =9°.5 (166 mrad). 
     This limit (θ-p) 2  may also be fixed by the field available in elevation in the display through which or in which the ground to be flown over is observed. 
     In the case of FIG. 5, instead of considering the distance h of vector V passing above the points of the passage (V being of unvarying direction) we consider the height H of the aircraft with respect to these landscape elements. 
     It will then be desirable to pilot H-H o  (where H o  is the desired or reference height) instead of piloting h-h o , that is to say that instead of piloting according to the direction of the speed vector, piloting will be carried out according to the height of the center of gravity of the aircraft. 
     In this case, we will have: Ω=V/D sine (θ-p) and, since ##EQU5## 
     
         Ω=V/D sine θ sine (θ-p)                  (6) and 
    
     
         H=V/Ω sine θ sine (θ-p)                  (7) 
    
     As before, by defining a reference height H o  and the associated angular speeds Ω o , we obtain the relationship: ##EQU6## 
     The above considerations apply then in the same way to this variant, except that sine θ sine (θ-p) is used instead of sine 2  (θ-p) in calculating the values Ω, H, Ω o , H o . 
     The advantage of this variant resides in the fact that the imposed trajectory is practically independent of the difference, at a given time, with the desired trajectory. For example, in FIG. 6, the trajectories of aircrafts A 1  and A 2  join up rapidly with center of gravity control whereas they remain separate with speed vector control. 
     In addition, the top B is passed over horizontally whereas under speed vector control it is passed over with a slope, so with overshoot. 
     The drawback of this variant is that it requires a more tedious piloting with higher load factor differences. 
     Another advantsge of this variant is that it allows a stable servo control to be obtained by injecting a suitable and constant dose of the drift dH/dT of the difference H-H o  (in fact dH/dt=Vp). On the other hand, in the case of the speed vector control solution, the servo control for the automatic pilot as a function of the difference H-H o  requires a more delicate variable compensation. 
     In FIG. 7, a device is shown which may be used in the case of automatic ground tracking. 
     This device comprises an image correlator 1 for measuring the angular moving speed at each of the points of the considered zone of the image. This zone is defined by the values θ 1 , θ 2  and by the values ±Δg in lateral deflection about the trajectory of the aircraft (shown with a broken line in FIG. 8) which is predicted from the angle of elevation of the aircraft and the load factor. The correlation is made by small limited elements of this zone, as shown in FIG. 8, and allows the elevational shift Δ(θ-p) to be obtained or the shift along the vertical of the image, of any point of the zone explored between two successive times t and t+Δt, whence the angular moving speed ##EQU7## may be derived for all the points of the zone explored. 
     Since the object of the invention does not in itself relate to the correlator, and since this element is well known per se, particularly from French Pat. No. 1 504 656, it will not be described in greater detail here. 
     This correlation receives from the inertial guidance center 2 of the aircraft the data required for locating the different points of this image, namely the position of the horizon, the value of the incidence angle i of the aircraft, the value of slope p and the value of the angle of elevation of the aircraft (giving the vertical of the image) 
     The device shown in FIG. 7 also comprises processing means 3 for calculating at all points the difference with respect to the reference height: Δh=V/Ω sine 2  (θ-p)-h o  from the values θ-p and Ω supplied by the correlator, V, supplied by the inertial guidance center 2 of the aircraft and h o  supplied for example by a display means 4. 
     The processing means 3 comprise for that elementary circuits performing the elementary functions X,:, -, sine, etc. and adapted so as to perform the desired function. The processing means 3 also allow the value Δh min  to be detected, using known processing methods, from the set of values Δh obtained for the different points of the zone explored. 
     The point corresponding to Δh min  may be reinforced on the display. The values Δh min  and g are delivered to the automatic pilot. The drift d/dt (Δh) is also supplied so as to allow loop stabilization; it is obtained for example from the calculation: ##EQU8## 
     The device thus described corresponds to the case where the height h at which the different points on the ground are crossed is measured with reference to the direction of the speed vector. 
     The description corresponding to the case where the height H of crossing the different points on the ground is measured with reference to a horizontal passing through the center of gravity of the aircraft is derived from the preceding one by changing sine 2  (θ-p) by sine θ sine (θ-p) and (h,h o ) by (H,H o ).