IL180183A - Process enabling a projectile to be navigated and/or guided and/or steered towards a target and the device therefor - Google Patents

Description

A process enabling a projectile to be navigated and/or guided and/or steered towards a target and the device thereof Giat Industries C. 172129 A PROCESS ENABLING A PROJECTILE TO BE NAVIGATED AND/OR GUIDED AND/OR STEERED TOWARDS A TARGET AND THE DEVICE THEREOF The technical scope of the invention is that of processes and devices to navigate and/or guide and/or steer a projectile to a target.

Known projectiles are guided towards their target by a guidance device which produces acceleration correction orders to be applied to the projectile to direct it towards the target.

These correction orders are then used by a steering device which produces the orders to be applied to the steering organs so as to ensure the required correction.

In order for the guidance and steering to be properly ensured, both the position and attitude of the projectile need to be known within a terrestrial reference frame.

More often than not, the position of the projectile is known thanks to a satellite positioning system (more commonly known by the acronym "GPS" meaning "Global Positioning System") .

Such a device enables the projectile to locate itself during its trajectory. Furthermore, the projectile is programmed before firing with the coordinates of its target.

It thus determines its own actual position in flight and prepares, using the data supplied by an on-board inertial measurement unit and by means of suitable algorithms, the control orders intended for the elevons.

This inertial measurement unit comprises accelerometers and gyrometers (or gyroscopes), which supply (in a reference frame linked to the projectile) the components of the instantaneous spin vector and non-gravitational acceleration to which the projectile is subjected.

This inertial measurement unit is implemented to know the projectile's orientation and namely to determine the Euler angles enabling the passage from a reference frame linked to the projectile to the fixed terrestrial reference frame.

It also enables the projectile to be steered and contributes towards its guidance by merging the data from this unit with that supplied by the GPS.

If such a solution is well adapted to projectiles such as missiles, it cannot be used for cannon-fired projectiles because of the lack of robustness of the gyrometers and the excessive cost of these measurement components.

The aim of the invention is to propose a process- to navigate and/or guide and/or steer a projectile towards a target or objective and which overcomes such drawbacks.

Thus, the invention relates to a process enabling a projectile to be navigated and/or guided and/or steered towards a target, such process wherein a calculation of all or some of the projectile angles of Euler is used to enable the determination of the attitude and/or location of the projectile within a terrestrial reference frame, such process wherein: - before firing, at least one value is programmed into the memory or register of a projectile's computer of the three components of a reference frame magnetic field in a fixed, direct and orthonormed terrestrial reference frame, such reference frame being centred on the firing position and having a horizontal axis and a vertical axis, the orientation of the reference frame with respect to the target's direction being known and fixed or programmed, at least one azimuth angle value is built in or programme before firing into another memory or register of the projectile's computer, - at least one measurement of the three components of the magnetic field is made during the trajectory in an orthonormed reference frame linked to the projectile, - the roll angle and/or attitude angle is calculated during the trajectory from the values of the magnetic field measured, the value or values of the reference frame field and the azimuth angle value memorised for that part of the trajectory under consideration.

Advantageously, the fixed terrestrial reference frame will be selected, centred on the firing position and will have a horizontal axis oriented in the direction of the target.

The value of the reference frame magnetic field may be measured before firing at the firing position and memorised, the calculation of the angle or angles being thereafter made from this measurement.

The. attitude angle Θ may be calculated from at least one measurement of the component HPX of the magnetic field along axis Xp of the projectile, using the following formula: HpX - b cosO - c sinO = 0, formula in which b and c are coefficients which are functions of the components HMBOX HMBOY and HMBoz of the reference frame magnetic field in the fixed terrestrial reference frame such as programmed before firing and ψη being a value of the azimuth angle known for the part of the trajectory under consideration.

The roll angle φ can then be calculated from the attitude angle and by using at least one measurement of components HPY and HPZ of the magnetic field along axes YP and ZP of the projectile by the following formula: ch d and e are coefficient functions of components ΗΜΒΟ ? HMBOY and HMBOz- of the reference frame magnetic field in the fixed terrestrial reference frame such as programmed before firing and the coefficients of a matrix M (θ, ψ) of partial passage of the fixed terrestrial reference frame to the reference frame linked to the projectile, and where only the azimuth and attitude angles are integrated.

After calculation of the roll angle φ a linearization of its value will advantageously be performed.

According to one embodiment, for a ballistic flight phase, the azimuth angle ψη = ψι will be memorised or programmed before firing and will correspond to the azimuth angle given to the projectile when fired.

According to another embodiment, for a steered flight phase, the azimuth angle ψη adopted for the part of the trajectory under consideration will be a value memorised before firing and incorporated into a firing table which will be read in relation with a timer counted down from the instant of firing.

According to another embodiment, for a steered flight phase, the azimuth angle ψη adopted for the part of the trajectory under consideration will be a value which will be calculated during the trajectory on the basis of the projectile's aerodynamic coefficients, at least one measurement of the acceleration and at least one memorised or programmed azimuth angle value.

Additionally, the projectile's instantaneous spin vector components can be evaluated in a reference frame linked to the projectile from the calculated Euler angle values and their derivatives as a function of time.

Advantageously, so as to correct the measurements of the magnetic field from part of the disturbances caused by the environment of the sensors, at least one correction may be made during a phase which has a stable azimuth angle, such correction comprising a measurement of the deviation between the theoretical magnetic field and the measured magnetic field.

To perform such correction, at least one programmed or memorised value of the azimuth angle can be used as well as at least one programmed or measured value of the attitude angle .

The invention also relates to a device to ensure the navigation and/or guidance and/or steering of a projectile,, such device incorporating at least one magnetic sensor ensuring the measurement of the three components of the magnetic field in an orthonormed reference frame linked to the projectile, such device implementing the process according to the invention and wherein it comprises at least one computer incorporating an algorithm to calculate the Euler angles, such computer associated with memory means coupled with means supplying programming data before firing for said memories, the memory means being intended to store at least one value of the three components of a reference frame magnetic field in a fixed terrestrial reference frame, this programming data being used by the computer with the measurements of the magnetic field during the trajectory to determine, for at least part of the trajectory having a substantially constant azimuth angle, all or part of the Euler angles thereby enabling the navigation and/or guidance and/or steering of the projectile to be ensured.

The projectile may also incorporate inertial means.

The computer may incorporate means enabling the measurements of the magnetic field of part of the disturbances caused by the environment of the sensors to be corrected.

The invention will become more apparent from the following description of a particular embodiment, such description made with reference frame to the appended drawings, in which: - Figure 1 is a diagram showing a projectile implementing a navigation and/or guidance and/or steering device according to the invention, - Figure 2 shows an example of a projectile in flight towards a target and the principal data implemented in the different calculations, - Figure 3 shows the three successive spins selected with enable the passage from a fixed reference frame to a mobile reference frame, - Figure 4 is a functional block diagram of the device according to the invention, - Figures 5a and 5b show how the skid and incidence angles are located, - Figure 6 shows the linearization of the roll angle.

Figure 1 schematically shows one embodiment of a projectile 1 implementing a navigation and/or guidance and/or steering device according to the invention.

The projectile 1 is equipped at its rear part by a deployable stabilising tailpiece 2 and at its front part by four canard elevons 3, also deployable.

Steering means, or a servomechanism 4, ensure that the different canards 3 are driven in rotation to steer the projectile. These means are not shown in detail and they may incorporate two or four back-geared motors (one per canard or one per steering plane) .

This projectile is, for example, a projectile fired by a cannon at a target.

When the projectile is inside the barrel of a weapon (not shown) the tail fins and canards are folded back along the projectile body 1, or else are housed in the projectile body. They deploy upon exiting the barrel so as to fulfil their stabilisation or steering functions.

The means to deploy the tail piece or elevons as well as the means to drive the canard elevons are well known to the Expert and are not part of the present invention. Reference may be made to patents FR2846080 and FR2860577 which describe such mechanisms.

The pivoting of the canards is controlled by an on-board computer 5.

The projectile 1 also encloses a warhead 6, for example a hollow charge, an explosive charge or else one or several scatterable munitions.

According to one important characteristic of the invention, the projectile 1 incorporates a triaxial magnetic sensor 7 (a single sensor or else three magnetic or magneto-resistant probes spaced in three different directions of a measurement trihedron, for example three mutually orthogonal probes each preferably directed along one of the axes of the reference frame linked to the projectile GXP, GYP or GZP) .

This sensor enables the components of the terrestrial magnetic field H to be measured in a reference frame linked to the projectile 1.

The magnetic sensor 7 is linked to the computer 5 which processes the measurements and ensures their subseqμent utilization .

The projectile 1 also incorporates an interface 8 to programme the computer. This interface is intended to cooperate with programming means (not shown) integral with the weapon. It is linked to the computer 5.

Inductive programming means may, for example, be provided (such as those described in patents FR2703412 or DE3843476) . In this case of the interface 8 is constituted by an inductive loop positioned in the vicinity of the external wall of the projectile body.

More simply, zones of electrical contact may be provided which cooperate with the weapon's programmer.

The projectile 1 lastly encloses inertial means. These inertial means 9 comprise three accelerometers 10a, 10b, 10c respectively oriented along the roll axis (GXP) , the yaw axis (GYP) and the pitch axis (GZP) of the projectile.

The inertial means 9 are classically intended namely to enable the implementation of steering laws.

The inertial means 9 are linked to the computer 5 which processes the measurement made and ensures their subsequent utilization for the navigation, guidance and/or steering of the projectile.

Naturally, Figure 1 is only an explanatory diagram which does not anticipate the relative locations and dimensions of the different elements. In practical terms, a single projectile fuse may incorporate the computer 5, the. magnetic sensors 7 and the accelerometers 9.

The projectile may also comprise a target detector 11 so as to enable its tracking when the projectile is in the terminal phase of its trajectory.

Figure 2 shows one example of a trajectory 12 followed by a projectile 1 according to the invention between its firing platform 13 and a target or objective B.

A reference frame centred at 0 (point at which the firing platform is located or the theoretical firing position 13) and is shown on this Figure, and axes OXMBo, OYMBO and . OZMBo-This reference frame is the fixed terrestrial reference frame which is selected in this particular embodiment to implement the invention.

It is a direct and orthonormed reference frame with a horizontal axis OXMBO oriented towards the target B and a vertical axis OZMBo (axis OYMBO is deduced from the two others for the reference frame to be orthonormed and direct) .

It is with respect to this reference frame that the Euler angles which form the axes GXPYPZP of the reference frame linked to the projectile will be evaluated.

Figure 3 shows, by way of a reminder, the three successive spins enabling the passage from a fixed reference frame GXMBOYMBOZMBO (reference frame OXMBOYMBOZMBO centred at G after a translation of vector OG) to mobile reference frame GXpYpZp linked to the projectile.

A first spin of this point XMBOYMBO MBO is made around axis ZMBO, such spin defining angle ψ (azimuth angle) . The direct orthonormed reference frame obtained has axes Χχ, Yi (perpendicular to Xi and Zi) and Z1=ZMBO- Reference frame ΧχΥχΖχ is then made to pivot around axis Yi of angle Θ (attitude angle) . The new direct orthonormed reference frame obtained has axes Z2, Y2 (Y2=Yi) (perpendicular to Z2 and Y2) .

Lastly, reference frame X2Y2Z2 is then made to pivot around axis X2 of angle φ (roll angle) . The new direct orthonormed reference frame obtained has axes Xp=X2, YP and ZP.

The navigation of the projectile (its location in space) as well as the guidance and steering stages necessitate the Euler angles (ψ,θ,φ) of the projectile to be known at all times so as to be able to locate the projectile with respect to its target and to thus be able to control the effects of the orders applied- to the elevons.

Usually, this knowledge of the Euler angles is supplied by a full inertial platform comprising three accelerometers and three gyrometers.

Gyrometers are appliances which are costly and fragile, and which do not withstand easily being fired from a cannon.

The invention proposes a process and device which enables us to do without gyrometers.

In accordance with the invention, as a first step, at least one value for the three components of a reference frame magnetic field in the selected fixed terrestrial reference frame OXMBOYMBOZMBO will . be entered into the memory (or register) before firing.

This reference frame magnetic field will preferably be that measured at the firing position 0. It will be measured, for example, by a triaxial magnetic sensor linked to the firing platform 13 and with its detection axes oriented along the axes of the fixed reference frame selected (OXMBOYMBOZMBO) · Furthermore, at least one azimuth angle value (ψη) will be entered into another memory or register that will be considered to be constant for at least part of the trajectory.

Indeed, cannon fired projectiles incorporate at least part of their trajectory at a constant azimuth angle, this is their ballistic phase (Ti in Figure 2) .

For the non ballistic phases of the projectile's trajectory (steered phases) it is also possible to break the theoretical trajectory curve 12 down into several sections in which the azimuth angle may also be supposed constant. By way of example, Figure 2 shows a trajectory 12 which incorporates, after the ballistic phase Ti, six sections T2, T3, T4, T5, T6 and T7 during which the azimuth is substantially constant. It is naturally possible for a trajectory curve to be defined which incorporates a greater number of sections during which the azimuth is known.

The trajectory profile 12 is known a priori. It can be programmed before firing and may be correlated to a timer initialised during firing. This trajectory profile will thus be entered in the form of a firing table into a memory in the projectile's computer. Thus, it is also possible, in accordance with the invention, for a specific trajectory section with an azimuth angle ψι, ψ2, ψ3, ψ4, ψ5, ψε or ψ7 to be associated with a specific time.

This azimuth angle will be used by the algorithm proposed by the invention to calculate the different Euler angles for the portion of trajectory under consideration.

In accordance with the invention, the final step is to take measurements during the trajectory of the three components of the terrestrial magnetic field in the orthonormed reference frame linked to the projectile GXPYPZP.

The different programmed values as well as the measured values are then used to calculate the roll and attitude angles of the projectile.

To perform these calculations, it is considered that the matrices enabling the evaluated magnetic field vector to be transformed in the projectile reference frame into an evaluated vector in the fixed terrestrial reference frame.

The magnetic field vector measured on board the projectile is writtenHp . This vector has the components Hpx, Hpy and Hpz in the reference frame linked to the projectile.

The reference frame magnetic field vector is written H This vector has the components ΗΜ0Βχ, HMBOY and HMBOZ n the fixed terrestrial reference frame.

We can write: Hp = Μ Μ Μ Η , expression which means that we move from one vector to another by the product of three transformation matrices which can be written in a developed form as: We can see that we move from one vector to another by successive matricial products integrating the Euler angles which are required to be determined.

This transformation may also be noted as: Hp = Μ Μ Η expression in which 0ψ represents a matrix which is the product of the two matrices related to angles Θ and ψ. Matrix ΜΘψ has coefficients which will hereafter be noted as This may be written as: We see that with such a breakdown, if Θ and ψ are known, it is possible for the roll angle φ to be determined (the two expressions of the magnetic field being in. fact known anyway since they are either programmed (HB O) or measured (HP) .

The invention proposes to eliminate the uncertainty of these calculations by using at least one known value memorised or programmed before firing of the azimuth angle Ψ- We note that, in effect, the measurement of component Hpx of the magnetic field along axis Xp does not depend of the roll φ.

This, in fact, is written: Ηρχ = an HMBOX + Sl2 ¾B0y + 3l3 ¾Β0ζ· Or more explicitly: HpX - b cosG - c sinO = 0, formula in which: b = HMBOX cosy + HuBOy sin\j/ and c = HMB0Z- When the azimuth angle ψ is known, the attitude angle Θ is thus immediately deduced from the values of the magnetic field (pre-programmed HBMO and measured Hp) .

Since the azimuth and attitude angles are known, the roll angle φ can be easily calculated.

It will be calculated using the following formula: ta (φ) = ch d = (a2i HMBOX + &22 HMBOY) and e = (a3i HMBOX + &32 HMBOY + £33 ¾ΒΟΖ) These calculations for Θ and φ can be made for a whole portion of trajectory Ti in which an azimuth angle ψί can be considered substantially constant and thus be pre-determined.

This calculation may namely be made for the ballistic portion of the projectile trajectory (Ti) . In this case, the azimuth angle ψη = ψί, programmed before firing corresponds to the azimuth angle given to the projectile upon firing.

These calculations may also be made during the steered flight phase for the different azimuth angle ψη successively adopted.

It is thus simple to read the value of the azimuth angle from a firing table memorised in the computer 5 and read in relation with a timer counted down from the instant of firing.

Other ways to select the value of the azimuth angle are possible and are described in the following part of the present description.

We note that if HPY and HPZ are nil, it is a priori no longer possible to known the roll position of the projectile using the previous calculation. Such a configuration appears when the projectile travels with its axis Xp collinear to the terrestrial magnetic field. To avoid such a problem, it is sufficient to fire the projectile so as to avoid such a configuration. A suitable firing position 0 will be selected and a form of trajectory 12 leading to such co-linearity will be avoided.

Once the Euler angles ψ, Θ and φ have been calculated, the components of the instantaneous spin vector Qof the projectile can be easily evaluated in a mobile reference frame linked to the projectile.

It is essential to know this vector to be able to conduct the usual navigation, guidance and steering stages of the projectile.

The components of this vector are: dw ■ . άθ , dm Where ψ = ; Θ =— and φ =—— , the derivatives dt dt dt of the angles calculated with respect to time.

We can thus see that it is possible to obtain the three components of the spin vector without having to rely on a gyrometer .

We note that during the different phases of evaluation during which the azimuth angle is considered to be constant, namely during the ballistic phase of the projectile, the spin rate in roll p can simply be approximated as φ = , indeed, the . derivative of the azimuth angle is always nil during these phases .

Figure 4 shows an example of the structural and functional organisation of a device according to the invention.

The computer 5 incorporates different calculation modules made in the form of algorithms entered into memories or registers .

A first module 14 performs the calculations of angles ψ, Θ and φ from the programming data supplied before firing by the programming means 8 or pre-programmed in the computer 5 and from the measurements of the magnetic field components made during the flight of the projectile by the triaxial magnetic sensor 7.

The programming data incorporate a measurement of the three components of the reference frame magnetic field HMBo in the fixed terrestrial reference frame centred on the firing position, such measurement being stored in memory 15.

It also incorporates one or several values for the azimuth angle ψη considered as known a priori for a different part of the trajectory.

These values are associated, for example, with a firing table entered into memory 16 and utilized by the algorithm in relation with the time data given by the timer 17 which is initialised after the firing of the projectile has been detected (by classically means not shown, for example, a starting acceleration sensor) .

A second mode of calculation 18 derives the calculated angles and evaluates the components p, q and r of the projectile's instantaneous spin vector.

These elements are utilized by a third guidance and/or steering module which implements known guidance or steering laws which do not require further explanation here.

The guidance and/or steering module 19 furthermore utilizes the projectile's acceleration data supplied by the inertial means 9.

This module ensures the control of the servo-mechanism 4 driving the elevon 3.

The above description was made using a reference frame centred on the firing position and having a horizontal axis OXMBO oriented towards the target.

It would naturally be possible to choose a reference frame which is oriented differently. To implement the invention, it will in this case be necessary to define the axes of the reference frame with respect to direction OB (straight line joining the firing platform to the target) . Subsequent calculations will use, in this case, transfer matrices to ensure the passage from the reference frame to the reference frame whose axis OXMB0 is oriented towards the target to define the different angles. These orientation elements of the reference frame will, for example, be programmed before firing at the same time as the components of the magnetic field in the reference frame as well as the value or value of the azimuth angle ψ.

By way of a variant, it would be possible to define a weapon system (associating firing platform and projectile) designed such that the initial azimuth ψ0 is always nil. Such a variant enables us to avoid having to programme the initial value of the azimuth, this value ψ=0 which is fixed, will thus be incorporated into the computer 5 of the projectile as initial calculation data. It is, however, in this case, necessary to programme the orientation element of the reference frame if the latter is not selected with its axis OX oriented in the direction of the target.

In practical terms, adopting ψ = 0 equates to choosing always to fire in the vertical plane OXZ of the selected reference frame.

By way of a variant, it is possible for the memorising of a firing table giving the different azimuth angles to be replaced by a value ψη which is calculated continuously during the trajectory from the aerodynamic coefficients of the projectile and from a measurement of the acceleration.

This calculation is made in a specific azimuth evaluation module 20 which uses the acceleration measurements supplied by the inertial means 9 as well as an evaluation of the aerodynamic coefficient Aof the projectile which is memorised (Λ is generally called the incidence time constant of the projectile) .

This azimuth calculation is made from a calculation of the angles made by the projection of the velocity vector V of the shell in planes GXMBOYMBO and GXMB0ZMB0 of the fixed reference frame with respect to the axes of such reference frame .

Figure 5a thus shows the skid plane GXMBOYMBO in which the projection VY of the velocity vector of the shell makes an angle β (skid angle) with axis GXP of the shell and an angle εγ (aerodynamic azimuth) with axis GXMBO- The azimuth angle ψ is also shown in this Figure.

Figure 5b shows the plane of incidence GXMBoZMBo in which the projection Vz of the velocity vector of the shell makes an angle a (incidence angle) with axis GXP of the shell and an angle εζ (aerodynamic slope) with axis GXMBo- The attitude angle Θ is also shown in this Figure.

The inertial means 9 enable the value of the components of the acceleration vector to be known at time t along axes GXP, GYP and GZP of the projectile.

The classical formula linking the value of azimuth angle ψ to the aerodynamic coefficient Λ and to angle εγ is implemented for a projectile steered by canard elevons.

Expression in which s is the Laplace operator ψ 1 + A s (derived with respect to time) and which may also be written: ψ = εγ + Λ d8Y/dt. Knowing angle εγ at a given time thus enables the azimuth angle ψ is be deduced.

An integration of the measured acceleration vector components enables the velocity vector components to be determined. The velocity vector is projected in the plane GXMBOYMBO by using a transfer matrix Μ(ψ, θ, φ) integrating the Euler angles.

We can see that this type of calculation of the azimuth angle requires this matrix to be known and thus also the azimuth angle. In fact, in accordance . with the invention, a known initial value will be taken for azimuth angle ψη (for example that which has been pre-programmed, or else built into the computer (such as ψ=0) and which corresponds to the ballistic phase) .

Then, step by step the values of the azimuth angle will be determined from the values calculated previously.

To do this, the steps described previously to determine angles Θ and φ are implemented using the measurements of the magnetic field as well as pre-programmed reference field values .

Then, successive iterations are made according to a given sampling period T.

Thus, at time t = n, the value of azimuth angle ψη will be determined from the acceleration values measured at t = n and with the coefficients of the matrix Μ(ψ, θ, φ) calculated at time t = n-T.

At a given time t = n, we therefore obtain: ψη = f (Yn , Hn , HMB0 , ψη-i θη - T r pn - τ ) from which we deduce: θη and This evaluation merely needs to be reiterated along the projectile's trajectory to have, with no gyrometers, the necessary parameters for navigation, guidance and steering.

This embodiment enables (for a projectile with canard elevons) the performances of the navigation process according to the invention to be improved by allowing most of the errors linked to magnetic and aerodynamic disturbances on the theoretical location of the constant azimuth zones to be overcome .

For a projectile steered by wings of elevons other than canards, there are other formulae to define the aerodynamic coefficients of the projectile. The Expert will easily be able to implement these formulae to calculate, for these types of projectile, the different values for the azimuth angle φη.

Simulations of the algorithms have been performed on several different configurations of the terrestrial magnetic field and have been compared to the theoretic data expected.

Excellent restitution of the different angles (ψ, θ, φ) has been observed as well as of the different spin rates (p, q, r) and this both for low projectile spin rates (of around 1 rev/sec.) and for higher spin rates (of around 15 revs/sec . ) .

Indeed, the deviations observed during disturbed trajectories are in any case less than 2° to 3° for each angle, which is enough for a projectile having a short range trajectory (less than 10km) .

These deviations may be less than one degree if the magnetic disturbances are corrected and if account is made of the ballistic disturbances (by introducing, for example, a bias during firing) .

Noise in the evaluation of the roll angle φ as well as in' the roll spin rate p has been observed.

Such noise is linked to the fact that the angle is estimated from an arc tangent function whose result is between -π and +π (or between 0 and 2π with a different choice of arc tangent function) and presents a discontinuity in moving to the value π. This discontinuity disturbs the calculation of the derivative.

So as to correct this disturbance an algorithm to linearize the value of the roll angle φ calculated is provided in module 14.

This linearization is based on the a priori knowledge of the amplitude of the jump in the roll angle which is of 2π each time.

Figure 6 thus shows the value of the roll angle φ calculated (saw-toothed curve 21) and the linearized value (curve 22) .

This linearization will be performed as follows: Sudden jumps in the curve 21 will be detected, for example by periodically calculating the deviations between two successively calculated angles (φ2, (pi) , the detection of a jump thus corresponding to the appearance of a deviation exceeding a pre-programmed threshold. The direction of mp will be estimated (by determining the sign of (φ2 - A counter will be incremented with the value of φ to which, at each jump, a deviation will be added enabling the jumps to be eliminated. This deviation will be, for example, calculated by adding (after a jump thus detected) a deviation corresponding to the value of the jump corrected of 2% to the value of the angle calculated and preceding said jump.

Someone skilled in the art will easily determine the type of calculation to be performed to obtain the required curve 21 and it is not necessary for the selected algorithm to be described in further detail here.

In a preferred manner, the arc tangent function used will be corrected in advance so as to give the calculated arc value a value of between 0 and 2π .

It is the linearized value of φ which will be used for άφ the different calculations, namely those integrating φ = -^- . dt Furthermore, the magnetic field measured by the triaxial magnetic sensor 7 is likely to be disturbed by the projectile's 1 electromagnetic environment.

The most disturbing elements are the motors of the servomechanism 4 since they implement permanent magnets.

The projectile body itself is also a source of disturbance of the measurements. Indeed, its metallic mass induces a localised deformation of the lines of the terrestrial magnetic field.

So as to correct the disturbances generated, means will be provided in the computer 5 to make at least one correction of the measures made. These means have been schematised in Figure 4 by block 23 .

So as to ensure such a correction, a flight phase may be used, for example, in which the azimuth angle ψ is stable (for example, the ballistic phase Τχ of the projectile) . During this phase, the magnetic field will be measured and compared to the reference value HMB0 memorised before firing.

When the azimuth is constant, the theoretical values of the components (Hpx, Hpy, Hpz) of the magnetic field vector in the reference frame linked to the projectile can be easily determined. Indeed, these values are given by the resolution of the vectorial equality seen previously: Hp = ΜΨΘΨ.ΗΜΒΟ in which Μψβφ is the frame transfer matrix integrating the Euler angles. It is easy to determine the theoretical value (HPXTH) expected for HPX from this equation. Indeed, as has been explained previously, angle φ is not integrated into the calculated of HPX as long as ψ is known since it relates to the theoretical firing azimuth which was pre-programmed and as long as the initial Θ (attitude angle) is also known.

Thus, the projectile's computer 5 will be programmed with a theoretical value for field HPXTH, then during the first instants of the ballistic trajectory, this theoretical value will be compared with the value actually measured HPXmes in accordance with the process according to the invention. A deviation will be deduced from this: Cor = HPXTH - HPXMES. This deviation will be applied thereafter to all the measurements of HPX and will enable the disturbances in the measurements along the projectile's longitudinal axis GXP to be corrected.

An analogous method may be applied to correct the measurements along the transversal axes GYp and GZp.

These corrections will integrate the roll angle φ. The correction factor can only be calculated when the projectile is spinning around its own axis (spin rate of at least 0.25 revs/sec), which will be the case a few seconds after firing and during the ballistic phase.

Correction along the transversal axes is based on the fact that the measurements HPY and HPZ must theoretically be sinusoidal and in phase quadrature. This means that theoretically, if a measurement along direction GYP is maximal it must also be nil along path GZP. Furthermore, the field module remains constant during the flight (and normed at 1 in the calculations) .

We can thus write: |H>|2 = 1 = (HPX2 + HPY2 + Hpz2)from which we deduce: Hpymax2 = Hpzmax2 = 1 - (HPX2 ) .

It then suffices to compare during the test the values actually calculated for the maxima of HPY and HPZ with the theoretical values deduced from the calculation of the value of (1 - (HPX2)), such calculation naturally being performed from the HPX corrected using the method previously described.

For the rest of the flight, all the values measured in Ηργ and HpZ are thus corrected by a bias which is equal to the deviation between the theoretical and actual values thus measured during the evaluation phase. To be complete, the correction will also take into account a scale factor which will be determined by performing at least a second comparative measurement of the theoretical and actual values of Ηργ and HPz.

Different variants are possible without departing from the scope of the invention.

It is thus possible for the invention to be implemented in a projectile which doesn't have a ballistic trajectory, for example a missile. For this, at least one value of the azimuth angle needs to be determined during the trajectory (or programmed before firing) which will be considered as known for at least part of the trajectory.

In all the examples described previously, the magnetic field HMBO measured at the firing platform 13 has been considered as a reference value. This field is considered to be constant over the whole trajectory planned for the projectile.

This is true is most case where the projectile is of moderate range (less than 100 km) . For projectiles of greater range, it will be necessary to replace the initial programming of a reference field by the introduction in the projectile of several reference magnetic field values which shall be adopted successively during the trajectory for the different calculations. These values may be adopted in connection with location data given by an onboard satellite positioning system (GPS) .

The invention will thus enable the guidance/steering of a missile or rocket to be simplified whilst also enabling the elimination of the gyrometers.

Claims (15)

f CLAIMS/

1. A process enabling a jTroTectile (1) to be navigated and/or guided and/or steered towards a target (B) , such process wherein a calculation of all or some of the projectile's Euler angles (ψ, θ, φ) is used to enable the determination of the attitude and/or location of the projectile (1) within a terrestrial reference frame, process wherein: - before firing, at least one value is programmed into the memory or register of a projectile's (1) computer (5) of the three components (H BOX, HMBoy, HMBOZ) of a reference frame magnetic field in a fixed, direct and orthonormed terrestrial reference frame, such reference frame being centred on the firing position and having a horizontal axis and a vertical axis, the orientation of the reference frame with respect to the target's (B) direction being known and fixed or programmed, - at least one azimuth angle (ψ) value is built in or programme before firing into another memory or register of the projectile's computer (5), - at least one measurement of the three components (Hpx, HPy, HpZ) of the magnetic field is made during the trajectory in an orthonormed reference frame linked to the projectile (1) , - the roll angle φ and/or attitude angle Θ is calculated during the trajectory from the values of the magnetic field measured, the value or values of the reference frame field and the azimuth angle value memorised for that part of the trajectory under consideration.

2. A navigation and/or guidance and/or steering process according to Claim 1, wherein the fixed terrestrial reference frame is selected centred on the firing position 0 and has a horizontal axis OXMB0 oriented in the direction of the target (B) .

3. A navigation and/or guidance and/or steering process according to one of Claims 1 or 2, wherein the value of the reference frame magnetic field (HMBox, H^oy, HMBOz) is measured before firing at the firing position 0 and memorised, the calculation of the angle or angles being thereafter made from this measurement.

4. A navigation and/or guidance and/or steering process according to Claim 3, wherein the attitude angle Θ is calculated from at least one measurement of the component HPX of the magnetic field along axis XP of the projectile, using the following formula: Ηρχ - b cos0 - c sinG = 0, formula in which b and c are coefficients which are functions of the components ΗΜΒΟΧ HMBOY and HMBOZ of the reference frame magnetic field in the fixed terrestrial reference frame such as programmed before firing and ψη being a value of the azimuth angle known for the part (Tn) of the trajectory (12) under consideration .

5. A navigation and/or guidance and/or steering process according to Claim 4, wherein the roll angle φ is calculated from the attitude angle and by using at least one measurement of components HPY and HPZ of the magnetic field along axes YP and ZP of the projectile by the following formula: formula in which d and e are coefficient functions of components HMBoxi HMBOY and HMBoz of the reference, frame magnetic field in the fixed terrestrial reference frame such as programmed before firing and the coefficients of a matrix M (θ, ψ) of partial passage of the fixed terrestrial reference frame to the reference frame linked to the projectile (1), and where only the azimuth ψ and attitude angles Θ are integrated.

6. A navigation and/or guidance and/or steering process according to Claim 5, wherein after calculation of the roll angle φ a linearization of its value is performed.

7. A navigation and/or guidance and/or steering process according to one of Claims 4 to 6, wherein for a ballistic flight phase, the azimuth angle ψη = ψι is memorised or programmed before firing and will correspond to the azimuth angle given to the projectile (1) when fired.

8. A navigation and/or guidance and/or steering process according to one of Claims 4 to 6, wherein for a steered flight phase, the azimuth angle ψη adopted for the part (Tn) of the trajectory (12) under consideration is a value memorised before firing and incorporated into a firing table (16) which is read in relation with a timer (17) counted down from the instant of firing.

9. A navigation and/or guidance and/or steering process according to one of Claims 4 to 6, wherein for a steered flight phase, the azimuth angle ψη adopted for the part of the trajectory under consideration is a value which is calculated during the trajectory on the basis of the projectile's aerodynamic coefficients, at least one measurement of the acceleration and at least one memorised or programmed azimuth angle value.

10. A navigation and/or guidance and/or steering process according to one of Claims 1 to 9, wherein the projectile's (1) instantaneous spin vector Ω components are evaluated in a reference frame linked to the projectile from the calculated Euler angle values and their derivatives as a function of time .

11. A navigation and/or guidance and/or steering process according to one of Claims 1 to 10, wherein, so as to correct the measurements of the magnetic field from part of the disturbances caused by the environment of the sensors (7), at least one correction is made during a phase which has a stable azimuth angle ψ, such correction comprising a measurement of the deviation between the theoretical magnetic field and the measured magnetic field.

12. A navigation and/or guidance and/or steering process according to Claim 11, wherein, to perform such correction, at least one programmed or memorised value of the azimuth angle ψ is used as well as at least one programmed or measured value of the attitude angle Θ.

13. A device to ensure the navigation and/or guidance and/or steering of a projectile (1), such device incorporating at least one magnetic sensor (7) ensuring the measurement of the three components (Hpx, Hpy, Hpz) of the magnetic field in an orthonormed reference frame linked to the projectile (1), device implementing the process according to the invention and wherein it comprises at least one computer (5) incorporating an algorithm (14) to calculate the Euler angles, such computer associated with memory means (15) coupled with means (8) supplying programming data before firing for said memories, the memory means being intended to store at least one value of the three components of a reference frame magnetic field (HMBox, HMBOY, HMBOZ) in a fixed terrestrial reference frame, this programming data being used by the computer (5) with the measurements of the magnetic field during the trajectory to determine, for at least part (Tn) of the trajectory (12) having a substantially constant azimuth angle (ψη) , all or part of the Euler angles thereby enabling the navigation and/or guidance and/or steering of the projectile (1) to be ensured.

14. . A device according to Claim 13, wherein the projectile incorporates inertial means (9) .

15. . A device according to one of Claims 13 or 14, wherein the computer (5) incorporates means (23) enabling the measurements of the magnetic field of part of the disturbances caused by the environment of the sensors to be corrected. For the Applicants EINHOLD COHN AND PARTNERS By: