EP1432958A1

EP1432958A1 - Method for guiding a rocket

Info

Publication number: EP1432958A1
Application number: EP02783193A
Authority: EP
Inventors: Michel Broekaert
Original assignee: Sagem SA
Current assignee: Safran Electronics and Defense SAS
Priority date: 2001-09-25
Filing date: 2002-09-23
Publication date: 2004-06-30
Anticipated expiration: 2022-09-23
Also published as: FR2830078B1; US20040245370A1; DE60214407T2; US7083139B2; DE60214407D1; WO2003027599A1; FR2830078A1; EP1432958B1

Abstract

The invention concerns a method for guiding a rocket (1) onto a target, wherein, the rocket (1) is equipped with guidance by self-reacting imaging device (10) and trajectory correction means (11); said method consists in: acquiring the target through a sighting device and determining its position; harmonizing the sighting device and the imaging device (10) of the rocket; stabilising the images of the imaging device (10) of the rocket; working out a guidance law; launching the rocket (1) and guiding it in accordance with the guidance law until it acquires the target by itself.

Description

Method for guiding a rocket

The invention relates to rocket guidance.

A rocket is a small rocket, without guidance. It is often used in the fight against tanks and it can be launched from a land, sea or air vehicle, for example from an airplane or a helicopter. However, the invention applies equally well to missiles and when we speak in the text of rockets, it will be necessary to take the term in its general sense and to consider that we also cover missiles.

The accuracy of a rocket is not very high. But it is also affected, in the event of a helicopter fire, by the wind of the blades which causes a deviation of trajectory.

Before launching a rocket, an operator first acquires the target in its sight, it identifies it, it follows it, to know its angular speed, then it ranges it, to know its distance and finally to know the position of the target in its benchmark. With this data and a flight model of the device, the fire calculator develops a future goal materialized by a reticle in the viewfinder.

It will be recalled that many missiles are equipped with self-guidance means, that is to say a deviation measurement system intended, depending on the result of the comparison between the images of the reference target and the images captured in flight by an imaging device, to actuate control surfaces or directional trajectory correction rockets.

Well, the present application aims to perfect the precision of rockets and, for this purpose, it relates to a method for guiding a rocket on a target in which, the rocket being equipped with self-guiding means with imaging device and trajectory correction means,

- the target is acquired by an aiming device and its position is determined,

- the aiming device and the rocket imaging device are harmonized,

- the images of the rocket imaging device are stabilized,

- we develop a guidance law,

- we launch the rocket and

- the rocket is guided according to this law until it acquires the target itself. It will be noted that the harmonization of the two aiming and imaging devices, of the launcher and of the rocket can be carried out in a completely simple manner, first by harmonizing the aiming and taking axes, respectively, then, by calculating the image of the launcher sight in the reference frame of the rocket imaging device.

It will also be noted that the stabilization of the images of the rocket imaging device makes it possible at least to overcome the disadvantages of the launcher before launching and therefore to stabilize these images in the absolute landscape of the target.

In a particular implementation of the method of the invention, before launching, an initial guidance law is developed and the rocket is guided until it acquires the target according to this initial law.

However, preferably before launch, an initial guidance law is developed and, after launch, a continuously variable guidance and trajectory correction law is developed until the rocket acquires the target.

20

More preferably, to harmonize the sighting device and the rocket imaging device, electronic harmonization is carried out according to which, in a terrestrial frame of reference, the images of the scene taken at the same times by the two devices are filtered in a low-pass filter, i:> to retain only the low spatial frequencies, and the equation of the optical flow between these pairs of respective images of the two devices is solved to determine the rotations and the variation of the ratio of the parameters respective zoom to subject these images to harmonize them on each other.

Preferably always, the images of the rocket imaging device are stabilized in a terrestrial frame of reference, on the landscape, even if stabilization by inertial unit is still possible.

In this case, it is advantageous, in this terrestrial frame of reference, to filter the images of the scene taken by the imaging device, in a low-pass filter, to retain only the low spatial frequencies, and to resolve the equation of the optical flow to determine the rotations to be subjected to the images to stabilize them on the previous images.

40 The invention will be better understood using the following description, with reference to the accompanying drawing, in which

- Figure 1 is a schematic axial sectional view of a rocket equipped with self-guiding means with imaging device and path correction means;

- Figure 2 is a block representation of the electrical, electronic and optical functional means of the rocket of Figure 1;

- Figure 3 illustrates the geometry of the movement of a camera; - Figure 4 is a block diagram of the rocket imaging device allowing the implementation of electronic stabilization of its images and harmonization with the aiming device;

- Figure 5 is a representation of the image of the rocket imaging device showing the various fields of view and - Figure 6 is a schematic view illustrating the method of guiding a rocket on a target from a helicopter.

The rocket comprises a body 1, of which only the front part has been shown, the rear part comprising the payload and the trajectory correction members, which may be control surfaces or small directional rockets, and a nose 2, covered of a cap 3. The cap 3 carries a first lens which acts as an aerodynamic window and which focuses the image on the detector using the rest of the optics which are discussed below. The cap 3 carries a first lens which acts as an aerodynamic window and which focuses the image on the detector using the rest of the optics which are discussed below. The rocket is a self-directing spinnated rocket, partly in the nose, partly in the body, as will be seen below, but the nose 2 and the body 1 of which are decoupled in rotation, the nose 2 carrying, by means of a hollow shaft 4, a flywheel 5 disposed in the body 1 creating a differential spin between the nose 2 and the body 1, so that the nose 2 is only rotated very slowly if not at all.

The hollow shaft 4 therefore extends on either side of the joint plane 6 between the nose 2 and the body 1, in rolling bearings 7 and 8 respectively in one 2 and the other part 1 rocket. The self-steering of the rocket comprises, in the nose 2, behind the cap 3 and a fixed optical system 9, an imaging device 10 and in the body 1, u ^'n equipment 11 Trajectory correction controlled by the device 10.

The equipment 11 also ensures, after launch, the comparison of the image taken by the imaging device 10 with the stored large and small field images of the scene taken, before launch, with the wearer's aiming device, it will be discussed below.

The imaging device 10 comprises a camera 13, with its conventional proximity electronic circuits 14, an analog-digital converter 15 and an image transmission component 16. The device 10 is supplied from the body of the rocket and through the hollow shaft 4, by a rechargeable battery 12. The camera 13 can be a camera or video or infrared device. The transmission component 16 can be a laser diode or an LED (light-emitting diode). This component 16 can be placed in the imaging device 10 and then, the transmission of images through the hollow shaft 4 and the flywheel 5 is effected by optical fiber 17 extending along the roll axis 30 of the machine. But the image transmission component 22 can be placed in the inertia flap 5, opposite a diode 24 receiving the transmitted images and then the signal transmission between the imaging device 10 and the component 22 s' made by wires through the hollow shaft 4. The imaging device is cooled by Peltier effect, if necessary.

The flywheel 5, symbolized in FIG. 2 by the two vertical dashed lines, carries the secondary 19 of a transformer 18 for coupling the power supply to the nose 2 of the rocket connected to the battery 12, a wheel 20 of an optical encoder 21 and a laser diode 22, or an LED, as the case may be, for transmitting in the body 1 of the rocket images of the device 10.

The trajectory correction equipment 11 of the rocket body comprises the transceiver 23 of the optical encoder 21, the diode 24 receiving the transmitted images, the primary 25 of the transformer 18, with its source 26, and circuits 27 for processing the received images and for guiding and controlling the control surfaces 28 of the rocket, connected to the receiving diode 24 and to the transceiver 23 of the encoder 21. The circuits 27 include a computer of edge.

The encoder 21 indicates the relative angular position between the imaging device 10 and the body 1 of the rocket. The rocket is guided using the computer of circuits 27, as a function of this angular position and of the comparison between the images received from the imaging device and stabilized in circuits 27 and the stored images previously supplied by example by a viewfinder.

The guidance commands are applied in synchronism with the rocket's own rotation, also taking into account the location of the control surface.

Before launching the rocket, the operator, using a sighting device, takes a wide field image 52 of the scene, which is memorized, and which will serve, in the case of low spatial frequencies, to be determined the approximate direction of the target (Figure 5). It also takes a small field image 53 which is also stored.

With reference to FIG. 5, the overall view is a view 50 of the navigation field, with, inside, a view 51 of the rocket's auto-director, then a view 52 of the wide field, then again more inside, a view 53 small field.

Referring to Figure 6, there is illustrated the example of an operator, who is in a helicopter 60 equipped, on each of its two flanks, with a rocket 1, 2, to be guided on the target to be reached constituted in this case a tank 61. This FIG. 6 shows a sighting device 62 and a firing computer 63 of the helicopter as well as the angle of field θ of the seeker of the rocket from right, corresponding to view 51, and the angle of small field v of the sighting device 62 of the helicopter, corresponding to view 53, angles in which the tank 61 is located.

Thus, the fire control operator, shooter of the helicopter 60, begins by acquiring the target 61 using his aiming device 62, that is to say that it proceeds to the determination of the position, the distance and the speed of the target 61 which will enable it subsequently, in combination with a flight model and with the aid of the fire calculator 63, to develop a law of guidance, or command, initial. During this time, the pilot of the helicopter brings back at best the axis of the helicopter in the direction aimed by the shooter thanks to a repeater.

After acquisition of the target 61 and its designation by the operator, the on-board computer, proceeds to the harmonization of the aiming device 62 and of the imaging device 10 of the rocket then to the stabilization of the images of the device rocket imagery, before developing the optimal rocket guidance law.

For reasons which will become apparent later, the description will first be made of the step of stabilizing the images of the rocket imaging device.

Let us consider the observation and guidance camera 13 of the rocket of FIG. 1. It can be a video camera or an infrared camera.

If the scene is stationary, the points of the scene seen by the camera between two images are connected by the trajectory of the carrier.

The Cartesian coordinates of the scene in the reference frame of the carrier are P = (x, y, z) ', the origin is the center of gravity of the carrier, with the z axis oriented along the main roll axis, l' x axis corresponds to the yaw axis and the y axis to the pitch axis.

The camera is in a three-dimensional Cartesian or Polar coordinate system with the origin placed on the front lens of the camera and the z axis directed along the viewing direction.

The position of the camera relative to the wearer's center of gravity is defined by three rotations (ab, vc, gc) and three translations (Txc, Tyc, Tzc). The relationship between the 3D coordinates of the camera and those of the wearer is:

(x ', y', z ')' = (ac, bc, gc) * (x, y, z) '+ T (Txc, Tyc, Tzc) OR

• R is a 3 x 3 rotation matrix,

• T is a 1 x 3 translation matrix.

The trajectory of the center of gravity is characteristic of the evolution of the state of the system and can be described by the system of differential equations

x (t) = F (t) .x (t) + u (t) + v (t)

x = state vector of dimension n F (t) = matrix function of t, of dimension n u = input vector function of t known v = n-dimensional white Gaussian noise

The state of the system is itself observed using the camera and the resolution of the optical flow equation, by m measures z (t) linked to state x by the observation equation:

z (t) = H (t) .x (t) + w (t)

where H (t) is a matrix m x n function of t and w is a white Gaussian noise of dimension m, which one can assimilate to the angular and linear vibrations of the camera compared to the center of gravity of the carrier.

The discrete model is written: x _{k + 1} = F _k * x _k + u _k + v _k z _k = H _k * x _k + Wk

or x _k = _[k aP, aV _k, bP _k, _k bV, gP _k, gV _k, _k xP, xV _k, _k yP, yV _k, zP _k, _k zV] ^τ is the state vector at the instant k of the trajectory, composed of the angles and speeds yaw, pitch, roll and positions and speeds in x, y and z.

x _{k + I} is the state vector at time k + 1 with t ₊₁ - t = Ti.

u _k is the known input vector function of k; it is the flight or trajectory model of the wearer's center of gravity.

v _k is the n-dimensional white Gaussian noise representing the acceleration noises in yaw, pitch, roll, positions x, y, z.

If the angles and translations to which the camera is subjected relative to the center of gravity are not constant during the trajectory, in a viewfinder for example, it suffices to describe their measured or controlled values (ac (t), bc (t ), gc (t), Txc (t), Tyc (t), Tzc (t) as a function of t or k.

As the trajectory of the wearer's center of gravity is defined by the vector x _{k +} ι, the trajectory of the camera can be defined by a vector

xc _{k + 1} = R (ac, bc, gc) * (F _k * x _k + u _k + v _k ) + Te

Between the instants of observation k and k + 1, the camera undergoes pure 3D rotations and three translations, the values of which are provided by the vector x ' _{k +} ι.

Consider the situation where the elements of the scene are projected in the image plane of the camera and only these projections are known.

Figure 3 shows the geometry of the camera movement in 3D space in the real world.

The camera is in a three-dimensional Cartesian or Polar coordinate system with the origin placed on the front lens of the camera and the z axis directed along the viewing direction. Two cases of different complexities exist:

• The scene is stationary while the camera zooms and rotates in 3D space.

• The scene is stationary while the camera zooms, rotates and translates in 3D space.

Let P = (x, y, z) '= (d, a b)' be the Cartesian or polar camera coordinates of a stationary point at time t

• x = d.sin (a) .cos (b)

• y = d.sin (b) .cos (a) • z = d.cos (a) .cos (b)

and P '= (x', y ', z') '= (d', a ', b') 'the corresponding camera coordinates at time t' = t + Ti.

The camera coordinates (x, y, z) = (d, a, b) of a point in space and the coordinates in the image plane (X, Y) of its image are linked by a perspective transformation equal to :

X = Fl (X, Y) .x / z = Fl (X, Y) .tg (a)

Y = Fl (X, Y) .y / z = Fl (X, Y) / tg (b)

where F1 (X, Y) is the focal length of the camera at time t.

(x ', y', z ')' = R (da, db, dg) * (x, y ₃ z) '+ T (Tx, Ty, Tz)

or

• R = R _γ R _β R _α is a 3 x 3 rotation matrix and alpha = da, beta = db, gamma = dg are, respectively, the yaw angle, the pitch angle and the roll angle of the camera between time t and t ' • T is a 1 x 3 translation matrix with Tx = x '- x, Ty = y' - y and Tz = z - z ', the camera's translations between time t and t'.

The observations by the camera being made at the frame frequency (Ti = 20 ms) it can be noted that these angles change little between two frames and that certain calculations can be simplified accordingly.

When the focal length of the camera at time t changes, we have:

F2 (X, Y) = s.Fl (X, Y)

where s is called zoom parameter, the coordinates (X ', Y') of the image plane can be expressed by

• X '= F2 (X, Y) .x7z' = F2 (X, Y), tg (a ')

• Y = F2 (X, Y), y '/ z' = F2 (X, Y) .tg (b ')

If we want to distinguish more precisely the camera movements deduced

-> o of those of the carrier and the real movements of the camera, we will say that the carrier and the camera have the same trajectory, but that the camera undergoes in addition to the linear and angular vibrations.

(x ' ₅ y *, z') '= R (da + aw, db + bw, dg + gw) * (x, y, z)' + 25 T (Tx + xw, Ty + yw, Tz + zw )

or

aw, bw, gw, xw, yw, zw are the angular vibrations.

30

These linear and angular vibrations can be assimilated to noises with zero averages, white or not depending on the spectrum of the carrier considered.

35 The equation of the optical flow is written:

- _, - _{. ^ ^} a (image, rX, Y)), _v „,. ,., _. c (image _lc (X, Y ⁾ ) _A _,, . im _a g _e ,., (X, Y) = image, X) + - ^ Hd ^ XH - ^.d Y _k _, ⁽ XΛ ⁾

or: imagek + 1 (Ai, Aj) = imagek (Ai, Aj) + GradieπtX (Ai, Aj) .dAi .pasH + GradientY (Ai, Aj). dAj .pasH with GradieπtX and GradieπtY the derivatives according to X and Y of imaςek (X.Y).

To estimate the gradients only adjacent points are used. As we are only looking for the overall movement of the landscape image, we will only be interested in the very low spatial frequencies of the image and therefore filter the image accordingly. Thus, the gradients calculated are significant.

Low-pass filtering consists, in a conventional manner, of dragging a convolution kernel from pixel to pixel of the digital images of the camera, a kernel on which the origin of the kernel is replaced by the average of the gray levels of the pixels of the kernel . The results obtained with a rectangular core of 7 pixels high (v) and 20 pixels wide (H) are very satisfactory on normally contrasted scenes. On the other hand, if one wants that the algorithm also works on some isolated hot spots, it is better to use a kernel which preserves the local maxima and does not create discontinuity in the gradients. Wavelet functions can also be used as the averaging kernel.

We therefore used an averaging nucleus in the shape of a pyramid (triangle along X convolved by triange along Y). The complexity of the filter is not increased since a rectangular kernel with sliding average of [V = 4; H = 10]. Wavelet functions can also be used as the averaging kernel.

Only dX and dY are unknown, but if we can decompose dX and dY as a function of the parameters of the state vector that interest us and of X and Y (or Ai, Aj) so as to have only as parameters the parameters of the state vector, we will be able to write the equation in a vector form B = A * Xtrans, with A and B known. Each point of the image can be the object of the equation, we are in the presence of an overdetermined system, A * Xtrans = B, which we will be able to solve by the method of least squares.

The optical flow equation measures the total displacement of the camera. We saw above that we could distinguish more finely the movements of the camera deduced from those of the wearer and the real movements of the camera by saying that the wearer and the camera have the same trajectory, but that the camera undergoes in addition to the linear and angular vibrations.

(x ', y', z ')' = R (da + aw, db + bw, dg ⁺ g) ^• (x. y, z) '+ T (Tx + xw, Ty + yw, Tz + z)

OR aw, bw, gw, xw, yw. zw are the angular and linear vibrations

However the displacements due to the trajectory of the camera (da, db, dg, Tx, Ty, Tz) are contained in the state vector x ' _{k +} ι of the camera, or rather in the estimation that one can make it, by averaging it, or by having a Kalman filter which provides the best estimate.

As the equation of the optical flow measures the totality of the displacements, we will be able to deduce from it the angular and linear vibrations aw, bw, gw, xw, zw, for stabilization purposes.

It should be noted that, except in extremely specific configurations, the linear vibrations can never be seen taking into account the observation distance, or their small amplitudes compared to the movements of the carrier. We will therefore observe: da + aw, db + bw, dg + gw, Tx, Ty, Tz.

Take the equation of the optical flow:

£ J 5S≥ £ y_idχ _k _ _l ( _X, Y). g (ima ^e , (Y ⁾⁾ _dY ^ ₍ _Xj y) ima ¹ gSe ^c ,! ^c .-, l (X- ',' Y) J = i "m" ag => e -, '<(VX , 'Y') ⁱ + - - χ. ^α ^ -> ^ ^I ^ _δ γ

OR image,., (X 4- dX _t ., (X, Y), Y + dY, ^, Y)) = im e, (X, Y) If we carry out this operation, we see that we will absolutely stabilize the images of the sequence. Unlike an inertial stabilization where the line of sight is tainted with bias, drifts and errors of scale factor, we can create a representation of the scene not tainted with bias and drifts if it is stabilized along three axes and if the optical distortion defects have been compensated. The fourth axis (zoom) is not necessarily necessary but it may prove to be essential in the event of optical zoom but also in the case where the focal length is not known with sufficient precision or when the focal length varies with the temperature ( IR, Germanium optics, etc.) or pressure (air index).

This can interest applications where we want to accumulate frames without dragging, or if we want to keep an absolute reference of the landscape (the dynamic harmonization of a self-director and a viewfinder by example).

But this can also concern applications where we will seek to restore landscape information in an optimal manner by obtaining an image free of the effects of sampling and detector size.

An improvement in spatial resolution and a reduction in temporal noise or fixed spatial brait can be obtained simultaneously.

We can notice that the same equation can also be written:

image _{k + 1} (X, Y) = image (X - dX _{k +} ι (X, Y), Y - dY _{k + 1} (X, Y))

The values dX _{k +} ι (X, Y), dY _{k +} ι (X, Y) are obviously not known at time k. On the other hand, using the camera motion equations we can estimate them at time k + 1.

This provides better robustness in the measurement of speeds and allows large dynamics of movement. As the same point P of the landscape, of coordinates X _k , Y in the image k, will be found at the coordinates X _{k +} ι Y _{k +} i in the image k + 1 because of the three rotations aV _k + ι. Ti, bV ₊ ι. TL, gV _{k +} ι.Ti, and the change of focal length, it is therefore necessary to undergo opposite rotations and zoom factors to absolutely stabilize the image k + 1 on the image k.

Let us now examine the particular case of a stationary scene and no camera translation.

When the camera undergoes pure 3D rotations, the ratio between the Cartesian coordinates 3D camera before and after the camera movement is:

(x ', y', z ')' = R * (x, y, z) '

where R is a 3 x 3 rotation matrix and alpha = da, beta = db, gamma = dg are, respectively, the yaw angle, the pitch angle and the roll angle of the camera between the tramps t and you.

In 3D camera polar coordinates, the ratio before and after the camera movement is:

(d ', a', b ') * = K (da, db, dg) * (d, a, b)'

The scene being stationary, we have:

d '= d for all points of the landscape

X = F1 (S, Y). X / z = F1 (X, Y). tg (a)

Y = F1 (X, Y), y / z) F1 (X, Y) .tg (b)

When the focal length of the camera at time t changes, we have:

F2 (X, Y) = s. Fl (X, Y) where s is called the zoom parameter, the coordinates (X ¹ , Y ') of the image plane can be expressed by

• X '= F2 (X, Y). x '/ z' = F2 (X, Y). tg (a ')

• Y = F2 (X, Y). y '/ z * = F2 (X, Y). tg (b ')

So there are four parameters that can vary.

Let us consider the practical case, to solve the optical flux equation, of the estimation of the yaw, pitch and roll speeds and of the focal change.

B (,:, 1) = imagek + 1 (Ai, Aj) -imagek (Ai, Aj)

If we set: AA ((::,:, 1) = Derivative Y (Ai, Aj). (1 + (Aj.pasV / F1 (X, Y)) ^Λ 2)

A (,:, 2) = DeriveeX (Ai, Aj). (1 + (Ai.pasH / F1 (X, Y)) ^Λ 2)

A (,:, 3) = DeriveeY (Ai, Aj) .Ai. pasH / pasV - DeriveeX (Ai, Aj). .Aj .notV / pasH

A (:,:, 4) = DeriveeX (Ai, Aj). Ai + Derivative Y (Ai, Aj). aj

Xtrans (1) = F1 (0.0). bVk + 1.Ti / pasV

Xtraπs (2) = F1 (0,0) .. aVk + 1 i / pasH

Xtrans (3) = gVk + 1.Ti

Xtrans (4) = (s-1) .Ti

we will try to solve the equation:

A * Xtrans - B = 0

We use the method of least squares to minimize the norm.

We can write the equation for all the points of the image. But to improve the precision and limit the calculations, we can notice that in the equation A * Xtrans = B, the term B is the difference of two successive images and that we can eliminate all the values too weak or close to the noise. In the tests carried out, all the points between +/- 0.6Max (B) and +/- Max (B) were kept. For the sequences studied, the number of points changed from a few tens to around 1500. We can also take a fixed number of the order of 1000 among the sequences, close to the maximum.

Referring to Figure 4, we will now briefly describe the imaging system allowing the implementation of the stabilization step.

The shooting camera 13 delivers its video signal from images to a filter

10 low-pass 42 as well as to a processing block 43 receiving on a second input the stabilization data and supplying as output the stabilized images. On its second input, the block 43 therefore receives the rotational speeds to be subjected to the images taken by the camera 13. The output of the filter

42 is connected to two buffer memories 44, 45 respectively storing the

15 two filtered images of the present instant t and of the past instant t-1. The two buffer memories 44, 45 are connected to two inputs of a computation component 46, which is either an ASIC or an FPGA (field programmable gate array). The calculation component 46 is connected to a working memory 47 and, at the output, to the processing block 43. All the electronic components of the

20 systems are controlled by a management microcontroller 48.

Having now described the stabilization stage, we can now approach the harmonization stage.

? The harmonization implemented in the guiding method of the invention is an extrapolation of the stabilization step, the sighting device and the rocket imaging device being, before launch, mounted on the same carrier.

The stabilization of the images of the rocket imaging device is a self-stabilization process, in which the image of time t is stabilized on the image of time t-1. In other words still, we can say that we harmonize each image of the imaging system on the previous one. To harmonize the two devices, at the same instant t, the two images of the two devices are taken and they are stabilized one on the other, that is to say that the two devices are harmonized.

Harmonizing is tantamount to confusing the optical axes of the two devices as well as matching the pixels of the two images two by two, and preferably also confusing these pixels.

Naturally, the two devices to be harmonized according to this process must be of the same optical nature, that is to say operate in comparable wavelengths.

In this case, the two devices both taking images of the same scene, in a terrestrial frame of reference, the images of the scene taken at the same times by the two devices are filtered in a low-pass filter, for n ' retain only the low spatial frequencies, and the equation of the optical flow between these pairs of respective images of the two devices is solved, to determine the rotations and the variation of the ratio of the respective zoom parameters to be subjected to these images for the harmonize on each other.

The initial guidance law, as recalled above, is developed by means of the position, distance and speed of the target, on the one hand, and a flight model on the other.

Having developed the initial rocket guidance law, the operator of the firing control proceeds to launch it. Up to a certain distance from the target 61, until the rocket acquires the target, the image taken by the imaging device 10 of the rocket is compared with the stored wide field image 52 of the scene taken at the start with the aiming device 62, that is to say that the rocket guidance is permanently controlled.

After acquisition of the target 61 by the rocket, the guidance of the rocket is continued until the terminal phase, by comparison of the image taken by the imaging device 10 of the rocket with the small field image 53 also stored.

Claims

L- Method for guiding a rocket (1) on a target, in which the rocket (1) being equipped with self-guiding means with imaging device (10) and path correction means (11) ,

- the target is acquired by an aiming device and its position is determined,

- the aiming device and the imaging device (10) of the rocket are harmonized,

- the images of the imaging device (10) of the rocket are stabilized, - a guide law is developed,

- we launch the rocket (1) and

- the rocket is guided according to this law until it acquires the target itself.

2. Guidance method according to claim 1, in which, before launching, an initial guidance law is developed and the rocket (1) is guided until it acquires the target according to this initial law.

3. Guidance method according to claim 1, in which, before launching, an initial guidance law is developed and, after launching, a continuously variable guidance and trajectory correction law is developed until the rocket (1) acquires the target.

4.- Method according to one of claims 1 to 3, wherein, to harmonize the sighting device and the imaging device (10) of the rocket, an electronic harmonization is carried out according to which, in a terrestrial frame of reference, the images of the scene taken at the same instants by the two devices are filtered in a low-pass filter (42), so as to retain only the low spatial frequencies, and the equation of the optical flow between these pairs of solves is resolved. respective images of the two devices for determining the rotations and the variation of the ratio of the respective zoom parameters to be subjected to these images in order to harmonize them one on the other.

- 5.- Method according to one of claims 1 to 4, wherein the images of the imaging device (10) of the rocket are stabilized in a terrestrial frame of reference, on the landscape.

6.- Method according to claim 5, wherein, in the terrestrial frame of reference, the images of the scene taken by the imaging device (10) are filtered in a low-pass filter (42), to retain only the low spatial frequencies, and the equation of the optical flow is solved to determine the rotations to be subjected to the images in order to stabilize them on the previous images.