Procedure and automatic control device for an airborne vehicle in low altitude overflight
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention concerns automatic control of an airborne vehicle in low altitude overflight. 
     2. Discussion of Prior Art 
     French Patent Application No. 84 10263, deposited on Jun. 28, 1984, in the name of the Applicant, described a technique that can be called &#34;terrain following.&#34; The question in that case was how to define the command signals for the airborne vehicle&#39;s servo motors so as to optimize the flight path flown by the airborne vehicle, while at the same time taking into consideration the relief, and other obstacles the vehicle may have to overfly. This prior Patent Application was concerned with what takes place in a vertical plane containing the airborne vehicle&#39;s speed vector. 
     SUMMARY OF THE INVENTION 
     The present invention goes well beyond the prior Patent Application, because now the behavior of the airborne vehicle, or, more simply put, the aircraft, in horizontal directions will be examined. Accordingly, the present invention includes &#34;terrain avoidance&#34; (abbreviated EDT). 
     Although the published technical literature contains very little on this subject, it is here considered that the manner in which the following steps are implemented is known: 
     set of real time aircraft parameters representing the airborne vehicle&#39;s instantaneous flight path is acquired; 
     set of terrain data representing the topography of the region the airborne vehicle is Setting ready to overfly is available; and 
     a route for the overflight of this region at low altitude as a function of the so-called flight path assigned to the airborne vehicle (the flight plan), is selected. 
     The main purpose of the present invention is to make a considerable improvement in this technique. 
     More precisely, the purpose of the invention is to enable an aircraft to optimize its flight path, with its flight plan, and the environment it will encounter, taken into consideration. 
     Another purpose of the invention, however, is to permit deviations from the flight path when an unexpected obstacle, such as a threat, appears ahead of the airborne vehicle, while at the same time avoiding, in so far as possible, having to climb, or to make a drastic course change. 
     To begin with, the invention provides a procedure of the type already cited, one that is remarkable for the following operations, which concern the components of the aforesaid route in a horizontal plane: 
     (a) the so-called flight path is stored in the form of digital coordinates of a sequence of target points (B i ), associated with each of which is at least one other suitable value so as to define a target zone around a target point, such as a circle (CB i ) centered in the target point (B i ), and for which the aforesaid other digital value is the radius; 
     (b) there is determined for this target point (B i ), toward which the aircraft is headed, at least one aiming envelope (DV) in a preselected geometric relationship with the target zone (CB i ), defined by an angular sector having an apex (PV) that points toward the downstream side of the aircraft&#39;s flight path, this sector approximately inscribing the entire target zone (CB i ), and its bisector approximately passing through the target point (B i ); 
     (c) terrain data are used to determine a digital performance function (FPR) for each direction, or bearing corridor, which constitutes an evaluation of the optimum advance, starting out in accordance with this direction, as a function of a preselected set of criterium that includes the flight altitude; 
     d) using the aforesaid aiming envelope, a digital degradation function (FPN) is determined, tending to maintain the aircraft within this aiming envelope (DV), or to bring it back into this envelope all the more firmly the more and more quickly it deviates, for each direction the aircraft can use at the present time; and 
     (e) the optimum route is determined from the performance and degradation functions associated with the corridors the aircraft can use at the present time. 
     By preference, operation (c) includes the following steps: 
     (c1)--a digital consequence function (FC), associating, at each range register for each bearing corridor, an evaluation magnitude based on at least one part of the aforesaid criterium, is determined; and 
     (c2)--a digital performance function (FPR) is determined in accordance with the weighted sum of the consequence functions found from a step-by-step progression among the successive range registers with a limited bearing change at each step. 
     In practice, operation (e) results in route slaving that determines the steering commands for the aircraft such as to slave the aircraft to the aforesaid optimum route. 
     Paralleling this is the determination, in accordance with the optimum route. of the terrain-following commands that concern piloting the aircraft in the vertical plane. Thus, the command sent to the aircraft integrates, in a combined way, the terrain-avoidance command and this terrain-following command to arrive at a definitive route (which can be different from the optimum route). 
     In accordance with a special aspect of the invention, operation (b) includes for each target (B i ) the establishment of a navigation envelope (DN), and a guidance envelope (DH), both in the form of an angular sector (PN, PH) pointing downstream, the navigation envelope (DN) open, and containing the route that joins the preceding target (B i  -1) to the present target (B i ), and the guidance envelope (DH) closed, being approximately coaxial with the direction from the present target (B i ) to the next target (B i  +1), the aiming envelope (DV) being first the navigation envelope (DN), then the guidance envelope (DH), when this latter is penetrated by the aircraft, and then the navigation envelope associated with the next target. 
     Very advantageously, entry is into the navigation envelope associated with the next target (B i  +1) when the aircraft has penetrated the present target (B i ) circle, and which it has approximated in a preselected manner from the axial route of the guidance envelope (DH), or when the aircraft has penetrated a mandatory switching envelope (DCF) outside the guidance envelope (DH), and the present target circle (CB i ), and the upstream limit of which is close to the bisector of the angle formed by the preceding target (B i  -1), the present target (B i ), and the next target (B i  +1). 
     In accordance with another aspect of the invention, operation (d) establishes a rigid, or constraining, command when the aircraft is in the guidance envelope, and a flexible command when the aircraft is in the navigation envelope. 
     Entry also is made into the navigation envelope (DN) associated with the next target (B i  +1) when the aircraft has penetrated the guidance envelope (DH), and there has been encountered a threat, at least such is the case for certain applications. 
     In one preferred version of the invention, the boundaries of the navigation envelope and of the guidance envelope make an angle (β max ) with the respective straight lines defined by the three targets considered. The guidance envelope (DH) is tangent to the associated target circle (CB i ), the whole being closed, at the outside of this target circle (CB i ) and on the opposite side of its apex (PH) by a preselected curve such that if the aircraft is tangential to this curve it can remain inside the guidance envelope (DH). The navigation envelope (DN), in turn, passes through the point of tangency (F 1 ) between the guidance envelope (DH) and the target circle (CB i ), on acute side of the angle formed by the three targets (B i  -1, B i , B i  +1), itself tangent at point (F 2 ) to the target circle (CB i ) on the other side. Finally, the mandatory switching envelope (DCF) includes the downstream half-plane delimited by the straight line joining the aforesaid points of tangency (F1, F2), exclusive of the guidance envelope (DH) and the target circle (CB i ). 
     In accordance with other aspects of the invention: 
     the determination (c1) of the consequence function includes leveling the altitude of any point at the maximum altitude that had been encountered at preceding points in the same bearing corridor; 
     the determination (c2) of the performance function takes into account lateral clearance of obstacles present in at least an adjacent corridor; 
     weighting at the (c2) step is in accordance with a hyperbolic function of range; 
     the determination of the degradation function at step (d) takes into account angle (β) between the straight line joining the aircraft (A) at apex (PV) of the aiming envelope (DV) and the axis (AV) of this aiming envelope, the temporal derivative (β) of this angle, and angle (α), formed by the aircraft&#39;s horizontal speed vector (VH) and the aforesaid straight line. 
     In accordance with another, important, aspect of the invention, the control signal for the aircraft combines the terrain-following and terrain avoidance commands in a different way, depending on whether or not the corresponding route is within a &#34;straight line&#34; sector with a predetermined angular width. 
     The invention also offers a device for implementing the above procedure, a device that includes: 
     facilities for the acquisition of instantaneous aircraft flight parameters and for determining the navigation parameters; 
     facilities for processing and mass storage suitable for data storage, in particular, data on threats the aircraft can encounter; 
     calculation and storage facilities suitable for cooperating with the processing facilities to define at least one envelope as a function of a present target (B i ), with the intention of the aforesaid facilitates that of determining navigation parameters; 
     target switching logic facilities as a function of aircraft position with respect to this, or to the aforesaid envelopes; 
     terrain data storage filled with data, such as that from a radar; and 
     a high-speed computer suitable for determining consequence performance, and degradation functions from flight path data relative to the envelope supplied to it by the processor and from the content of the terrain storage, in order to establish a controlled course change for the aircraft as a function of its current flight path. 
     This device is advantageously complemented by: 
     facilities supplying a terrain-avoidance command based on the controlled course change; 
     facilities generating a terrain-following command for each possible route; 
     facilities generating a definitive command signal for the aircraft as a function of the terrain-avoidance and terrain-following commands. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other characteristics and advantages of the invention will become apparent 
     study of the detailed description that follows in conjunction with attached drawings, where: 
     FIG. 1 is a general block diagram of a device implementing the invention; 
     FIG. 1A is a horizontal plane view of a geometric figure illustrating the definition of the navigation parameters relative to an envelope; 
     FIG. 2 is a general block diagram of the functioning of the device in accordance with the invention; 
     FIG. 3 is a geometric figure showing a sequence of envelopes relative to a present target; 
     FIG. 4 is a more precise block diagram of the functioning of the device accordance with the invention; 
     FIG. 5 is a block diagram that once again details one step of that in FIG. 4; 
     FIG. 6 is a consequence function diagram; 
     FIGS. 7 through 10 are diagrams concerned with the performance function; 
     FIGS. 11 (a, b, c, d), 12 (a, b, c), and 13 (a, b, c) are diagrams concerned with the degradation function; 
     FIG. 14 is a curve illustrating the role of the degradation function; 
     FIG. 15 is a block diagram of the combination of terrain-following and terrain-avoidance commands: and 
     FIGS. 16 and 17 are two diagrams illustrating the advantages of the aforesaid combination. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     It is evident that the attached drawings essentially define geometric, written data of a certain nature. They are, therefore, incorporated in this description to complete, and to contribute to, the definition of the invention. 
     FIG. 1 is a block diagram of a device for controlling an airborne vehicle terrain avoidance in accordance with the invention. 
     Aircraft parameters are supplied over line 10. They are the horizontal coordinates of point A, x A  and Y A , in which the aircraft is found, and the corresponding horizontal components of its speed vector, VH x  and VH y . 
     An aiming point being given (provisionally designated PV, FIG. 1A), circuit 11 deduces, in a known manner, the navigation (or aiming) parameters, which are: 
     α, the aiming angle formed by the aircraft&#39;s horizontal speed vector, VH, and the direction of the aiming point (with respect to the aircraft); 
     β, the angle formed by the straight line A-PV in a preselected direction, AV, and passing through PV; and 
     the temporal derivative of β, designated β. 
     An envelope, DV, begins at point PV, and is delimited by an angular sector defined by its half-angle at the apex, β max , and the direction, AV, of its axis of symmetry. This envelope is turned toward the aircraft and thus toward the upstream side with respect to the aircraft&#39;s flight path (it will be seen in what follows that PV is a point at which the aircraft aims). 
     Envelope storage 31 contains the digital values defining the envelope. This storage reads out at the rate of 10 Hz (all rates are presented as examples) such data useful to computer 11 as: 
     PV coordinates; 
     components of the unit vector of direction AV. 
     Computer 11 reads out (at 20 Hz) digital values (α, β, β) to a target switching logic unit, designated 20. The function of this unit is to define the instant the target changes, and does so as will be described in what follows. 
     This takes place by taking into account threats, or other possible obstacles, that can appear suddenly in front of the aircraft. Detection of such threats can be accomplished in many ways, all known to the expert. It is taken that data concerning threats detected arrive over line 22 as received. These data (threat situation parameters, and parameters on the zone in which the threat is active) are accumulated in mass storage 21, which forms the &#34;threat data file.&#34; Storage 21 also can contain advance data on fixed, and known, threats, as well as other pertinent information. 
     Data file storage 21 is managed by general maintenance unit 40 through a file maintenance circuit, 41. The role of unit 40 here is to keep the file up to date by interpreting data received over line 22, and in this way ensure (at a 20 Hz rate) the presentation to logic unit 20 of threat data that are pertinent to that unit. Unit 40 also ensures transfer to assembly 100 (described in what follows) navigation and threat data used by it. 
     Logic 20 indicates the present aiming point (PV) (at a 10 Hz rate) to maintenance unit 40, which, at the same rate, receives already cited parameters (α, β, β) from computer 11. 
     The expert will understand that the calculation of the navigation parameters, and threat maintenance, take place in parallel at the rate of 20 Hz, whereas these two operations take place in series upon operation of target switching logic 20, but here the response is at 10 Hz. 
     Finallv, the maintenance unit transmits data on the present aiming point at a lesser rate (0.5 Hz) to envelope computer 30, which updates the content of envelope storage 31. 
     Given all of the above, general maintenance 40 transmits all data on the present aiming point and the navigation parameters for the airborne vehicle to a high-speed computer, 100. 
     During this time, terrain storage 51 is filled with data on the topography of the terrain it is expected the aircraft will be overflying. These data are provided primarily by the airborne radar, from its extractor output, 50 (digital output), or from other sensors. These data also can be prerecorded. 
     Unit 100, which functions as a 2 MHz clock, and is by preference, wired, includes: 
     computer 101, that calculates a function called &#34;consequence&#34;; 
     if necessary, a corridor interpolation storage, 112 (depending on the fineness of the resolution, into bearing corridors available in terrain storage 51); 
     computer 111, that calculates a function called &#34;performance&#34;; 
     a read/write circuit, 113, in two storages, 114,115, functioning alternately (for reading on one side and writing on the other); 
     computer 121, that calculates a function called &#34;degradation; 
     computer 131, that calculates the anticipated course change for the airborne vehicle. 
     The output from computer 131 is supplied at the rate of 0.5 Hz to circuit 60, which generates command signals concerned with terrain avoidance (horizontal plane). 
     Circuit 60 is associated with circuit 70, which latter generates terrain-following commands (vertical plane). 
     A final unit, 80, couples the two commands, combining them to deliver definitive commands to the aircraft. 
     The functions described above have been diagrammed in the order in which they take place in FIG. 2, where the response of the servo mechanisms in the aircraft appear as 295. An initialization step for the first target appears at 200. The first of the units in FIG. 2, while their numbers appear in the 200s, correspond to those shown in FIG. 1. 
     Reference now is made to FIG. 3. The so-called flight path flown a priority the aircraft is defined by a series of targets B i , that has already taken into account geographic and/or military data. It is considered that the aircraft is found between target B i  -1 and target B i , which will be followed by target B i  +1. 
     A target zone here the target circle CB i , centered in target B i , and of preselected radius, variable for each target, provided it remains greater than a threshold, is defined in the horizontal plane around each target B i . that a sector of apex PH and of the half-angle at the apex, B max , exactly 
     A guidance, or homing point, PH, is defined on the B i , B i  +1 axis, such that a sector of apex PH and of the half-angle at the apex, β max , exactly inscribes target circle CB i . Note that F 1  is the point of tangency of this sector with target circle CB i , of the acute side of the angle formed by the three targets B i  -1, B i , and B i  +1. The sector of apex PH forms a guidance or homing envelope, DH, which, as opposed to point PH, is closed by the continuous arcs of a circle (here three), the preselected radii of which are, preferably, selected as a function of the maximum in the range of minimum banked turn radii authorized for the airborne vehicle. 
     Defined in addition is navigation point PN, such that a sector of apex PN, turned toward the aircraft (and thus upstream), and the half-angle at the apex, β max , passes at previously cited point F 1  on one side to form angle β max  with the direction B i  +B i  and tangent on its other side to the B i  target circle at point F 2 . This sector forms a navigation envelope, designated DN, open toward the upstream side. 
     Previously cited point PV, the associated envelope DV, and its axis AV, thus will be: 
     PN, DN, and its axis parallel to B i  -1, B i  ; 
     or PH, DH, and the B i , B i  +1 axis; 
     or a new PN, DN . . . , but associated with target B 1  +1 and its target circle CB i  +1, and so on. 
     The half-angles at the apex of DN and DH can be different in practice. 
     Another envelope, different in nature, is illustrated in FIG. 3. The line F 1 ,F 2  forms the upstream boundary, LCF, of a mandatory switching envelope, DCF, consisting of a half-plane delimited by LCF, and containing point PH, but not guidance envelope DH. 
     Consider now tile sequential diagram in FIG. 4, which goes back to the functions in FIG. 2, but with more precision. The FIG. 4 diagram includes: 
     the slow functions, illustrated by the long broken line; 
     the low-frequency functions (0.5 Hz), illustrated by the axis line; 
     the fastest functions (about 10 Hz), illustrated by the continuous line. 
     Initialization operation200 (FIG. 2) now appears as 300 (FIG. 4), as does 310 for 210, and 322 for 222, with the observation that the threats can be picked up by the airborne radar as well. 
     It now is understood that the operator of computer 11 (FIG. 1) will take into account the navigation, DN, and the guidance, DH, envelopes associated with the present target, and possibly with the next target as well. 
     Step 326 thus consists of aircraft Localization with respect to these envelopes. 
     This step is followed by step 320, which condenses the functioning of target switching logic 20 (FIG. 5). Step 330 calculates the new envelopes, DN and DH (30, FIG. 1), if the target is changed, and return is to 325. 
     FIG. 5 explains the sequence diagram in accordance with which logic 20 operates. In this figure, the aiming envelope, DV, is that of envelopes DN and DH used to establish the aircraft controls. 
     The assumption, looking at the diagram, is implicit that the aircraft takes the navigation envelope, DN, associated with the present target, B i , for the &#34;aiming envelope.&#34; 
     Step 400 tests to see if the aircraft has in fact already penetrated guidance envelope DH. This thus already is the aiming envelope. Step 401 tests to see if the aircraft is in the process of penetrating the guidance envelope, DH, and, if this is the case, step 405 establishes that the guidance envelope is now taken as the aiming envelope, and terrain avoidance (EDT) is hardened, that is, that the aircraft will be more firmly constrained, as far as its flight path is concerned, than when the aiming envelope is the navigation envelope, with EDT flexible. 
     Steps 410 and 411 test to see if the aircraft has already penetrated target circle CB i , or if it is in the process of doing so. If &#34;yes&#34; step 415 determines the deviation |.increment.r| between the route followed by the aircraft and the guidance route to the future target (the B i , B i  +1 axis FIG. 3), and tests to see if this deviation is below a preselected threshold. .increment.r m , in absolute terms. If &#34;yes&#34; unit 450 switches targets, that is, B i  is replaced bv B i  +1 , with all the consequences that results from doing so in the envelopes. 
     When the aircraft is controlled with DV =DH (the aiming envelope is the guidance envelope: the output from 411 is &#34;no&#34;) target switching takes place if: 
     threat appears (step 420); 
     the aircraft has just departed the aiming envelope (step 430). 
     The situation is the same when the aircraft is controlled with DV =DN the aiming envelope always is the navigation envelope for target B i  ; the output from 401 is &#34;no&#34;), seeing that the aircraft enters the mandatory switching envelope, DCF (step 440). 
     The final return step, 460, returns to step 320 with a &#34;yes&#34; or a &#34;no&#34; result in all cases. 
     The functions illustrated in FIG. 5 are realizable by the expert, who will be able to draw up a detailed block diagram, it being observed that: 
     steps 400,401,410, 411,430 and 440 and possibly 420, involve what are, primarily, tests of geometric magnitudes; 
     step 405 includes a change in the aiming point, and a new calculation of the aiming envelope, the establishment of firm, or &#34;hard,&#34; terrain avoidance (EDT), and, of course, the calculation of new aircraft navigation parameters with respect to the guidance, or homing, envelope, DH, which becomes the aiming envelope; 
     step 415 can be in the form of a test of the angular deviation between the route being flown and the route toward the future target. 
     All of this describes in detail step 320 in FIG. 4, the sequence of which will now be examined. 
     Test 340 returns the device to 310 if the calculation of a new terrain-avoidance (EDT) command is not necessary, the restoration rates taken into consideration. 
     In the contrary case, step 355 calculates the three functions, consequence, performance, arid degradation (calculating the digital values deriving from the functions). This calculation is made: 
     in series for the consequence and performance functions; 
     in parallel with the two preceding functions for the degradation function. 
     The remainder of the calculation, made in series with the set of function calculations, includes: 
     in 356, the calculation of the envelope of admissible routes, with, in 357, a test to see if there is, or is not, an admissible corridor in this envelope, then, following this test: 
     in 358, the aircraft is forced to fly an extreme controlled route; the limit of the envelope that is the closest to the guidance, or homing route to the present aiming point, or else 
     in 359, the best overall &#34;performance&#34; corridor, and the one closest to the route for homing on the aiming point, is selected. 
     These calculations now will be described in more detail. 
     Consequence function 
     The consequence function is calculated with all range registers used to avoid the terrain for all corridors in actual fact scanned by the radar. 
     Depending on the type of radar, two cases can be presented: 
     (a) a radar with low bearing resolution, supplying a reduced number of corridors typicallly three or four corridors each about 10° wide with a 2° overlap: 
     (b) a radar with high bearing resolution, supplying numerous narrow corridors with little absolute overlap. 
     The Applicant has observed that it is necessary to introduce lateral clearance in order to obtain correct terrain avoidance, the clearance such that the aircraft does not pass too close to an obstacle. This clearance, in case (a), is provided by the overlap between adjacent corridors. 
     This overlap is inadequate, in case (b), and lateral clearance is introduced (combined with the vertical clearance of terrain following) in the form of a transverse elliptical function, defined by the isorange registers. 
     The consequence function calculation thus includes two stages: 
     corridor by corridor processing at ascending ranges: 
     in any register i, elevation Z i , that detected by the radar, is, if necessary, increased to the maximum elevation encountered at shorter ranges, and is written 
     
         Z.sub.i →sup [Z.sub.i, sup (Z.sub.k when k&lt;i] 
    
     the effect of which is illustrated in FIG. 6; 
     transverse processing for all isorange registers in view of a flight altitude &#34;prediction&#34;: 
     case (a), low resolution radar, the consequence function is 
     
         FC.sub.ji =Z.sub.ji +h.sub.o -z.sub.a 
    
     where 
     j is the corridor subscript: 
     i is the range subscript (range register); 
     h o  is an overflight altitude instruction; 
     z a  is the aircraft&#39;s instantaneous altitude; 
     case (b), radar with a goodly number of corridors: 
     the aforecited elliptical function, an example of which is illustrated in FIG. 7, yields flight altitude z ji  in the vicinity of an obstacle. The consequence function then is written 
     
         FC.sub.ji =z.sub.ji -z.sub.a. 
    
     It is apparent that the function FC ji  thus determined represents the &#34;consequence&#34; in terms of the increase in altitude for each corridor, and for each range in this corridor. It is possible, of course, to integrate other criterium in this consequence function, in particular: 
     threats that are, or are not, assimilated to the relief; 
     a distinction, in accordance with which a relief point is in a shadow zone, or in an illuminated zone, for a detector, of the airborne type, for example; 
     the intervention of other factors, changing, such as fuel reserves, or schedule imperatives, in particular; 
     so that it is possible to deduce an artificial modification of the real relief (an excessive increase in altitude, most often). 
     Performance function 
     Designated FPR, this function is calculated for each corridor between range registers i min  and i max  (these ranks are preselected). The corresponding consequence function, FC ij , is weighted for each value of i as a function of the range by coefficients μ i  of a hyperbolic law (FIG. 8). Their sum for a corridor is a constant, designated S.sub.μ. In the (a) case (few corridors), the consequence function is interpolated beforehand so as to obtain 1000 fictitious corriders for bearing resolution .increment.θ of the order of 1 milliradian. 
     FIG. 9 shows the manner in which the performance function is calculated. The start is made initially from range register i=i min  in some given corridor (subscript j o ). Then, three range registers that are in the corridor where j is found, and in the two adjacent corridors j-1 and j+1 are scrutinized iteratively each time at the next subscript, i+1. The register retained each FIG. 9, that finally, extends from  min  to i max . This is repeated, in time is the one with the least weighted consequence function (and, if necesprinciple, for each of the beginnings of the corridors at i min , such as j 0 . A performance function that is the sum of the consequence functions encountered along the advance determined is associated with the beginning of each corridor, j o , for example. This can be written ##EQU1## where k is the set of subscripts for the corridors (as a function of i) that traverses the aforesaid advance successively, or the line of least consequence. 
     It is quicker, in practice, to do this backwards, beginning with the maximum ranges (the subscripts i max ). 
     Looking at FIG. 10, which corresponds to the view of an obstacle on flat terrain, note that the zone of obstacle influence is longer in range the greater the discrimination of the corridors in bearing. In order to obtain a reaction range for the aircraft that is satisfactory (while avoiding the terrain), it has turned out to be necessary to adapt bearing discrimination by multiplying the number of corridors by any interpolation between two initial corridors. This adaptation leads, incidentally, to less drastic (or flexible) course change controls. 
     Degradation function 
     Designated FPN, this function is determined in order to maintain the aircraft in the present aiming envelope, DV, while at the same time providing best direction of the aircraft toward the aiming point, PV (with DV =DN or DH, and PV =PN or PH, respectively). 
     Consideration also can be given to all other angle data that can exert an influence on the desirable flight path for the aircraft (threats or directional jamming, in particular). The further question is to seek to cancel aiming angle α (FIG. 1A), and this more especially since ##EQU2## 
     The preferred degradatation function can be written 
     
         FPN.sub.j =ρ×ξ[(β.sub.max -|β|, |β|]·λ(θ.sub.j +α.sub.O) 
    
     where 
     ρ is an adaptation coefficient that can be taken equal to h o  ; 
     ξ is a terrain-avoidance hardening coefficient; 
     λ is the actual degrading function centered in -α o  (α o  being tied to the existing route), and 
     θ j  is the angular displacement of the corridor with respect to the route being flown. 
     The λ function 
     The λ function is, by preference, simply defined by two segments of a straight line (FIG. 11a) broken at α o  (for θ j  =-α o ), that is 
     
         λ=μλ·|θj+α.sub.O | 
    
     Centering this function in α o  has the following effects: 
     favors the aiming direction toward PV when the aircraft is in the aiming envelope (DN or DH). It then can be taken that 
     
         α.sub.o =αif β.sub.max -|β|≧O 
    
     where 
     αis the angle between the present route and the aiming route; 
     returns the aircraft to aiming envelope DV, when the aircraft has left it. The definition of α o  then is different. 
     First to be determined is 
     
         α.sub.test =α&#39;k(β) [|β|-β.sub.max +.increment.β.sub.O ] 
    
     where 
     k(β) is a function of the form shown in FIG. 11b; 
     .increment.β o  is a control parameter. 
     If ε is given the sign of β, then ##EQU3## 
     This defines k, with νk, β min , and k o , as the control parameters. 
     Finally, α o  is defined as follows (FIG. 11c); ##EQU4## where α o  max is a control parameter. 
     The expert will understand that if |β|&gt;β max  (the aircraft leaves DN or DH), the aiming route will be displaced (with respect to the normal route of PV =PN or PH) by a quantity that is proportional to the angular deviation |β|-β max , the proportionality factor being greater the greater |β|, that is, β varies more quickly. 
     The hardening coefficient ξ 
     The Applicant, by using this coefficient, seeks to emphasize the degradation as a function of the urgency of aircraft reaction. This emphasis must come into play on the side of the aiming direction in which the aircraft is in danger of departing from the DV. The coefficient thus must be applied to just one group of numbers for the function λ. Two cases must be distinguished: ##EQU5## 
     Now .increment.β.sub.ρ, or .increment.β N , can be designated .increment.β PN , depending on the case. Then, 
     
         ξPN(.increment.βPN, β)=ξ.sub.1 (.increment.βPN)+γ(.increment.βPN)·ξ.sub.2 (ε·β) 
    
     FUIGS. 12a, 12b, and 12c show the functions ε 1 , γ, and ξ 2 , respectively. Their analytical expressions are: ##EQU6## control parameters τ 10 , ξ max , .increment.β 1  ##EQU7## control by γ o  and .increment.β 2  ##EQU8## control by ν.sub.μ and β o . 
     What is finally obtained in the general case is an asymmetrical degradation function, FPN, the shape of which is shown in FIG. 14. 
     It is advantageous to complete the function FPN, with one, or several additive terms, taking into account other factors associated with the angle, such as: 
     air-air threats, enemy fighter planes, for example; 
     directional jammers; 
     evaluation as a function of the angle of the chances of survival faced with a volume made up of a dense set of threats (ground-air, in particular). 
     In practice, if it is desired to interdict the aiming angle, α, in order to surpass a maximum value, α max , it is sufficient to select the assigned course change from among the set of corridors such that 
     
         |θj+α.sub.o |&lt;α.sub.max 
    
     The result will be a set of admissible routes for the aircraft (step 356, FIG. 4). This is the intersection of this set with the possible routes for the aircraft. 
     If this intersection contains several routes, the expert will understand that the system described retains the best instantaneous route (step 359). This route is &#34;revived&#34; at the 0.5 Hz rate. 
     In the contrary case (step 358), the assigned route (by default) is de-fined as the boundary of the aiming envelope (DV =DN or DH) closest to the route running directly to the aiming point (PV =PN or PH). 
     Thus, the invention makes it possible in all cases to define a terrain avoidance command (EDT route) in the horizontal plane. In parallel, the aircraft is fitted with terrain-following equipment, as described, for example, in the above-cited Patent Application, in which a terrain-following command (SDT route) in the vertical plane is defined. Here the combination these two bits of data will be described with the help of FIG. 15 (the functions performed by unit 80, FIG. 1). This detailed description is given for a high-resolution radar [case (b), mentioned above]. 
     The Applicant has observed that during an EDT [terrain-avoidance]maneuver it is not necessary that the SDT [terrain-following]function compel the aircraft to climb. 
     But, for different reasons (late discovery of a threat, too much of a change in course with respect to the aiming direction, for example), the aircraft can be caused to leave its flight path for a lower altitude. It is necessary to ensure that the aircraft will be able to clear any obstacle that should appear unexpectedly during this maneuver. Two approaches, complementing each other, are anticipated to bring this about: 
     the path flown by the aircraft takes into consideration at all times possible neighboring reliefs in the vertical plane by providing lateral protection (partial, so as not to cause degradation in flight altitude). This lateral protection is obtained by azimuth scanning (radar) of an angular sector encompassing the aircraft&#39;s speed vector; 
     any course change maneuver is subject to prior system authorization. Authorization is forthcoming if the terrain-following commands calculated for the relief to be overflown do not reach prohibitive values (a load factor in excess of the aircraft-pilot couple possibilities). 
     In accordance with an important aspect of the invention, this latter is obtained by distinguishing if the aircraft is flying on a straight line, or is in a turn. 
     Upon departure (step 500, FIG. 15), the path being flown by the aircraft, that is, the terrain-avoidance command supplied, is acquired to calculate the course change that remains to be made. 
     The terrain-following command is designated α (the vertical component of the load factor normal to the plane of the wings). The terrain-avoidance command is designated .increment.R c  (for the assigned course change). 
     Straight line 
     A straight line terrain-following command, η LD , is calculated in a bearing sector encompassing the speed vector, a sector that includes N LD  scanned corridors. If there is an obstacle in this sector, the terrain-avoidance function will generate the avoidance command in good time (thus taking this obstacle into account, at least partially). 
     The straight line terrain-following command thus can be written (step 501, FIG. 15) 
     
         η.sub.LD=inf.sub.1 η.sub.i, where i=1, N.sub.LD 
    
     and where 
     η i  is the terrain-following command, calculated in each corridor (in accordance with the already cited patent). 
     The expert will understand that an η LD  command such as this makes possible correct flight out of a &#34;blind&#34; valley (the most precarious case), without degradation of the altitude at which the aircraft is flying, in the general case. 
     The terrain-following command is η=η LD  (step 502, FIG. 15) as long as the assigned course change, .increment.R c , is within the &#34;straight line&#34; sector. 
     Meanwhile, step 503 tests to see if α&gt;α max . If yes (step 504), the assigned route is stored but is replaced by the next route ±.increment.R f  (.increment.R f  defining an outright course change that is less than that assigned in accordance with what has been described above). 
     Test 505 then is made to see if the assigned route is within the sector of the straight line. If yes, passage to final step 520 confirms the fact that η=η LD  for the terrain-following command; and that 
     .increment.R=.increment.R c  for the terrain-avoidance command. 
     The contrary case is a turn. 
     Turn 
     In this case the calculation also includes a supplementary terrain following command for the turn, designated η VI , from η j  n each of the corridors separating the assigned route from the neighboring boundary of the straight line sector (FIG. 16). 
     A terrain-following command, η j , is determined as before in each corridor, subscript j, of sector &#34;VI,&#34; but the scanning range is reduced with respect to that used for all preceding η i . 
     This range reduction makes it possible to avoid &#34;climbing&#34; an obstacle that should be avoided. Then (step 510) ##EQU9## 
     It will be recalled that &#34;inf&#34; and &#34;sup&#34; are functions designating the smallest and the largest elements of a set, respectively. 
     Step 511 then compares η VI  with a maximum permissible η VI  max. If the test is favorable, step 512 establishes the terrain-following command as: 
     and this process too passes to final step 520, which adds the corresponding terrain-avoidance command, unmodified, to the assigned route. The inclusion of η in the &#34;sup&#34; function means that any preceding η value calculated will not be reduced in a turn, for safety reasons the expert will understand. 
     On the other hand if the output from test 511 shows that η VI  exceeds the maximum, it is considered that the aircraft cannot make the turn under adequate safety conditions. The acceptable load factor in a turn is designated η VI . Step 513 provisionally fixes 
     
         η=sup [η, η.sub.LD, inf (η.sub.VI, η.sub.VI)] 
    
     Test 514 then determines if α exceeds the threshold α max , and if it does the assigned terrain-avoidance route is reestablished by 515 (thus accepting, reluctantly, a climb in altitude) and final step 520 confirms the provisional terrain-following command η (step 513) and the terrain-avoidance command. 
     If α&lt;α max , test 514 is exited via step 517, which reduces the assigned course change .increment.R c , and the result is a new assigned route that is closer to the sector containing the straight line. The calculation of η VI  is repeated by step 505 until an η VI  value acceptable to test 511 is obtained, or until the sector containing the straight line is retrieved. 
     Noting that in a turn, η is defined by the largest η j , a reduction in the width of sector &#34;VI&#34; will avoid having to take into consideration an obstacle of reduced size present in this sector. Further, any significant course change is avoided by making this reduction toward the sector containing the straight line. 
     A ticklish problem arises when an aircraft is faced by a cliff (or other wide and steep obstacle). The expert will understand that the use of the block diagram in FIG. 15 (particularly steps 503,513, and 514) will enable the aircraft to clear the cliff without being completely diverted, as illustrated by the arrow in FIG. 17. 
     It will be understood that the present invention is not limited to the mode of realization described, and that it can be the subject of variants, particularly by using equivalent facilities.