GB2266286A - Method of piloting an aircraft to avoid its colliding with the ground. - Google Patents

Abstract

Method of piloting an aircraft to avoid its colliding with the ground, according to which: a numbered three-dimensional model (66k) of a terrain is memorised (65), a future position (11) of the aircraft is determined, and as a function of this (11) a position, in relation to the ground, of a precursory volume (41) in which the aircraft may pass is determined and then the existence of an intersection between the said precursory volume (41) and the terrain is sought, and if this is affirmative, a piloting signal (70, 71) is generated. A GPS receiver (1) supplies an input to the system, and a memory (30) supplies an aircraft manoeuverability signal (31). <IMAGE>

Description

20d-o"".66286 METHOD OF PILOTING AN AIRCRAFT IN ORDER TO AVOID ITS

COLLIDING WITH THE GROUND The present invention relates to a method for piloting an aircraft in Order to avoid its collision with the ground.

The pilot of an aircraft must maintain a sufficient distance between the trajectory followed by the aircraft and the high ground of the terrain over which he is flying or which he is skirting so that if the aircraft deviates, within a certain limit, from his planned trajectory he does not run the risk of hitting the ground. Prior to the flight the trajectory has been defined by the pilot or has been supplied to him by a preparation centre and during the flight he controls the following of this path or entrusts this task to an automatic piloting system, the purpose of which is to prevent the trajectory followed from deviating too much from the one prescribed, taking into account the accuracy of the aircraft navigation system, if it exists.

The aircraft must remain permanently in a flight volume which is approximately in the form of a tube which is centred on the prescribed trajectory, the distance of the envelope of the flight volume from the trajectory being the value of the deviation referred to above.

The flight volume in which the aircraft must manoeuver, is surrounded concentrically by a safety volume of tubular shape, and of a thickness equal to the selected safety distance in which neither the aircraft nor a contour of terrain must be situated. The safety volume thus provides a safety margin. When the trajectory is chosen, its path is defined in such a way that the safety volume does not contain any ground contour.

Adherence to the prescribed trajectory in the flight volume requires that the pilot, or the automatic piloting system, periodically knows the position of the trajectory of the aircraft in order to determine the deviation of its trajectory in relation to the prescribed trajectory and, if necessary, to correct any unacceptable deviation in sufficiently good time, that is, before entering the safety volume, in which the distance from the ground may be less than the safety distance, and, imperatively, before coming out of it while continuing to deviate from the prescribed trajectory, for there is no longer a guarantee with regard to the absence of mountainous terrain. The time available for correcting the deviation is all the more reduced the more rapidly the aircraft travels, which is the case with an aeroplane.

1 The position of the aeroplane can be determined by means of a receiver, situated in the aircraft, which, on receiving signals from radio emitters which are fixed in relation to the Earth, compares these signals in order to determine its position. Thus a system known as the GPS (Global Positioning System) makes it possible for the receiver to determine its position by means of emissions originating from several satellites of known trajectories, with an accuracy close to one hundred metres.

Should the necessity arise, the aeroplane has available a radio-altimeter which gives it the value of its vertical distance in relation to the ground. The pilot can thus check that he knows the required safety distance vertically to the position which he occupies, even if the aeroplane has deviated from the prescribed trajectory.

In military applications, for a cruise missile, for example, the distance from the ground registered by the radio-altimetre provides a profile of the ground over which the missile is flying which is memorised and compared periodically with the profiles of the terrains adjoining the prescribed trajectory, determined from a three-dimensional map or numerical model of the terrain which is memorised in the missile. That profile of the model which most resembles the profile over which the flying object passes is selected and the centre of inertia is reset so that it provides a position indication which is in accordance with the real trajectory followed. However, whereas the radio-altimeter makes it possible to know the altitude in relation to the terrain over which the aircraft is flying, it does not ensure that mountainous terrain situated on the future trajectory is detected.

This danger was emphasised in the article IIGPS Ground Proximity Warning Syster01 by Mark A. Sturza, in the "Proceedings of the National Technical Meeting" (Anaheim, California, January 20 - 23, 1987) edited by The Institute of Navigation, Washington, which states that this can be remedied by having available on board an aeroplane equipped with a GPS receiver, an electronic memory which contains an electronic description of the relief of the Earth, whose surface is divided into geographical cells, covering a surface of around several thousand square kilometres, whose maximum altitude is indicated by the description. The basic system presented in this article thus compares the altitude of the aeroplane with that of the cell over which it is flying and with that of the neighbouring cells and

2 generates an alarm if the altitude of the aircraft is less than that of at least one of the cells, increased by a safety margin linked to the accuracy of the altitude provided by the GPS system. In order to anticipate the detection of a risk of collision with a mountain, the author proposes to improve the system by determining, from the speed of the aircraft, its short term future position, by increments of 10 seconds, over a period of 2 minutes, and by checking that this future position is above the mountainous terrain in the cell over which the aircraft will be flying, with, again, a safety margin. This amounts to the aircraft's being preceded by a pinpoint electronic UshielC which must not "bump into" the high ground. In the improved system the size of a cell can be reduced to an area whose side measures about 8 km.

A method of this type is suitable for navigating at medium altitude, in which a mountain, whose presence is exceptional, will be avoided by passing at a wide distance from it in an adjoining cell whose ground configuration is lower, or else by gaining altitude. However, in the case of a low altitude flight, in valleys, for example, such a method would provide a permanent alarm, which would therefore be useless, since the cell is clearly of a greater size than a valley. In this latter case, the method described in the article points out the navigation problems, but cannot propose a solution.

However, since the shield is pinpoint, the presence of a lateral safety margin is not checked. In fact, for a given geographical cell, the altitude is assumed to be equal to that of the highest point, which guarantees the detection of a possible collision. However, if the future position of the aircraft is at the edge of a low altitude cell which is close to a steeply sloped mountain which belongs to another cell, the trajectory of the aircraft can pass close to this high ground without the pilot being warned, since in the improved system of the previous technology discussed here only a horizontal margin of 150 metres around the cell over which the aircraft is flying is provided.

Moreover, the above method does not take into account the possible dispersion of the trajectory of the aircraft around its future position, due to a manoeuver, on drifting or on breakdown, with the result that the pilot does not know the safety margin available to him, whether lateral, as stated above, or vertical. In the case of change of trajectory, for example, a sudden descent, he will indeed be warned, but this warning may indicate to 3 him that there will be a collision in the next ten seconds, which is too late for the aircraft to react.

Thus in normal flight the pilot of an aircraft avoids a mountain if, when he is too low, he is warned of its presence, but, if he is initially at barely sufficient altitude, he does not know of its existence and only discovers this at the last moment in the case of a sudden loss of altitude, whilst he could have avoided this risk by going another way. The improved method, still using this previous technology, is thus dangerous because it gives the pilot an impression of medium term "visibility" in front of the aircraft, which, as a result of a manoeuver, can suddenly disappear.

By following a trajectory prepared before the flight, the purely conventional piloting method has the disadvantage of offering to the pilot, or the piloting system of the aircraft, in the event of an unacceptable deviation appearing in the trajectory followed in relation to the prescribed one, a reaction time, for example, for circumventing a mountain, which depends on the safety distance chosen initially between the prescribed trajectory and the mountain. Now, at the time of choosing the prescribed trajectory, it is not always possible to increase the safety distance with a view to tolerating a large discrepancy on the trajectory followed.

In fact, in an enclosed valley, any deviation from the trajectory of one of the valley slopes, will bring it nearer to the other, which would then be at a flight volume distance less than the safety distance. The aircraft is thus forced to follow the prescribed trajectory without a rapid modification of this trajectory being possible.

Now, if the speed of the aircraft is greater than the prescribed speed, the time that the pilot has available is proportionally reduced, for the value of the safety distance is constant, with the result that, if the aeroplane penetrates the safety volume, the pilot risks not having the time to react, whilst, in addition, the aircraft cannot manoeuver suddenly because of its speed. Thus there is a risk of the aircraft coming out of the safety volume and hitting the ground.

Thus the above method is not adaptable in relation to the speed of the aircraft nor its trajectory. If the latter must be modified during the flight, the pilot must estimate the position of the safety volume on the chart, for the new tPajectory, in relation to the prescribed speed, which 4 requires a certain amount of time, which he does not always have, especially in the case of a critical flight situation. Moreover, if the trajectory is normally defined on the ground by a preparation centre, the pilot on board the aircraft can only make a manual definition which is inaccurate and liable to error.

The present invention aims to solve these problems.

It therefore relates to a method of piloting an aircraft in such a way as to avoid its colliding with the ground, characterised by the fact that:

- a three-dimensional numbered model of a terrain over which the aircraft is to fly is memorised, - the trajectory of the aircraft is determined up to its present position and from this is deduced a future position reached after a duration which is determined in relation to the instant of its passage to the present position; - the position, relative to the ground, of a precursory volume in which the aircraft may pass after the said determined duration is determined, as a function of the future position referred to above; - the existence of an intersection between the said precursory volume and the terrain is sought by comparing the position of this volume and the model of the terrain, and, if this is affirmative, a piloting signal is generated.

Thus the position of the precursory volume can be defined automatically, in a very short space of time, by suitable means of calculation, without the specific intervention of the pilot, since it is from the trajectory followed that the future position of the aircraft is determined, without the pilot's needing to foresee explicitly a future trajectory. The aircraft is thus preceded by a precursory volume which extends a sufficient distance in front of the aircraft to allow for the trajectory to be modified in good time if the precursory volume "collides with" a mountain.

The precursory volume fulfils its function basically owing to its surface which is turned towards the front of the trajectory, which forms a sort of shield which precedeb the aircraft and which, in the absence of the piloting signal, guarantees that the precursory volume does not contain any mountainous terrain. If the aircraft then crosses the volume previously occupied by this precursory volume, it does not run the risk of colliding with the ground. The volume itself, as opposited to its front surface, can, however, have a use for detecting a mountain. In fact, in the case of a sudden Panoeuver of large amplitude, the aircraft can take a new trajectory intersecting a mountain nearer to the aircraft than the front surface of the precursory volume is from this. Now, if the latter is of limited size, it risks no longer being crossed by the new trajectory, and thus can no longer carry out its warning role, having passed beyond the near mountain before the manoeuver by passing to the side. The mountain will then be detected by the fact that it is situated inside the precursory volume.

In order to improve the information supplied to the pilot or to the automatic piloting system, the method can be applied, in a manner which is interlaced in time, to several precursory volumes whose volume the aircraft is likely to cross. These precursory volumes can be separate or intersecting, they can have independent sizes and shapes from one volume to the other and correspond to determined durations which are independent from one another.

In a particular case, the precursory volumes are approximately concentric, for example spherical, around the future position of the aeroplane, the probability that the latter crosses the position which one of these occupies without crossing that of the others being just as slight as that this volume should occupy an eccentric position. The pilot can thus know that a piloting signal corresponding to an eccentric precursory volume is only of interest if the aeroplane suddenly changes trajectory and that the prescribed trajectory passes in the vicinity of or inside this precursory volume.

In particular, it is of advantage that the determined duration depends on the speed of the aircraft. In fact, although at a fixed determined duration an increase in the speed of the aircraft is expressed in terms of a proportional increase of the distance between the aircraft and the position of the precursory volume, the manoeuverability of the aircraft, for a change of direction or altitude, varies with the speed. Because of this, in order to keep the performances of the method approximately constant, it is preferable to define a determined duration, for each speed of the aircraft, which takes into account both its speed and the corresponding 6 manoeuverability which were previously memorised.

In order to optimise its shape and position, the precursory volume can have an orientation which depends on the present position of the aircraft. it can then be possible to define a vector which proceeds from the present position to the future position of the aircraft and which serves as a reference for determining the orientation of the direction of the precursory volume, if the latter does not present a symmetry of revolution in relation to the vertical. In particular, it is advantageous that it should have a shape which is approximately a cone which is open towards the future position, centred on the vector and with an apex which is situated in the vicinity of the present position of the aircraft, and should contain the most probable trajectories of the latter. In addition to a distant precursory volume, for example approximately centred on the future position of the aircraft (base of the cone), a precursory volume of this shape also comprises a volume, close to the apex, which corresponds to future positions of the aircraft which are nearer to the aircraft's present position. This, as explained previously, is especially useful in the case of a gyroplane which can rapidly change its course and altitude.

It is an advantage if the piloting signal consists of information which represents the position of the said intersection. The pilot can then locate a mountain accurately in the precursory volume and manoeuver if necessary.

Similarly, the piloting signal can consist of information which represents the maximum altitude of the relief of the terrain situated in the precursory volume. Then, if necessary, depending on the position of the precursory volume in relation to the prescribed trajectory, and should the need arise, depending on the exact position of the mountain if it is supplied to him as indicated above, the pilot can gain altitude or circumvent the mountain.

Since piloting requires constant attention on the part of the aeroplane pilot in order for him to detect the presence of the piloting signal or of an excessive deviation in relation to assigned flight parameter values which he has supplied to an automatic piloting device and since the trajectory followed can be affected by a strong cross wind or even by a breakdown of the automatic piloting system, or since the speed can drift, the invention provides the pilot with an audio alarm signal when there is a piloting signal.

7 The invention will be better understood with the help of the following description of a preferred method of implementing the method according to the invention, with reference to the attached drawing, in which:

- figure 1 is a diagram showing the different stages of the method according to the invention; - figure 2 is a perspective view showing the shape and the position of two precursory volumes which are used to detect highground in the terrain over which an aeroplane using the method of the invention will fly; - figure 3 shows another precursory volume; - figure 4 is a cross section, perpendicular to a future trajectory of the aircraft, of several adjoining precursory volumes and - figure 5 is a perspective view from above, of the precursory volumes of figure 4.

The method of the invention is applied to the piloting of an aeroplane which, in the example described above, is equipped with a GPS receiver, which is numbered 1 on figure 1. This receiver provides regularly, every second in the example under consideration, to a calculating unit 2 of a computer 3, the present position of the latter, given in a pinpoint manner in figure 2 and numbered 10, in the form of the latitude, longitude and altitude of the aeroplane, as well as its speed vector 15 and the indication of the associated instant tO. The calculation unit 2 provides the table of the trajectory which has been passed, in terms of the positions 10, speeds 15 and associated instants tO.

A calculation unit 4 of the computer 3 receives from the calculating unit 2 the signals of the speed 20 and of the trajectory which has been passed 21 and determines the most probable future trajectory of the aeroplane by extrapolation of the past trajectory, defined by the signal 21, and taking into account its curvature along three orthogonal axes orientated according to the parallel, meridian and vertical positions. In this example the future prescribed trajectory also takes into account the accelerations which the aeroplane has undergone just before reaching its last position 10, the calculation of these accelerations being possible because a known instant, 8 such as to, as indicated above, is associated with a position such as 10.

The calculation unit 2 also sends the signal 20 to an electronic memory 30 of the calculator 3, which contains information relating to the manoeuverability of the aircraft, in particular in relation to its speed, that is, an indication of its ability to change the trajectory and speed quickly. It will be understood that other flight parameters, such as the attitude of the aircraft, for example, can also be taken into account by the memory 30.

In response the memory 30 supplies an aircraft manoeuverability signal 31 to another input of the calculation unit 4. By applying the signal 31 to a correspondence table which it includes, the calculation unit 4 determines a determined duration Ti between the position 10 and a future position 11 on the future prescribed trajectory 22, and, according to the speed of the aircraft, also determines the value of a distance 23 which separates the position 10 from the position 11. It will, however, be noted that the presence of the memory 30 is not obligatory in order to apply the method and that the duration Ti does not have to depend on the manoeuverability of the aircraft.

Throughout the description, a position "in front" of another position means that it is situated at a greater distance from the position 10 than the other position is, and approximately in the direction of the future trajectory 22.

Starting from the signals 20, 21 and 31, the calculation unit 4 supplies a position signal 32 indicating the future position 11 which the aeroplane will occupy on its future trajectory 22 at the end of the determined period of time Ti after the instant to which corresponds to the position 10.

In addition, the computer 3 includes an electronic memory 35 which contains data 36 which define the shape and size of the volumes known as precursory volumes. These volumes are optimised in relation to the trajectory, to the flight parameters of the aeroplane and to specific parameters relating in particular to the terrain. In this first detailed example, a precursory volume 41 is used which has an envelope 51 which is approximately spherical in shape. Data 37, forming part of the data 36 define, in relation to the position of a reference point 50 which is the centre of the sphere 51, the relative position of numerous N points 51k (k = positive entity from 1 to N) 9 situated on the envelope 51.

A calculating unit 5 of the computer 3 receives the signal 32 as well as the data 37 and then, by means of calculation, places the centre 50 of the precursory volume 41 on the position 11. It then adds to the three coordinates of each point 51k the homologous coordinate of position 11, which supplies in the form of a signal 61k, the absolute coordinates: latitude, longitude and altitude, of each of the points 51k. A circumference 56 of the sphere 51, perpendicular to the future trajectory 22 at position 11, thus represents a surface which will be crossed by the real trajectory if, during the time period Ti which follows the instant tO, it does not deviate from the prescribed trajectory by more than the length of the radius 49 of the circumference 56.

The calculating unit 5 then sends to a first input of a comparator 6 of the computer 3 and to an electronic memory 65 of the computer 3, the signal 61k, of which only the information concerning the latitude and the longitude are used by the memory. The memory 65 is a volume memory containing a numbered three-dimensional model of the relief of the Earth's surface which has been divided into adjoining geographical cells of approximately square shape whose sides measure approximately 500 metres. To each cell is associated an altitude corresponding to that of its highest point. In response to the signal 61k, the memory 65 supplies to a second input of the comparator 6, a signal 66k which indicates the value of the memorised altitude of the cell which corresponds to the vertical of the point 51k. The comparator 6 thus generates a piloting signal 71 if the altitude of the above cell is greater than that of the point 51k, which indicates the existence of an intersection between the precursory volume 41 and the terrain which the aeroplane is to fly over. The above operations are repeated for each of the points 51k.

In this example, the piloting signal 71 consists of two supplementary items of information. In fact, by means of the signal 71, the comparator 6 transmits the signal 66k which indicates the altitude of the cell which has been "bumped into" by the point 51k. In addition, the comparator 6 also transmits, by means of the signal 71, the signal 61k which indicates the position of the point 51k. The pilot can thus know the position and the altitude of a dangerous mountain.

It is understood that, although only the points 51k of the envelope 51 of the precursory volum 41 are used, in this example it is a question of a volume detection, since, if a point inside the precursory volume 41 is at an altitude less than that of the cell in question, at least one point of the envelope 51 is situated even lower, approximately close to its vertical, and its UcollisionU with the ground will be detected before, or simultaneously to that of the internal point.

The above method can be applied in an interlaced manner in time, by changing each time the determined duration Ti, for example by 15 seconds to 2 minutes by increments of 15 seconds, and by adapting the size and if necessary the shape of the precursory volume.

It is also possible, in relation to the future position 11, to define the position of several precursory volumes, of increasing sizes and shapes whether similar or not. For example, the precursory volume 41 can be surrounded by another spherical precursory volume 42 of envelope 52. In this case, the circumference 56 and its opposite number 57 of the precursory volume 42 are concentric, the fact that a piloting signal 72 is generated for what there is of external precursory volume 42, without the piloting signal 71 being generated indicates that a mountain 75, which is the cause of the piloting signal 72 is only situated in the volume which separates the two envelopes 51 and 52. The mountain 75 is thus situated either approximately on the future trajectory 22 in front of the position 11, in which case the internal precursory volume 41 will also reach it and the piloting signal 71 will also be generated, or else the mountain 75, as shown in figure 2, is situated laterally to the future trajectory 22, at a distance from the latter which is greater than the length of the radius 49, and the piloting signal 71 will not be generated, since the mountain 75 does not present any danger in the absence of large deviation of the future trajectory in relation to the prescribed trajectory 22.

In the detailed example given above, in which the precursory volume 41 is not orientated on course, if several differently-orientated precursory volumes are not used in succession, this necessitates that the precursory volume 41 has a shape with symmetry of revolution around the vertical, for a flat precursory volume which moves parallel to its surface would only cross a too restricted volume around the future trajectory 22. A precursory volume can, however, have a shape which is elongated towards the bottom, in such a way as to define a section which is equivalent to the circumference 56, through which the most probable trajectories pass and taking into account the fact that the aircraft can more easily nosedive than gain 11 altitude, and thus that a cone, with an apex 10, containing a divergent cluster of possible trajectories, must flare outwards towards the bottom.

In the following second detailed example, the computer 3 is set to determine an orientation of a precursory volume 80 in relation to the position 10. The precursory volume 80, represented in figure 3, is then orientated in latitude and longitude. It is in the shape of a cone with symmetry of revolution centred on a vector 88 which proceeds from position 10 to position 11, with an apex coinciding with position 10, and making a tangent, by the conical periphery of its envelope 53 in the vicinity of its base 81, with the circumference 56. The base 81 of the cone 80 is of a spherical shape centred on the position 10 and making a tangent with the envelope 51 at a point 55 which is approximately situated on the future trajectory 22, a little in front of position 11.

The cone 80 thus quite well represents the volume occupied by the possible future trajectories 22 in the case of a manoeuver of the aircraft, the cone 80 becoming wider as the distance of a point of its envelope 53 increases from position 10, The cone 80 approximately encompasses the precursory volume 41 and thus offers the same protection, but also includes a part nearer to the aircraft, which is useful in the case of sudden change of course or slope of the line of flight which would make the prescribed trajectory come out of the volume already crossed by theface of the precursory volume 41 which is turned towards the front of the future trajectory 22.

It will be noted that, although we have referred to the volume and even insisted on the fact that this was a matter of a volumic detection, it is also possible, when the direction of the future trajectory 22 which is close to the direction defined by the positions 10 and 11 is approximately known, to reduce the precursory volume to a surface which is directed laterally to the future trajectory 22, like the circumference 56 which must not Ubump into" a mountain. A plurality of surfaces of this type, at different distances in front of the aircraft, and corresponding to various flight durations Ti, also makes it possible to define a precursory volume, like the cone 80 for example, which contains the above surfaces.

As shown in figures 4 and 5, which are a cross section in a plane perpendicular to the future trajectory 22 at position 11, seen from position 10, and a view from above corresponding to figure 4 respectively, several 12 adjoining precursory volumes 82, 83, 84 and 85 can be provided, which define conical lobes with apex 10 and, whose base is turned forward, and arranged against the cone 80 and all around, above, to the right, below and to the left respectively. Similarly, a precursory volume 86 in the shape of a cone is formed from the cone 80 and extends it forward. A piloting signal relating to the cone 80 indicates the presence of a mountain close to the future trajectory 22, whilst a piloting signal relating to one of the cones 82 to 85, which are eccentric, indicates the presence of a mountain situated at a greater distance from the future trajectory 22, and thus not presenting any danger in the absence of a large deviation by the actual trajectory in relation to this. A piloting signal due solely to the cone 86 indicates the presence of a mountain which is still far away, but which is likely to be dangerous. This signal commands the flashing of an indicator light, the emitting of a vocal warning message and the display of an alert message.

13

Claims (8)

1. Method of piloting an aircraft to avoid its collision with the ground characterised by the fact that:

- a numbered three-dimensional model (66k) of terrain over which the aircraft must fly is memorised; - the trajectory of the aircraft is determined up to its present position (10) and a future position (11) reached after a determined duration (Ti) in relation to the instant (tO) of its passage to the present position (10) is deduced from it; - the position of a precursory volume (41) in which the aircraft can pass at the end of the said determined duration (Ti) is determined as a function of the said future position (11).

- the existence of an intersection between the said precursory volume (41) and the terrain is sought, by comparing the position of this volume (41) and the model of the terrain (66k), and if this is affirmative, a piloting signal (70, 71) is generated.

2. Method according to claim 1, in which stages are effected, in a manner which is interlaced with respect to time, in relation to several precursory volumes (41, 42).

3. Method according to one of the claims 1 and 2, in which the said determined duration (Ti) depends (31) on the speed of the aircraft.

4. Method according to one of the claims 1 to 3, in which the precursory volume (80) is orientated in relation to the present position (10) of the aircraft.

5. Method according to one of the claims 1 to 4, in which the piloting signal (71) consists of an item of information (61k) which represents the position of the said intersection.

6. Method according to one of the claims 1 to 5, in which the piloting signal consists of an item of information (66k) which is representativ6 of the maximum altitude of the terrain situated in the 14 precursory volume (41).

7. Method according to one of the claims 1 to 6, in which the GPS system is used to determine the position of the aircraft.

8. Method according to one of the claims 1 to 7, in which an audio signal is emitted when a piloting signal (70, 71) is generated.