METHOD AND DEVICE FOR CALIBRATING AN INERCY UNIT

Description

[0001] The technical field of the invention is that of the methods and devices enabling the recalibration of an inertial unit equipping a platform, such as a vehicle and in particular a land vehicle.

[0002] It is known that inertial navigation systems use accelerometers and gyroscopes to detect the displacements and rotations of the axes of a geometric frame linked to the platform relative to a terrestrial reference frame.

[0003] However, inertial measurement units (IMUs) drift over time, and therefore periodic recalibration is necessary to guarantee a minimum level of accuracy. They also need to be initialized at startup.

[0004] Today, alignment can be performed using GPS signals. However, when the control unit needs to be initialized and the vehicle must remain stationary, GPS information is insufficient for aligning the control unit with the vehicle. Furthermore, when these signals are disrupted, alignment is performed by dedicated teams equipped with measurement tools that allow them to determine the platform's position and orientation, enabling them to align it with the inertial measurement unit.

[0005] These interventions are disruptive. They immobilize the platform for a certain period of time and require additional qualified personnel.

[0006] It is also known in the field of intercontinental ballistic missiles to recalibrate the missile's inertial navigation system using radar images of the terrain overflown, on which at least three reference points are located, the coordinates of which were entered before launch. This device is the subject of US patent 5755400.

[0007] However, it is associated with a vehicle operating in flight over the terrain and equipped with a surveillance radar. This system is not suitable for a platform such as a land vehicle, which must also be able to recalibrate its inertial navigation system several times during a mission, without any well-defined reference points being known beforehand.

[0008] US patent 5809457 also describes a device and method for inertially pointing a target whose coordinates are known from a fixed or mobile platform. In this device, the inertial measurement unit is decoupled from the platform and mounted directly on the pointing equipment. However, this pointing method systematically uses the GPS positioning system, which is susceptible to jamming or being inaccessible in the field.

[0009] Patent application EP 3495837 A1 discloses a collaborative platform registration system based on the principle of position registration using landmarks.

[0010] US patent 8260567 B1 discloses a method for determining the position of a moving platform based solely on calculated angles corresponding to the positions of at least three landmarks on an image acquired using an onboard camera.

[0011] The aim of the invention is to propose a method and device for recalibrating an inertial navigation system that can easily be implemented by the vehicle crew without requiring specific equipment or tools and without imposing any particular skills.

[0012] Thus, the invention relates to a method for recalibrating an inertial navigation system equipping a land vehicle, the vehicle being equipped with at least one turret orientable in elevation and azimuth to observe the terrain around the vehicle and determine at least one pointing direction of at least one object present on the terrain, angular measurement means capable of providing the elevation and azimuth orientation angles of at least one pointing direction in a reference frame (RV) linked to the vehicle, or vehicle reference frame, the vehicle also being equipped with computing means to which the angular measurement means are connected and a cartographic database enabling the coordinates of various landmarks present on the terrain to be known in a terrestrial reference frame (RT), the method comprising the following steps: We carry out the pointing, from at least one turret, of at least three landmarks present on the ground and whose geographical coordinates in the terrestrial frame (RT) are known; we record the elevation angles (β) and bearing angles (α) of each pointing direction in the vehicle frame (RV); we determine coefficients of an M TV matrix of transition from the terrestrial frame (RT) to the vehicle frame (RV) by implementing at least one calculation of the direction of the landmarks in the vehicle frame (RV) thanks to the measurements of the angles of the pointing directions and the knowledge of the coordinates of the landmarks in the terrestrial frame (RT); we calibrate the inertial unit of the vehicle from the orientation values ​​and possibly the position of the vehicle frame (RV) relative to the terrestrial frame (RT) thus calculated.

[0013] According to a first embodiment, and when the vehicle is equipped with a satellite positioning system (or GPS), the process comprises the following steps: We calculate the three vectors (Amer i) of the pointing directions of the Amers in a reduced vehicle frame (R TV) from the positions of the Amers in the terrestrial frame (RT) which are given by the cartography and the position of the vehicle in the terrestrial frame (RT) which is given by the GPS, the reduced vehicle frame (R TV) is a frame linked to the vehicle but whose axes are parallel to the axes of the terrestrial frame (RT), the calculation is therefore carried out considering that the orientation angles of the vehicle frame (RV) with respect to the terrestrial frame (RT) are zero; we calculate the three vectors (Amer iV) of the pointing directions of the Amers in the vehicle frame (RV) from the measured elevation and bearing angles;From this, we deduce the orientation angles of the vehicle frame (RV) with respect to the terrestrial frame (RT) by solving the three matrix products (one per landmark): Amer i = R ox *R oy *R oz * Amer iV = M TV * Amer iV, an expression in which Amer i is the pointing vector of a landmark (P i ) in the reduced vehicle frame (R TV ) as deduced from the mapping, Amer iV is the pointing vector of the same landmark (P i ) in the vehicle frame (RV ) calculated from the measured elevation and bearing angles, and R ox , R oy and R oz are the rotation matrices allowing the transition from the terrestrial frame to the vehicle frame, the product R ox *R oy *R oz of these matrices constituting the transformation matrix M TV from the terrestrial frame (RT ) to the vehicle frame (RV), a matrix which is reduced to only the rotation coefficients and gives the orientation angles sought.

[0014] According to a second embodiment, and when the vehicle is equipped with a rangefinder mounted on the turret, the process comprises the following steps: We calculate the coordinates (P iV) of the landmarks (P i ) in the vehicle frame (RV) from the measured elevation and bearing angles and the distances (D i ) measured by the rangefinder; we determine the coordinates (P iT) of the landmarks (P i ) in the terrestrial frame (RT ) from the map;From this, we deduce the orientation angles of the vehicle frame (RV) with respect to the terrestrial frame (RT) as well as the coordinates of the vehicle in the terrestrial frame (RT) by solving the three matrix products (one per point): P iT = M TV * P iV, an expression in which P iT is the vector giving the coordinates of a point (P i ) in the terrestrial frame (RT ), P iV is the vector giving the coordinates of the same point (P i ) in the vehicle frame (RV ), and M TV is the transformation matrix from the terrestrial frame (RT ) to the vehicle frame (RV ), the coefficients of this matrix giving on the one hand the orientation angles of the vehicle frame (RV ) with respect to the terrestrial frame (RT ) and on the other hand the position of the vehicle frame (RV ) with respect to the terrestrial frame (RT ). ;

[0015] Advantageously, in order to avoid measurement errors, the inertial unit will only be recalibrated when the vehicle is stationary, this stationary state being detected by appropriate means (odometer, speedometer).

[0016] Advantageously, one can choose a recalibration calculation method using the satellite positioning system or a recalibration calculation method using the rangefinder.

[0017] According to a particular method of execution, after the choice of landmarks by an operator, the foreseeable angular uncertainties for the angles of side slope, slope and heading are determined by reading pre-established precision charts and the operator can then be asked to modify his choice of landmarks if the uncertainty(s) are greater than at least one threshold.

[0018] The invention also relates to a device for recalibrating an inertial navigation system equipping a land vehicle, a device implementing the method according to the invention, a device characterized in that it comprises calculation means and a cartographic database enabling the determination of the coordinates in a terrestrial frame of reference (RT) of at least three landmarks present on the ground, the device also comprising means for angular measurement of the elevation and azimuth orientation of at least one turret enabling observation of the terrain around the vehicle and determination of at least one pointing direction, the angular measurement means being able to provide the elevation (β) and azimuth (α) orientation angles of at least one pointing direction (di) in a frame of reference (RV) linked to the vehicle, or vehicle frame of reference.The calculation methods allow for combining measurements of observation angles in site and bearing with the coordinates of known landmarks from the cartographic base, to determine coefficients of an M TV matrix for the transition from the terrestrial frame (RT) to the vehicle frame (RV), the calculation methods being connected to the inertial measurement unit to be able to recalibrate it from the orientation and possibly position values ​​of the vehicle frame (RV) relative to the terrestrial frame (RT) thus calculated.

[0019] According to a particular embodiment, the device includes a satellite positioning system (or GPS), the latter being used by the computing means to determine the positioning of the vehicle reference frame (RV) relative to the terrestrial reference frame (RT).

[0020] According to another embodiment, the device includes a rangefinder carried by the turret, the latter being used in combination with the angular measurement means of the elevation and azimuth orientation of the turret to allow the calculation of the coordinates (P iV) of the landmarks in the vehicle frame (RV).

[0021] Advantageously, the recalibration device may include a Human-Machine interface giving an operator the choice between a recalibration calculation mode using the satellite positioning system and a recalibration calculation mode using the rangefinder.

[0022] According to a particular embodiment, the device may incorporate at least one nomogram allowing the determination of the foreseeable angular uncertainties for the angles of side slope, slope and heading after the choice of the landmarks.

[0023] According to a particular embodiment, the device may include a Human-Machine interface enabling an operator to be informed if the uncertainty(s) are greater than at least one threshold.

[0024] Advantageously, the Human-Machine interface will be able to require the Operator to modify their choice of landmarks if the uncertainty(s) are greater than at least one threshold.

[0025] The invention will be better understood upon reading the following description of particular embodiments, a description made with reference to the attached drawings, in which: [ Fig. 1 ] shows a platform on the ground positioned relative to three landmarks; Fig. 2 ] is a diagram representing the recalibration device according to the invention; [ Fig. 3 ] is a flowchart illustrating a variant implementation of the process according to the invention.

[0026] By referring to the figure 1 A platform 1 is positioned on the ground. Platform 1 here is an armored ground vehicle, but it could be another type of vehicle, for example a truck, a command vehicle, a light vehicle or a vehicle equipped with a weapon such as an artillery system.

[0027] In all cases the vehicle 1 which implements the device according to the invention is equipped with a turret 2, that is to say a mobile system which can be oriented in elevation and azimuth in a reference RV linked to the vehicle.

[0028] Here, turret 2 is the main turret of the armored vehicle, equipped with a large-caliber gun (greater than 50mm). Turret 2 could also be a medium-caliber gun turret (caliber between 20mm and 50mm), a small-caliber gun turret (caliber less than 20mm), or even an unarmed observation turret equipped with a sighting system.

[0029] In a classic way and as schematically shown in the figure 2 The turrets 2 are equipped with a first motor M1 allowing the turret 2 to rotate about a vertical axis 4 for azimuth aiming (arrow G). They are also equipped with a second motor M2 to rotate the weapon (or the sighting system for an unarmed turret) about a horizontal axis 5 for elevation aiming (arrow S).

[0030] The pointing is controlled by a pointing servo system 6 and the values ​​of the pointing angles in bearing α and in elevation β are measured by encoders: a bearing encoder 7 and an elevation encoder 8.

[0031] We were also represented at the figure 2 an inertial unit 9 whose information feeds a computer 10 of the vehicle which supervises the various functions of the vehicle and in particular the pointing servo system 6.

[0032] According to the invention, the inertial navigation system 9 recalibration device comprises computing means 11 and a mapping database 12.

[0033] The computing resources 11 were isolated on the figure 2 for the sake of clarity of presentation, but it is clear that these means will most often be incorporated into the computer 10 of the vehicle whose computing resources make it possible to carry out the processing necessary for the implementation of the process according to the invention.

[0034] The cartographic database 12 is also represented in isolation but it will be advantageously integrated into an internal memory of the computing means 11 (or of the computer 10).

[0035] Map databases are standard and commercially available. These are digital databases that allow the creation of paper maps or digital maps, which are now incorporated into vehicle navigation systems (civilian or military) or accessible on the internet.

[0036] It is clear that the accuracy of the cartographic database 12 will condition to some extent the accuracy of the recalibration of the inertial navigation system 9 that it will be possible to ensure.

[0037] The computing means 11 are also supplied by the positioning signals in bearing α and in elevation β which come from the encoders in bearing 7 and in elevation 8. We have shown here cables coming out of the pointing servo system 6 and carrying the signals to the computing means 11.

[0038] It is clear that specific wiring could come directly from encoders 7 and 8, if the computing means 11 incorporate the calibration and shaping devices to exploit the signals from encoders 7 and 8.

[0039] According to a first embodiment of the invention, the computing means 11 are also powered by signals provided by a satellite positioning means 13 (means more commonly known by the Anglo-Saxon acronym GPS or "Global Positioning System").

[0040] In accordance with the invention, and with reference to the figure 1 , we will seek to know the positioning and orientation, in a fixed terrestrial frame RT, of a frame RV linked to the vehicle.

[0041] This positioning and orientation data will then be provided to the inertial measurement unit 9 to realign it with the Earth-based reference frame RT and thus compensate for its drifts. The Earth-based reference frame RT has its center O and its axes OX (north), OY (east), and OZ (towards the center of the Earth).

[0042] The vehicle reference frame RV is centered at V and has axes Vx (towards the front of the vehicle), Vy (towards the right of the vehicle), and Vz (towards the ground). Point V is an arbitrary point on the vehicle corresponding to the origin of the line of sight d from turret 2.

[0043] We will calculate the positioning and orientation of the RV reference frame linked to vehicle 1 using three landmarks P1, P2, and P3 present on the ground. Turret 2 will be successively pointed towards each landmark (Pi), and the elevation and azimuth angles for each landmark (αi, βi) will be provided to the calculation system 11. The angles will be provided by the pointing servo 6 or directly by the encoders 7 and 8.

[0044] The landmarks will be chosen by the person in the vehicle who implements the inertial navigation system recalibration process. These landmarks can therefore be different from one mission to another and they are not fixed a priori but chosen at the time of recalibration.

[0045] However, the Amers P i must be identifiable in the map database 12. It will be advantageous to choose clearly identifiable landscape features that can be precisely pointed to by turret 2: buildings, monuments, mountain peaks...

[0046] According to this first embodiment, the computing means 11 have the following input data: The coordinates (P iT) of the landmarks in the RT terrestrial frame which are provided by the map database 12; The coordinates (VT) of the vehicle in the RT terrestrial frame which are provided by the GPS; The elevation and bearing pointing angles (α i , β i ) for each landmark, i varying from 1 to 3.

[0047] Following the procedure, we will first calculate the three direction vectors (Amer i) of the pointing directions of the landmarks in a reduced vehicle frame (RT ), which is positioned in V and whose axes are parallel to those of the terrestrial frame RT . This calculation is performed from the positions of the landmarks in the terrestrial frame (RT ), which are given by the map, and the position of the vehicle in the terrestrial frame (RT ), which is given by the GPS.

[0048] The calculation is therefore carried out considering that the orientation angles (φ,θ,ψ) of the vehicle frame (RV) with respect to the terrestrial frame (RT) are zero.

[0049] For the sake of clarity figure 1 , only one vector Amer 1 is represented.

[0050] The map database provides the coordinates of the landmarks in the terrestrial reference frame RT, and the GPS provides the vehicle's coordinates in the same terrestrial reference frame. It is therefore possible to determine the pointing directions between the vehicle and each landmark in the reduced vehicle reference frame R TV, a reference frame having the same center V as the vehicle reference frame Rv but with x, y, and z axes parallel to the axes of the terrestrial reference frame.

[0051] The direction vectors of these pointing directions are easy to determine since the distance between the vehicle and each landmark can also be calculated from the map database.

[0052] To perform this calculation, it is sufficient to first determine the distance between vehicle 1 and each Amer P i on the three directions di of pointing of the Amers by turret 2.

[0053] Let i be the number of the landmark: i = {1,2,3}; (VX ,VY ,VZ ) the coordinates of the vehicle in the terrestrial frame RT; (P iX ,P iY ,P iZ ) the coordinates of a landmark (i) in the terrestrial frame RT; (dX i ,dY i ,dZ i ) the coordinates in the terrestrial frame RT of the direction di joining the vehicle to the landmark (i) considered.

[0054] Therefore, we calculate: dX i = − V X − P iX dY i = − V Y − P iY dZ i = − V Z − P iZ

[0055] Then the total distance D i between the vehicle and each Amer: D i = sqrt (dX i 2< + dY i 2< + dZ i 2< ) (Note: in this expression “sqrt” means square root).

[0056] We can then deduce, in the reduced vehicle frame R TV, the direction vectors (Amer i ) of the direction di of turret 2 when it points towards each Amer: Amer i = (Xt i, Yt i, Zt i) with: Xt i = dX i / D i Yt i = dY i / D i Zt i = dZ i / D i

[0057] Furthermore, we calculate the three direction vectors (Amer iV) of the pointing directions of the Amers in the vehicle frame (RV) from the measured site and bearing angles.

[0058] If we denote α i as the bearing angle for the landmark (P i ) and β i as the elevation angle for the same landmark (P i ), we can calculate: X i = cos α i . cos β i y i = sin α i . cos β i z i = − sin β i

[0059] So the vector Amer iV has coordinates: (xi ,yi ,zi ) in the vehicle frame RV .

[0060] It is then possible to deduce the orientation angles of the vehicle frame (RV) with respect to the terrestrial frame (RT) by solving the three matrix products (one per Amer): Amer i = R ox * R oy * R oz * Amer iV = M TV * Amer iV

[0061] In this expression: Amer i is the pointing vector of an Amer in the reduced vehicle frame (R TV ) as deduced from the mapping: Amer i = (Xt i , Yt i , Zt i ) ; Amer iV is the pointing vector of the same Amer in the vehicle frame (RV ) calculated from the measured elevation and bearing angles Amer iV = (xi ,yi ,zi ) ; Ro ox , Ro oy and Ro oz are the rotation matrices allowing the transition from the terrestrial frame RT to the vehicle frame RV .

[0062] In a classic way: Rox = 1 0 0 0 cos φ − sin φ 0 sin φ cos φ Roy = cos θ 0 sin θ 0 1 0 − sin θ 0 cos θ Roz = cosψ 0 − sinψ sinψ 1 cosψ 0 0 0

[0063] Expressions in which: φ is the cant angle (rotation of the vehicle frame RV around the ox axis of the terrestrial frame RT); θ is the slope angle (rotation of the vehicle frame RV around the oy axis of the terrestrial frame RT) and Ψ is the heading angle (rotation of the vehicle frame RV around the oz axis of the terrestrial frame RT).

[0064] The product R ox *R oy *R oz of these matrices constitutes the M TV matrix of transition from the terrestrial frame (RT ) to the vehicle frame (RV ), a matrix which is here reduced to only the rotation coefficients (and which therefore does not include translations from one frame to the other).

[0065] We thus obtain three equations for each Amer and solving the nine equations will allow us to calculate the three orientation angles (φ,θ,ψ) we are looking for.

[0066] Thanks to these calculations, we know the angles (φ, θ, ψ) between the axes of the vehicle's frame of reference RV, and we also know the vehicle's location via GPS. It is therefore easy to recalibrate the inertial measurement unit 9 with this new information on the vehicle's positioning and orientation.

[0067] The recalibration itself is a standard operation that does not need to be described in detail. All commercially available inertial navigation systems are equipped with a signal input that provides updated positioning and orientation data.

[0068] This embodiment of the invention is particularly discreet since the landmarks are located solely by the turret's pointing of the vehicle.

[0069] However, it requires an operational GPS system, meaning one that is not jammed.

[0070] The second embodiment of the invention differs from the first in that the turret 2 is equipped with a rangefinder 14 ( figure 2 ), which is similar to weapon 3, and which allows you to know the distance D between a target it points to and vehicle 1.

[0071] The signals provided by the rangefinder 14 are used by the fire control 6 associated with the computer 10.

[0072] In accordance with this embodiment of the invention, these signals are also sent to the computing means 11 (wired link D).

[0073] As in the previous embodiment, it is understood that the structural separation between the vehicle's computer 10 and the computing means 11 dedicated to recalibrating the inertial measurement unit 9 serves only to make the description clearer. The recalibration process according to the invention can implement algorithms that will be incorporated into the computer 10 (thus eliminating the need for dedicated computing means 11), and the computer 10 will then perform the function of the computing means 11.

[0074] According to this second embodiment, the computing means 11 have the following input data: The coordinates (P iT) of the landmarks in the RT terrestrial frame which are provided by the cartographic database 12; The elevation and bearing pointing angles (α i , β i ) for each landmark, i varying from 1 to 3; The distances D i between the vehicle and each landmark P i provided by the rangefinder 14.

[0075] Initially, we will use turret 2 to perform successive pointings of the three landmarks (P i ) and the pointing angles in elevation and bearing for each landmark (α i , β i ) will be provided to the computing means 11.

[0076] During each pointing, a telemetry of the Amer in question will also be carried out and the value of the distances (D i ) measured by the rangefinder 14 will be transmitted to the calculation means 11. This value is provided to the calculation means 11 by the pointing servo 6 or directly by the rangefinder 14.

[0077] This information allows the calculation of the coordinates of the landmarks (P iV ) in the vehicle frame (RV ) - or in other words the definition of the coordinates of the three vectors VP i linking the vehicle (frame RV with center V) to each of the three landmarks (P i ).

[0078] We will write the coordinates (xi, yi, zi) of each vector P iV in the vehicle frame (RV): x i = D i . cos α i . cos β i y i = D i . sin α i . cos β i z i = − D i . sin β i

[0079] The coordinates of the three landmarks (P iT ) are also determined in the terrestrial reference frame (RT ) from the map 12.

[0080] The coordinates of the landmarks are thus defined in the RT terrestrial frame of reference and in the RV vehicle frame of reference.

[0081] It is then possible to calculate the orientation angles of the vehicle frame (RV) with respect to the terrestrial frame (RT) as well as the coordinates of the vehicle in the terrestrial frame (RT).

[0082] To do this, we solve the three matrix products (one per Amer): P iT = M TV * P iv

[0083] Expression in which: P iT is the vector giving the coordinates of the point P i in the terrestrial frame (RT); P iV is the vector that gives the coordinates of the point P i in the vehicle frame (RV); M TV is the transformation matrix from the terrestrial frame (RT) to the vehicle frame (RV).

[0084] In a classical way the coefficients of the change of reference matrix M TV give, on the one hand the orientation angles (φ,θ,ψ) of the axes of the vehicle reference frame (RV ) with respect to those of the terrestrial reference frame (RT ), and on the other hand the position of the vehicle reference frame (RV ) with respect to the terrestrial reference frame (RT ), therefore the coordinates in the terrestrial reference frame (RT ) of the point V, center of the vehicle reference frame RV .

[0085] Matrix products are usually written as: M TV = T* R ox *R oy *R oz an expression in which the matrices are expressed in the following way: T = 1 0 0 X v 0 1 0 Y v 0 0 1 Z v 0 0 0 1 Rox = 1 0 0 0 0 Cosφ − Sinφ 0 0 Sinφ Cosφ 0 0 0 0 1 Roy = Cosθ 0 Sinθ 0 0 1 0 0 − Sinθ 0 Cosθ 0 0 0 0 1 Roz = Cosψ − Sinψ 0 0 Sinψ Cosψ 0 0 0 0 1 0 0 0 0 1

[0086] The advantage of this embodiment of the invention is that it allows not only the recalibration of the vehicle's inertial measurement unit, but also the provision of the vehicle's updated position without the need for a GPS.

[0087] It is clear that it is possible to equip a vehicle with computing means comprising both types of algorithms and thus allowing to operate, at choice, one or the other of the two embodiments of the process according to the invention.

[0088] The GPS-associated method can be used when the GPS signal is not jammed. One advantage is the discreetness of the positioning without telemetry.

[0089] The method associated with the rangefinder can be used when GPS is not available, however, with the loss of the vehicle's discretion.

[0090] The choice of algorithm to use will be offered to the operator on the screen of their Human-Machine Interface.

[0091] Whether the first or second method of implementation is used, inaccuracies in measurements inevitably lead to seeking solutions to equations that are not exactly verified.

[0092] The calculations will therefore preferably be carried out in the form of optimization calculations, for example in the form of successive iterations seeking the values ​​of the parameters giving the minimum for a difference criterion F between the two members of each equation to be solved.

[0093] For example, for the first embodiment, criterion F will be equal to: F = ∑ i = 1 3 M TV ∗ Amer iV − Amer i 2

[0094] Expression in which, as detailed previously, Amer i is the pointing vector of an Amer in the reduced vehicle frame (R TV); Amer iV is the pointing vector of the same Amer in the vehicle frame (RV) calculated from the measured elevation and bearing angles and M TV is the transformation matrix from the terrestrial frame (RT) to the vehicle frame (RV), a matrix reduced to only the rotation coefficients.

[0095] For the second embodiment, criterion F will be equal to: F = ∑ i = 1 3 M TV ∗ P iV − P iT 2

[0096] Expression in which P iV corresponds to the coordinate vector of the Amer P i in the vehicle frame (RV) and P iT corresponds to the coordinate vector of the Amer P i in the terrestrial frame (RT) and M TV is the transformation matrix from the terrestrial frame (RT) to the vehicle frame (RV).

[0097] In both cases, we will seek to minimize the F criterion, for example by applying the well-known gradient method.

[0098] In both cases, it is necessary that the vehicle or platform be stationary in order for the inertial measurement unit to be recalibrated.

[0099] To avoid any error in determination, the device will be associated with a means of verifying that the vehicle has stopped before starting the inertial measurement unit recalibration process.

[0100] For example, the device can be connected to a vehicle's odometer or speedometer.

[0101] We saw earlier that the accuracy of the calculations depended on the accuracy of the digital mapping used.

[0102] Accuracy will also depend on the accuracy of the means of measuring the orientation angles in elevation and bearing of the pointing or observation direction di.

[0103] Depending on the implementation method chosen, it will ultimately depend on the accuracy of the GPS 13 used and / or that of the rangefinder 14.

[0104] It is clear that it is possible to define a platform 1 which is equipped with means of measuring pointing angles, a GPS and / or a rangefinder as well as digital mapping with a minimum precision allowing to ensure a priori the recalibration with a desired level of precision.

[0105] However, there remains an uncertainty linked to the choice of the P i Amers by the operator.

[0106] Indeed, if the landmarks are too close to each other or if they are too close to the vehicle, the accuracy of the calculation will be poor.

[0107] In order to ensure in all cases the desired level of precision and according to a variant of the execution of the first or second embodiment, before the integration of the software into the platform or the vehicle, numerical charts 15 will be produced giving the uncertainty on the calculated attitude angles (side slope, slope and heading) as a function of the locations of the landmarks relative to the vehicle (elevation angles and bearing of the pointing direction of each landmark in the vehicle frame and distances measured or calculated relative to the vehicle).

[0108] These charts (15) are easily created by performing preliminary calculations of the uncertainties obtained when varying the locations of the landmarks within a given geometric domain. Naturally, the uncertainty calculations will also take into account the accuracy of the measurement methods and the mapping.

[0109] There figure 3diagrams the main steps of this variant of the execution of the process according to the invention.

[0110] Block A corresponds to the operator's selection and marking of the three landmarks. Steps B, C, and D correspond to the extraction of uncertainties associated with these choices from the nomograms 15. Step B: uncertainty in cross slope (φ), step C: uncertainty in gradient (θ), and step D: uncertainty in heading (ψ).

[0111] Each test T1, T2, and T3 compares the obtained angular uncertainty with a predefined and programmed threshold L. The uncertainty threshold is advantageously the same for each test, but it would be possible to choose a different threshold for each angle.

[0112] If the uncertainty obtained is greater than the threshold L for at least one of the angles (OR gate), a red indicator light V1 is switched on and the operator is asked to change their choice of Amer (return to step A).

[0113] If all uncertainties are below the threshold L (AND gate) a green indicator V2 is switched on and the process can continue with the actual calculation of the inertial measurement unit recalibration data.

[0114] Displaying red or green indicator lights will be a valuable feature on the Human-Machine Interface (HMI) for selecting landmarks. This could be done, for example, on a screen or a dedicated indicator light. The screen will be connected to a control device (keyboard, mouse) and could be a touchscreen.

[0115] The illumination of the red V1 indicator can be accompanied by the display of advice for the operator: "choose more distant bitters" "choose more distant bitters".

[0116] These messages will be advantageously incorporated into the 15 nomograms for defining uncertainties since, in practice, these nomograms allow us to know the areas that give the desired precision, so the location of the three landmarks in a nomogram allows us to know what modifications must be made to one or more landmarks to get closer to the areas of the nomogram giving the desired precision.

[0117] As an alternative, it is also possible to define a second threshold L' (L'>L) which will be checked at each output >L of tests T1, T2 and T3. This check will be done by three new tests T1', T2' and T3' (not shown) which will make it possible to check whether the inaccuracy is less than or greater than the second threshold L'.

[0118] This will allow, if the inaccuracy is greater than L but less than L', for example to trigger the lighting of an orange light and thus allow the operator to start the recalibration of the inertial measurement unit with a slightly degraded accuracy.

[0119] If the inaccuracy remains greater than L', the operator will have to modify their choice of landmarks.

[0120] This variant helps to overcome the potential difficulty of obtaining good Amers in certain areas of the terrain.

Claims

1. - A method for resetting an inertial navigation system (9) fitted to a land vehicle (1), the vehicle being equipped with at least one turret (2) which can be oriented in elevation and in bearing in order to observe the field around the vehicle and to determine at least one pointing direction (di) of at least one object present on the field, angular measurement means (7, 8) being able to give the angles of orientation in elevation and in bearing of the at least one pointing direction (di) in a reference frame (RV) linked to the vehicle, referred to as the vehicle reference frame, the vehicle being also being equipped with calculation means (11) to which the angular measurement means (7, 8) are connected and with a map database (12) making it possible to know the coordinates of various landmarks (Pi) present on the field in a terrestrial reference frame (RT), the method comprising the following steps, carried out when the vehicle is stationary: - performing pointing, from the at least one turret (2), of at least three landmarks (P1,P2,P3) present on the field and whose geographical coordinates in the terrestrial reference frame (RT) are known; - recording the elevation angle (β) and bearing angle (α) of each pointing direction (d1,d2,d3) in the vehicle reference frame (RV); - determining coefficients of a matrix MTV for passing from the terrestrial reference frame (RT) to the vehicle reference frame (RV) by implementing at least one calculation of the direction of the landmarks (P1,P2,P3) in the vehicle reference frame (RV) by means of the measurements of the angles of the pointing directions and the knowledge of the coordinates of the landmarks in the terrestrial reference frame (RT); - resetting the inertial navigation system (9) of the vehicle on the basis of the thus-calculated values of orientation and possibly of position of the vehicle reference frame (RV) relative to the terrestrial reference frame (RT).

2. - The method for resetting an inertial navigation system according to claim 1, method in which the vehicle (1) is equipped with a satellite positioning system (GPS) (13), the method comprising the following steps: - calculating the three vectors (Ameri) of the pointing directions of the landmarks (Pi) in a reduced vehicle reference frame (RTV) from the positions of the landmarks in the terrestrial reference frame (RT), which are given by the mapping, and from the position of the vehicle (1) in the terrestrial reference frame (RT), which is given by the GPS (13), the reduced vehicle reference frame (RTV) being a reference frame linked to the vehicle but whose axes are parallel to the axes of the terrestrial reference frame (RT), the calculation being therefore carried out by considering that the orientation angles of the vehicle reference frame (RV) relative to the terrestrial reference frame (RT) are null; - calculating the three vectors (AmeriV) of the pointing directions of the landmarks (Pi) in the vehicle reference frame (RV) from the measured elevation and bearing angles; - deducing the orientation angles of the vehicle reference frame (RV) relative to the terrestrial reference frame (RT) by solving the three matrix multiplications (one per landmark): Ameri = Rox*Roy*Roz * AmeriV = MTV * AmeriV, where Ameri is the pointing vector of a landmark (Pi) in the reduced vehicle reference frame (RTV) as deduced from the mapping, AmeriV is the pointing vector of the same landmark (Pi) in the vehicle reference frame (RV) calculated from the measured elevation and bearing angles, and Rox, Roy and Roz are the rotation matrices making it possible to pass from the terrestrial reference frame to the vehicle reference frame, the product Rox*Roy*Roz of these matrices constituting the matrix MTV for passing from the terrestrial reference frame (RT) to the vehicle reference frame (RV), said matrix being reduced to the rotation coefficients only and giving the desired orientation angles.

3. - The method for resetting an inertial navigation system according to claim 1, method in which the vehicle (1) is equipped with a rangefinder (14) carried by the turret (2), the method comprising the following steps: - calculating the coordinates (PiV) of the landmarks (Pi) in the vehicle reference frame (RV) from the measured elevation and bearing angles and the distances (Di) measured by the rangefinder; - determining the coordinates (PiT) of the landmarks (Pi) in the terrestrial reference frame (RT) from the mapping; - deducing the orientation angles of the vehicle reference frame (RV) relative to the terrestrial reference frame (RT) as well as the coordinates of the vehicle in the terrestrial reference frame (RT) by solving the three matrix multiplications (one per landmark): PiT = MTV * PiV, where PiT is the vector giving the coordinates of a landmark (Pi) in the terrestrial reference frame (RT), PiV is the vector giving the coordinates of the same landmark (Pi) in the vehicle reference frame (RV), and MTV is the matrix for passing from the terrestrial reference frame (RT) to the vehicle reference frame (RV), the coefficients of this matrix giving, on the one hand, the orientation angles of the vehicle reference frame (RV) relative to the terrestrial reference frame (RT) and, on the other hand, the position of the vehicle reference frame (RV) relative to the terrestrial reference frame (RT).

4. - The method for resetting an inertial navigation system according to claims 2 and 3, wherein the method includes choosing a resetting calculation mode that uses the satellite positioning system (13) or a resetting calculation mode that uses the rangefinder (14).

5. - The method for resetting an inertial navigation system according to one of claims 1 to 4, method in which, after an operator has chosen the landmarks (Pi), the predictable angular uncertainties for the tilt, slope and heading angles are determined by reading preestablished accuracy nomographs (15), and the operator is offered the possibility to modify his choice of landmarks (Pi) if the one or more uncertainties are higher than at least one threshold (L).

6. - A device for resetting an inertial navigation system (9) fitted to a land vehicle (1), the device implementing the method according to one of claims 1 to 5, the device being characterised in that it includes calculation means (11) and a map database (12) making it possible to know the coordinates, in a terrestrial reference frame (RT), of at least three landmarks (P1,P2,P3) present on the field, the device also including means (7, 8) for angular measurement of the orientation in elevation (β) and in bearing (α) of at least one turret (2) fitted to the vehicle (1) and making it possible to observe the field around the vehicle (1) and to determine at least one pointing direction (di), the angular measurement means being able to give the angles of orientation in elevation (β) and in bearing (α) of the at least one pointing direction (di) in a reference frame (RV) linked to the vehicle, referred to as the vehicle reference frame, the calculation means (11) making it possible to combine the measurements of the angles of observation in elevation and in bearing and the coordinates of the landmarks (Pi) known from the map database (12), to determine coefficients of a matrix MTV for passing from the terrestrial reference frame (RT) to the vehicle reference frame (RV), the calculation means (11) being connected to the inertial navigation system (9) in order to be able to reset it on the basis of the thus-calculated values of orientation and possibly of position of the vehicle reference frame (RV) relative to the terrestrial reference frame (RT).

7. - The device for resetting an inertial navigation system according to claim 6 and enabling the implementation of the method according to claim 2, wherein the device includes a satellite positioning system (GPS) (13), the latter being used by the calculation means (11) to determine the position of the vehicle reference frame (RV) relative to the terrestrial reference frame (RT).

8. - The device for resetting an inertial navigation system according to claim 6 and enabling the implementation of the method according to claim 3, wherein the device includes a rangefinder (14) carried by the turret (2), the rangefinder being used in combination with the means (7, 8) for angular measurement of the orientation in elevation and in bearing of the turret to enable the calculation of the coordinates (PiV) of the landmarks in the vehicle reference frame (RV).

9. - The device for resetting an inertial navigation system according to claims 7 and 8, wherein the device includes a human-machine interface giving an operator the choice between a resetting calculation mode that uses the satellite positioning system (13) and a resetting calculation mode that uses the rangefinder (14).

10. - The device for resetting an inertial navigation system according to one of claims 6 to 9, wherein the device incorporates at least one nomograph (15) making it possible to determine the predictable angular uncertainties for the tilt, slope and heading angles after the landmarks (Pi) have been chosen.

11. - The device for resetting an inertial navigation system according to claim 10, characterised in that it includes a human-machine interface making it possible to indicate to an operator if the one or more uncertainties are higher than at least one threshold (L).

12. - The device for resetting an inertial navigation system according to claim 11, characterised in that the human-machine interface imposes on the operator to modify his choices of landmarks (Pi) if the one or more uncertainties are higher than the at least one threshold (L).