Aircraft navigation assistance method and device

Abstract

The present invention relates to a method (200) for assisting in the navigation of an aircraft (100) comprising a GNSS receiver (110) and a barometric altimeter (120), the method comprising providing (210) a mesh that comprises a plurality of cells (M_i) in an operating zone of the aircraft, wherein the mesh comprises a current position (A(t)) of the aircraft at a current time (t); calibrating (220) the mesh, comprising assigning, to each cell, a calibration model (C_i(t)) on the basis of GNSS signals and a barometric measurement at the current time, as well as a model for the propagation of uncertainty between the current position and the cell at a geometric altitude (Z_geo,rec(A',t_est)) that would be determined at a position (A') of this cell (M_i) on the basis of a barometric measurement; and estimating (230), for a planned future position of the aircraft (Aplan(tplan)), an uncertainty (DPV(Aplan,tplan)) at a geometric altitude (Z_geo,rec(Aplan,tplan)) that would be determined on the basis of a barometric measurement (Z_baro(Aplan,tplan)) at the future position and the retained calibration of the mesh.

Description

[0001] TITLE: Method and device for assisting the navigation of an aircraft

[0002] The present invention relates to a method for assisting the navigation of an aircraft comprising a GNSS signal receiver and an altimeter.

[0003] The present invention also relates to a corresponding device for assisting in the navigation of an aircraft, as well as to a computer program comprising instructions which cause such a navigation aid device to execute the method for assisting in the navigation of an aircraft.

[0004] The present invention belongs to the field of navigation aids for aircraft involving a receiver of navigation signals from a constellation of GNSS (Global Navigation Satellite Systems) type satellites.

[0005] The performance of such navigation aids is assessed in particular using two indicators: precision and integrity.

[0006] Accuracy describes the position (or speed) error observed during nominal operation, i.e. in the absence of failure or signal loss.

[0007] Positioning integrity describes the degree of confidence the user can place in the position (or velocity) information provided by the navigation aid system. Integrity includes the system's ability to provide a loss of integrity alarm.

[0008] Integrity can be expressed by expressing, for each calculated position or speed, a confidence interval that the actual position (or speed) may be outside this interval due to an undetected failure of the navigation system. The confidence interval can be defined according to a risk level P raccepted by the user. Integrity can also be reflected by the emission of an alarm signal in the event of an excessive confidence interval compared to a given limit for carrying out the operation.

[0009] The navigation aid system makes it possible, in particular, to establish an altitude in a given geometric reference point and to associate it with a level of geometric vertical protection.

[0010] The geometric vertical protection level corresponds to half the length of a segment on the vertical axis (i.e. the axis perpendicular to the horizontal plane of a reference ellipsoid, for example the WGS-84 ellipsoid) of an ellipsoid whose center is the current measured position of the aircraft and which describes the region assured to contain the position of the aircraft with a given confidence level. Depending on the type of aircraft, a more or less low vertical protection level is imposed by the regulations that apply to the flight territory. For example, for unmanned aircraft such as drones, the required vertical protection level is typically of the order of a few tens of meters.

[0011] Satellite systems are supplemented by so-called augmentation systems to ensure vertical integrity to a few dozen meters because the redundancy of GPS signals alone does not necessarily guarantee this level of integrity on the vertical position. Augmentation systems calculate and transmit correction messages for satellite data. Correction messages make it possible to increase the accuracy of measurements and / or to guarantee the integrity of the navigation system at a level of autonomous failure risk P r particularly low, typically of the order of 10' 7 per flight hour.

[0012] However, in the event of loss of integrity of the navigation solution, the required level of vertical protection may no longer be guaranteed for all or part of the future trajectory.

[0013] Document EP 1 462 761 describes a method for assisting with augmented navigation in vertical integrity in which altitude measurements are obtained, in the event of loss of a satellite signal among the n normally accessible satellite signals, by hybridization of GPS measurements and barometric measurements. The barometric measurements are regularly recalibrated using the GPS measurements and possibly temperature compensation.

[0014] However, since this process relies only on the redundancy of GPS signals and not on augmentation systems or on complementary measurements provided by other GNSS constellations, it does not guarantee vertical integrity with a high level of risk of failure, for example 10' 7per hour and with a vertical protection of a few tens of meters. In addition, this process does not make it possible to limit the drift in accuracy of the altitude estimated from barometric measurements alone in the absence of a temperature sensor, so that navigation based temporarily only on barometric measurements is only possible in an extremely limited way.

[0015] An aim of the invention is then to propose a navigation aid method making it possible to predict the evolution of the level of vertical protection associated with barometric measurements when an integrated altitude measurement from the signals of the satellite constellation is no longer available, in particular in order to allow optimal decision-making on the continuation of a mission to be carried out by an aircraft by means of barometric altitude measurements.

[0016] To this end, the subject of the invention is a method for assisting the navigation of an aircraft comprising at least one GNSS signal receiver and a barometric altimeter, the method comprising the following steps:

[0017] - providing a mesh of an aircraft evolution zone comprising a current position of the aircraft at a current date for calibration, the mesh comprising a plurality of cells;

[0018] - calibration of the mesh, comprising the allocation to each mesh of a calibration model on the basis of: i) GNSS signals received on the current date making it possible to establish a vertical protection level in relation to a given risk and, where applicable, at least one date prior to the current date, ii) at least one barometric measurement carried out on the current date and, where applicable, on said date prior to the current date, and iii) a model of propagation of uncertainties between the current position and the mesh on a geometric altitude which would be determined at any position of this mesh from a barometric measurement; and

[0019] - for at least one planned future position of the aircraft included in the aircraft's evolution zone, estimation of an uncertainty on a geometric altitude which would be determined from a barometric measurement at the future position and the calibration of the mesh retained at the current date.

[0020] At any point of interest in the evolution zone, the calibration model of the mesh in which this point of interest is located makes it possible to know, from the uncertainty propagation model, the precision on the determination of the geometric altitude from the barometric altitude which would be determined from barometric measurements at the point of interest and the current calibration of the altimeter.

[0021] Thanks to the step of assigning a calibration model, this precision is also the best possible precision in the presence of the set of measurement data collected on the current date, but also prior to the current date if applicable.

[0022] Ultimately, it is therefore possible to predict, for a plurality of points of a portion of the aircraft's trajectory to come, the minimum vertical protection limit that can be associated with each of these points in the event of loss of integrity of the GNSS signals leading to a purely barometric altitude measurement, the vertical protection limit being estimated on the basis of the GNSS and barometric measurements available on the current date. Thanks to this prediction, the aircraft can be kept in flight as long as the vertical protection limit thus predicted allows it, navigation being ensured from the barometric measurements as long as the integrity of the satellite signals is not restored.

[0023] It is therefore understood that the method according to the invention makes it possible to extend the flight time of the aircraft in many cases of loss of integrity of the satellite signals, while respecting local regulations, the flight being carried out on the basis of barometric measurements alone and an optimized calibration of these measurements as long as the integrity of the signals is not restored.

[0024] According to other advantageous aspects of the invention, the method comprises one or more of the following characteristics, taken individually or in all technically possible combinations:

[0025] - the method comprises a decision step on the continuation of a mission to be carried out by the aircraft from the current position, the decision step comprising the evaluation of an integrity performance with the GNSS signals on the current date, and:

[0026] I) if the assessment is positive, the continuation of the mission, and if not,

[0027] (ii) the estimation of a remaining flight time under the assumption that the uncertainty on the barometric altitude of the aircraft for all future positions planned from the current position during this remaining flight time remains below a predetermined threshold from the last calibration model available for each mesh, and

[0028] (iii) depending on the remaining flight time, the provision of a landing instruction or an instruction to continue the mission for a maximum duration calculated from the remaining flight time;

[0029] - the mesh calibration step comprises: a) determining a first geometric altitude of the current position from the GNSS signals received on the current date and a first barometric altitude of the current position from the barometric altitude measurement taken on the current date, b) for each mesh, constructing a candidate calibration model on the current date configured to estimate, on an estimation date identical to or later than the current date, a geometric altitude of the aircraft at any position in the mesh from a barometric measurement taken at this position on the estimation date, and c) for each mesh: a) if a previous calibration model, corresponding to a candidate calibration model on a date prior to the current date, has been assigned to the mesh on the previous date,and if an uncertainty in a geometric altitude estimated at a reference point of the mesh with the prior calibration model is less than that of the candidate calibration model, the assignment of the prior calibration model to the respective mesh, and otherwise, P) the assignment of the candidate calibration model to this mesh;,

[0030] - the evolution zone is subdivided into meshes according to a vertical direction in which the altitude is measured;

[0031] - the subdivision of the evolution zone into meshes is also carried out according to two horizontal directions perpendicular to the vertical direction;

[0032] - the calibration and estimation steps are repeated periodically;

[0033] - the mesh supply step is carried out before the first iteration of the calibration step, the evolution zone being chosen so that a predetermined trajectory to be followed by the aircraft is entirely included in the evolution zone;

[0034] - the mesh supply step is carried out before each iteration of the calibration step, the evolution zone comprising for each supply step at least a portion of a predetermined trajectory to be followed by the aircraft from the current position;

[0035] - the calibration step comprises, after step c), a grouping step d), during which, for each of the outermost cells of the mesh in at least one predetermined direction, if the same calibration model has been assigned at the current date to said cell and to a neighboring cell in the predetermined direction, only the calibration model of the neighboring cell is retained;

[0036] - a number of mesh cells is fixed for all the iterations of the mesh supply step, and for an iteration of the supply step subsequent to a grouping step d), the volume of the evolution zone for said iteration of the supply step is equal to the volume of all the cells retained at the end of said step d).

[0037] - in the calibration step, the GNSS signals received on the current date making it possible to establish a vertical protection level in relation to a given risk and, where applicable, on at least one date prior to the current date, are signals transmitted to the receiver by at least one auxiliary GNSS signal reception device independent of the aircraft, these signals transmitted to the receiver having been received by the at least one auxiliary reception device prior to the current date, the method then being implemented in a predictive mode.

[0038] The invention also relates to a navigation aid device for an aircraft comprising:

[0039] - at least one GNSS signal receiver,

[0040] - at least one barometric altimeter, - a meshing module, configured to implement the step of providing a mesh of an area of ​​evolution of the aircraft of the aircraft navigation aid method as described previously,

[0041] - a calibration module, configured to implement the mesh calibration step of the aircraft navigation aid method as described previously

[0042] - a determination module, configured to estimate, for at least one planned future position of the aircraft included in the area of ​​evolution of the aircraft, an uncertainty on a geometric altitude which would be determined from a barometric measurement at the future position and a calibration of the mesh retained at a current date by the calibration module.

[0043] According to other advantageous aspects of the invention, the device comprises a decision module, configured to implement the decision step of the method for assisting the navigation of an aircraft as described previously.

[0044] The invention also relates to a computer program comprising instructions which cause the device as defined above to execute the navigation aid method as defined above.

[0045] The invention will appear more clearly on reading the description which follows, given solely by way of non-limiting example, and made with reference to the drawings in which:

[0046] [Fig. 1] Figure 1 is a schematic representation of an aircraft comprising a navigation aid device according to the invention;

[0047] [Fig. 2] Figure 2 is a flowchart representation of one embodiment of a method implemented by the device of Figure 1;

[0048] [Fig. 3] Figure 3 is a representation of the process of Figure 1, showing the input and output data of the various stages;

[0049] [Fig. 4] Figure 4 is a two-dimensional schematic representation of the evolution of a fixed mesh provided at the end of a mesh provision step of the method of Figure 2 or Figure 3, for an example of an aircraft trajectory between two times t1 and t2;

[0050] [Fig. 5] Figure 5 is a two-dimensional schematic representation of the evolution of an adaptive mesh provided at the end of a mesh provision step of the method of Figure 2 or Figure 3, for an example of an aircraft trajectory between two times t1 and t2; [Fig. 6] Figure 6 is a two-dimensional representation of the optional steps of grouping and refining a vertical mesh at a current date t of a calibration step of the method of Figure 2 or Figure 3.

[0051] The invention relates to a method for assisting the navigation of an aircraft 100.

[0052] The aircraft 100 is shown schematically in Figure 1.

[0053] The aircraft 100 comprises a receiver 110 of GNSS (Geolocation and Navigation by a Satellite System - in English, “Global Navigation Satellite Systems”) signals, a barometric altimeter 120 and a navigation aid device 130 according to the invention.

[0054] Alternatively, the aircraft 100 comprises several GNSS signal receivers 110 forming a navigation solution which combines the measurements of these receivers 110.

[0055] The receiver or, where appropriate, the receivers 110 are configured to receive satellite signals, or equivalently GNSS signals, designated by S(GNSS) in the figures, from a plurality of navigation satellites of one or more GNSS systems.

[0056] The processed signals can be emitted by one or more constellations among GPS, Galileo, GLONASS or even Beidou.

[0057] The satellite signals include navigation messages, the receiver 110 being configured to determine at least three geographic coordinates of the aircraft 100 from the navigation messages.

[0058] The receiver 110 may also be configured to receive augmented navigation messages from one or more satellite geolocation accuracy augmentation systems.

[0059] The precision augmentation system can be ground-based. In this case, it is a LAAS or equivalently GBAS (Local Area Augmentation System or Ground Based Augmentation System).

[0060] Alternatively or in addition, the precision augmentation system may include geostationary satellites. This is then a SBAS (Spatial Based Augmentation System) type system, such as EGNOS, WAAS, MSAS, etc.

[0061] In the following, the expression "GNSS signals" refers to augmented or unaugmented signals.

[0062] The augmented navigation messages include, for example, information for correcting geolocation measurements made by the receiver 110 and information characterizing the integrity of the GNSS system(s). The information characterizing the integrity of the GNSS system may include instructions for rejecting the use of one or more satellites identified as faulty, and / or information on the level of vertical and / or horizontal protection.

[0063] Alternatively or in addition, the receiver 110, or where appropriate the navigation solution, may rely on the redundancy of the satellite signals and / or the measurements provided by the different receivers 110, and / or the integrity messages transmitted by the satellite signals, and / or the combination of the satellite signals on several frequencies to eliminate the ionospheric part of the signal propagation errors, so as to ensure vertical autonomous integrity (in English, “Vertical Advanced Receiver Autonomous Integrity Monitoring”).

[0064] The barometric altimeter 120 is configured to measure a pressure difference between the current position and a reference position.

[0065] The navigation aid device 130 is configured to receive data from the receiver 110 and the barometric altimeter 120.

[0066] The device 130 comprises means for implementing the navigation assistance method 200 described below.

[0067] The device 130 notably comprises a meshing module 130A, a calibration module 130B, a determination module 130C and a decision module 130D.

[0068] Each of the modules 130A, 130B, 130C, 130D is implemented at least partially by a programmable logic circuit of the FPGA type and / or by software.

[0069] In the latter case, the device 130 further comprises at least one memory storing such software and at least one processor enabling it to be executed.

[0070] The navigation aid method 200 is described with reference to FIGS. 2 to 6.

[0071] This method is implemented when, for example, the aircraft 100 begins a scheduled flight mission, or during the scheduled mission of the aircraft 100. The mission of the aircraft 100 can only begin or continue if a degree of vertical protection lower than the regulations imposed in the geographical area in which the aircraft's mission takes place is respected.

[0072] At a current date t of implementation of the method 200, it is considered that the aircraft is at a current position A(t). This position A(t) may be on the ground if the mission has not yet started or if the aircraft 100 has landed during the mission. During the mission, the aircraft is generally in flight at position A(t).

[0073] The navigation assistance method 200 comprises a step 210 of providing a mesh of an area of ​​movement of the aircraft comprising the current position A(t) of the aircraft 100. The step 210 of providing the mesh is implemented by the mesh module 130A.

[0074] The step 210 of providing the mesh comprises providing a set of N meshes MJ, N being an integer greater than 1, preferably strictly greater than 1.

[0075] The set of N meshes paves a predetermined geographic volume V, in which the current position A(t) of the aircraft 100 is included.

[0076] The predetermined geographic volume V is chosen so that a planned geographic area of ​​evolution of the aircraft 100 from the current position A(t) is included in the geographic volume V.

[0077] In particular, at least a portion, advantageously the entirety, of a planned trajectory TR of the aircraft from the current position A(t) may be included in the geographic volume V.

[0078] Advantageously, each MJ mesh is parallelepipedal.

[0079] Advantageously, all the MJ meshes have the same volume VJ = V / N.

[0080] If the mesh consists of equidistributed parallelepiped MJ cells, each MJ cell can be characterized by the geographic coordinates of a reference point of the cell, for example the altitude ZJ, the latitude LatJ, and the longitude LonJ of this reference point.

[0081] The reference point is, for example, the center of the rectangular parallelepiped forming the MJ mesh.

[0082] The mesh is at least vertical, as shown in Figures 4 and 5. In other words, the geographic volume V is subdivided into at least two meshes MJ of different altitudes ZJ according to a vertical direction Z with respect to which the altitudes are defined.

[0083] Advantageously, the meshing is also carried out along at least one direction among two horizontal directions X, Y orthogonal to the vertical direction, as represented in two dimensions in Figure 4.

[0084] Advantageously, the MJ meshes are equidistributed along at least one of the horizontal directions X, Y, preferably along both horizontal directions X and Y, as shown in Figure 4.

[0085] The mesh supply step 210 may comprise: a) the supply by an operator to the mesh module 130A of the device 130 of the predetermined geographical volume V and / or the planned trajectory TR and the number N of meshes MJ, and b) the subdivision of the geographical volume V into N meshes by the mesh module 130A, the geographical volume V being either supplied to the mesh module 130A or determined by the mesh module 130A from the planned trajectory TR. This latter case is shown in FIG. 4.

[0086] The mesh calibration step 220 follows the mesh supply step 210.

[0087] The calibration step 220 comprises the assignment to each MJ mesh of a calibration model C_i(t) at the current date t on which the method 100 is implemented.

[0088] The calibration step 220 is performed by means of a calibration module 130B of the device 130.

[0089] The calibration module 130B receives from the receiver 110 at least three geometric coordinates characterizing the position of the aircraft 100 at the current date t, determined from the satellite signals received by the receiver 110 at the current date t.

[0090] The three geometric coordinates are for example a geometric altitude Z_geo(t) according to the vertical direction Z, a latitude Lat_geo(t) and a longitude Lon_geo(t).

[0091] The calibration module 130B also receives at least one barometric altitude Z_baro(t) characterizing the position of the aircraft 100 on the current date, determined from at least one barometric measurement carried out by the barometric altimeter 120 on the current date t.

[0092] The calibration step 220 includes the construction of a candidate calibration model C_can(t) at the current date t.

[0093] The candidate calibration model C_can(t) is configured to estimate, at the estimation date t_est, a so-called “reconstituted” geometric altitude Z_geo, rec(A', t_est) of the aircraft 100 at any position A' which would be determined from a barometric altitude measurement Z_baro(A', t_est) carried out at this position A' at the estimation date t_est, as well as its uncertainty.

[0094] For example, at the current position A(t), if the GNSS signals are integral at the current date t, the following candidate calibration parameters are calculated from the GNSS signals:

[0095] - the candidate calibration itself, which is the difference 5_alt between the geometric altitude Z_geo(t) determined by the GNSS receivers and the barometric altitude Z_baro(t) measured at the current position A(t): 5_alt = Z_geo(t) - Z_baro(t);

[0096] - a reference standard deviation o_can(A,t) of the error of the candidate calibration at the current date t, which is equal to the vertical positioning standard deviation Ov,GNss(A(t)) of the geolocation by means of integrated GNSS signals, possibly including augmented navigation messages, received by the receiver 110 at the current position A(t): o_can = o v ,GNss(A(t)); and - a reference bias B_can of the candidate calibration at the current date t, which is equal to the estimation bias of the standard deviation of the vertical positioning error Ov,GNss(A(t)): B_can= B v ,GNss(A(t)) ; and

[0097] - memorization of position A and calibration time t.

[0098] The calibration module 130B implements an uncertainty propagation model between the current position A(t) and any mesh MJ, i ranging from 1 to N, the uncertainty propagation model being chosen to evaluate the uncertainty on the reconstructed geometric altitude Z_geo,rec (A', t_est).

[0099] For each MJ mesh, in order to select the best calibration between the previous calibration and the candidate calibration, we can then calculate the uncertainty, for example expressed in terms of vertical protection limit, on the precision of the reconstructed geometric altitude Z_geo,rec (A', t_est)i, from the uncertainty propagation model.

[0100] To select the best calibration, the estimation date t_est can be chosen equal to the current date t.

[0101] For example, the calibration uncertainty propagation model at any position A' and at the estimation date t_est may take into account one or more of the following terms, advantageously all of the following terms: a) a horizontal precision degradation OH, characterizing the degradation of the standard deviation due to the horizontal separation between the position A' and the current position chosen for the calibration A(t), which may be evaluated according to the following equation: where dn is the horizontal distance between position A' and the current position for calibration A(t) and kn a predetermined horizontal degradation rate. For example, kn can be equal to 2.7.10 -4 m / m; b) a degradation of temporal precision o t , characterizing the degradation of the standard deviation due to the temporal separation between the estimation date t_est and the current date for the calibration t, which can be evaluated according to the following equation: at,t est= k t . |t est - t| (2) where k t is a predetermined temporal degradation rate. For example, k t can be equal to 4.2.10' 3 m / s; c) a vertical precision degradation Ov, characterizing the degradation of the standard deviation due to the vertical separation between the position A' and the current position A chosen for the calibration, which can be evaluated according to the following equation: where kv is a predetermined vertical decay rate. For example, kv can be equal to 1,066.10 -1 m / m.

[0102] In the case where the three terms OH, o t ,t_ es tet Ov are calculated, the standard deviation o ca n associated with the candidate calibration model C _can(t) on the reconstructed geometric altitude Z_geo,rec (A', t_est) from a barometric measurement which would be carried out at point A' on date t_est is obtained with the following equation:

[0103] Advantageously, in the case where the signals received by the receiver 110 include augmented navigation messages:

[0104] - the total uncertainty associated with each candidate calibration model C_ can(t) includes uncertainty terms calculated from the augmented navigation messages, and / or

[0105] - the reference standard deviation o_can,A(t) of the error of the candidate calibration at the current date t and / or the reference bias B_can of the candidate calibration are calculated from the augmented navigation messages.

[0106] The vertical protection limit DPVj, ca n at the risk Pr of the barometric altitude at the reference point of the MJ mesh and at time t_est associated with the candidate calibration model C _can(t) of the MJ mesh will be provided by the following equation: PVi'Can Lat i , Lon i , Z i , t est ') B c + K ïn / . <j (jan ÇLcit^ tLon^ t Z^ t t es ^^ 5) where Kmd is the normalized threshold of missed detection at risk level P r , namely:

[0107] In the case where the mesh is only vertical, the calibration module 130B advantageously assigns to all the MJ meshes the latitudes and longitudes of the current position, so that Lat_i=Lat_geo(t) and LonJ =Lon_geo(t) for all the MJ meshes.

[0108] For each MJ grid, if a calibration model CJ(t') has been assigned at a date t' prior to the current date t, the calibration module 130B compares the uncertainty on the reconstructed geometric altitude Z_geo,rec(i, t_est) which would be determined from a barometric measurement at the reference point of the MJ grid at the date t_est associated with the candidate calibration model C_can(t) and that which is associated with the previous calibration model CJ(t').

[0109] For example, the calibration module 130B compares, for each MJ mesh, the vertical protection limit DPVj, can at risk Pr associated with the candidate calibration model C_can(t) and that which is associated with the previous calibration model C_i(t').

[0110] If the uncertainty in the altitude Z_geo,rec(i, t_est) associated with the previous calibration model CJ(t') is strictly lower than that of the candidate calibration model C_ can(t), the previous calibration model CJ(t') is assigned to the MJ grid at the current date t and becomes the current calibration model C_i(t).

[0111] If no previous calibration model has been assigned or if the uncertainty in the reconstructed geometric altitude Z_geo,rec(i, t_est) associated with the previous calibration model CJ(t') is greater than or equal to that of the candidate calibration model C_ can(t), the candidate calibration model C_ can(t) is assigned to the MJ grid at the current date t and becomes the current calibration model CJ(t).

[0112] This step is represented schematically in Figure 4 for which the geographical volume V containing the entire planned trajectory TR has been subdivided into eight parallelepiped meshes of the same volume.

[0113] At a first current position A(h) corresponding to a first date ti, a first calibration step 220 was carried out. First calibration models CJ(h) were assigned to each of the meshes Mi to Ms.

[0114] At a second date t2 after the date ti, the aircraft 100 is at a second current position A(t2) at which a second calibration step 220 is implemented while maintaining the same mesh.

[0115] In this example, a previous calibration model is therefore available for each of the meshes, namely the calibration model assigned to date ti.

[0116] After comparing the candidate calibration model C_can(t2) with the previous calibration model CJ(ti) for each MJ mesh:

[0117] - a new, more precise calibration model CJ(t2) is assigned to each of the meshes M4, Ms, M? and Ms thanks to the new satellite signals received on the second date t2;

[0118] - on the other hand, based on the satellite signals received at the first date h and at the second date t2, it appears that the previous calibration model CJ(ti) is more accurate for the meshes Mi, M2, M3 and Ms and must be kept as the calibration model at the second date t2.

[0119] Optionally, the calibration step 220 includes a grouping step 220A, which is shown in FIG. 6.

[0120] For the grouping step 220A, the calibration module 130B identifies the meshes M_b said to be external to the volume V. The meshes M_b are the meshes of which one face forms at least part of a face of the volume V in at least one determined direction.

[0121] - for example, the vertical Z direction if the mesh is vertical.

[0122] The grouping step 220A comprises, for each mesh M_b external to the volume V, the comparison of the current calibration model C_b(t) and C_vois,b(t) retained for the mesh M_b and for a neighboring mesh M_vois,b in the determined direction. If these calibration models are identical:

[0123] - the calibration module 130B links the mesh M_b to the neighboring mesh, which we will call the innermost mesh in volume V. This operation is symbolized by the angular arrow between the meshes M4 and M5 in figure 6.

[0124] - and only the calibration model C_vois,b(t) of the innermost mesh M_vois,b is stored in memory by the calibration module 130B.

[0125] If the mesh is both vertical and horizontal, the grouping step 220A may be performed in at least two predetermined directions.

[0126] The grouping step 220A makes it possible to reduce the memory cost of the process.

[0127] Alternatively, if the number of calibration models stored in memory at the end of the grouping step 220A is less than a predetermined number of meshes N provided in the providing step 210, the grouping step 220A can be followed by a mesh refining step 220B, as shown in FIG. 6.

[0128] The mesh refinement step 220B is implemented by the calibration module 130B. It comprises the subdivision into N sub-meshes of the volume covered by all the meshes for which a calibration model has been stored in memory at the end of the grouping step, followed by a calibration of the N sub-meshes, on the principle of the calibration described previously.

[0129] In the example of Figure 6, the geographic volume V containing the planned trajectory TR has been subdivided vertically into five parallelepiped meshes Mi to Ms of the same volume at the end of a mesh supply step 210.

[0130] At the current position A(t) corresponding to date t, a first calibration 220 was made according to the principle described previously. Calibration models C_i(t) were assigned to the meshes Mi to Ms.

[0131] During the grouping step 220A, it is observed that the mesh Ms which is on the edge of the volume V along the vertical direction Z has a calibration model Cs(t) identical to that of the neighboring mesh M4. The mesh Ms is therefore grouped with the mesh M4.

[0132] In the refining step 220B, the volume corresponding to the four meshes M1 to M4 for which a calibration model C_i(t) has been retained is subdivided into five new meshes M'1 to M's, with volumes smaller than that of the meshes M1 to Ms. A new candidate calibration model is then calculated for each of these meshes M'1 to Ms. Finally, a final calibration model C'i(t) to C's(t) is assigned to the meshes M'1 to M's according to the procedure described previously, the mesh Ms remaining ultimately linked to the mesh M's.

[0133] This arrangement makes it possible to obtain a more accurate calibration model across the entire geographic volume V at constant memory cost. Once the mesh has been calibrated, the determination step 230 is implemented by the determination module 130C for at least one planned future position of the aircraft 100 A pian at a date t pian later than the current date t included in the area of ​​operation of the aircraft 100.

[0134] The determination step 230 comprises the estimation of an uncertainty on a reconstructed geometric altitude Z_geo,rec (A pian , t pian ) which would be determined from a barometric measurement Z_baro (A pian , t pian ) to the future position Api an at date t pian and the mesh calibration retained at the current date t.

[0135] In particular, the vertical protection limit DPVk at risk Pr associated with the calibration model C_k(t) retained at the current date t for a mesh M_k in which the future position A is located pian , or whose reference point is closest to the future position, can be associated with the reconstructed geometric altitude Z_geo,rec (A pian , t pian ).

[0136] The difference between a reconstructed geometric altitude Z_geo,rec (A pian ) of the Apian position (tpi an >t) and its barometric altitude Z_baro (Apian , t pian ) can be estimated according to the following equation:

[0137] Alternatively, a vertical protection limit DPV(A pian , t pian ) can be calculated by interpolation of the vertical protection limits DPVj at the risk Pr associated with the calibration model C_i(t) retained at the current date t for a plurality of neighboring meshes M_p, including the mesh M_k in which the future position A is located pian (t pian ) or the closest to this future position, can be associated with the reconstructed geometric altitude Z_geo,rec(A pian , t pian ), according to the following equation, the set of neighboring meshes M_p being designated by MV: the difference between the geometric altitude Z_geo(A pian ) from position A pian and its barometric altitude can be estimated according to the following equation:

[0138] For a vertical mesh, the neighboring meshes M_p can include the two consecutive meshes whose reference point altitudes frame the current estimated altitude. For a vertical and horizontal mesh, the neighboring meshes M_p can include the eight meshes whose reference point positions are closest and frame position A pian (t p ian>t).

[0139] If these eight meshes are not available in the provided mesh, the number of meshes can be reduced to four, two or one depending on the location of the Apian position (t p ian> t) relative to the geographic volume V covered by the mesh.

[0140] Advantageously, the determination step 230 is carried out for a plurality of future positions distributed over a fraction or over the entire planned trajectory TR.

[0141] Advantageously, the uncertainty on a reconstructed geometric altitude Z_geo, rec(A p ian, tpian) is the largest uncertainty among all those calculated for passage dates included in an interval [t pian -At pian , t pian -At pian ], At pian being an uncertainty on the date of passage ti to the future position Api an .

[0142] Advantageously, the method 200 comprises a decision step 240 at the end of the determination step 230, carried out by means of the decision module 130D.

[0143] The decision step 240 comprises the evaluation of an integrity performance IN of the vertical positioning from the GNSS signals S(GNSS) received on the current date, that is to say of the vertical positioning based on the GNSS navigation solution alone, without taking into account the barometric measurements.

[0144] If the evaluation is positive, that is to say that the vertical protection level of the solution does not exceed a first predetermined threshold for the mission, the mission of the aircraft 100 is continued, this case being symbolized by the notation PM in FIG. 3, and the calibration 220, determination 230 and decision 240 steps will advantageously be repeated later.

[0145] Otherwise, the decision module 130D estimates a remaining flight time t_vol under the assumption that the uncertainty on the reconstructed geometric altitude Z_geo, rec(Api an , t pian ) of aircraft 100 for all future planned positions Api an from the current position A(t) during this remaining flight time t_vol remains below a second predetermined threshold.

[0146] Preferably, the first threshold and the second threshold are equal.

[0147] For this, the decision module 130D estimates the uncertainty on the reconstructed geometric altitude Z_geo, rec(Api an , t pian ) of a plurality of planned future positions Api an included in the planned trajectory TR from the calibration models last assigned to the MJ meshes.

[0148] Preferably, if no candidate calibration model has been previously calculated for one or more given MJ meshes, an initial uncertainty model, such as a model provided by the manufacturer and based on an uncertainty in the barometric altitude determined during the manufacture of the altimeter 120, is assigned to each of these meshes. The remaining flight time t_vol is then the time remaining before the planned date of passage at the last of the plurality of planned future positions for which the uncertainty in the reconstructed geometric altitude is less than a predetermined threshold.

[0149] For example, the remaining flight time t_vol is the time remaining before the planned date of passage to the last of the plurality of planned future positions for which the vertical protection limit DPV associated with the reconstructed geometric altitude is less than a predetermined threshold.

[0150] If the remaining flight time is less than or equal to a duration T_att necessary for the landing of the aircraft 200, the decision module 130D advantageously provides a landing instruction, in response to which the aircraft 100 is configured to begin the landing maneuvers.

[0151] If the flight time is strictly greater than the duration T_att, the decision module 130D provides an instruction to continue the mission of the aircraft 100 for a maximum duration equal to the difference between the remaining flight time t_vol and the duration T_att if the integrity of the GNSS signals is not restored.

[0152] The calibration 220, determination 230 and decision 240 steps are advantageously repeated later.

[0153] Advantageously, all of the calibration 220, determination 230 and decision 240 steps are repeated periodically.

[0154] In a particular embodiment, the step 210 of providing the mesh is also repeated.

[0155] For example, the mesh supply step 210 is repeated before each calibration step 220. This arrangement makes it possible to dynamically adapt the mesh as the aircraft 100 evolves along its planned trajectory TR.

[0156] In particular, if the calibration module 130B is configured to impose that the geographic volume V covers the entire trajectory to come from the current position A(t), this volume V decreases over time. Consequently, with a constant number of meshes N, the mesh provided can be refined over time. Alternatively, with a constant volume VJ of the meshes MJ, the number of meshes can be reduced as the aircraft 100 advances.

[0157] Alternatively, if only a portion of trajectory TR is covered by the geographic volume V, the mesh can be adapted according to the evolution of the aircraft 100.

[0158] This step is represented schematically in Figure 4, for which the geographical volume V contains at the first and second dates ti and t2 only a portion of the planned trajectory TR from the respective current position. The volume V(ti) was subdivided in a first step 210 of providing the mesh into three parallelepiped meshes of the same predetermined volume Vmaiiie, so as to cover all the planned altitudes of the aircraft on the portion of the trajectory concerned.

[0159] At the first current calibration position A(ti), corresponding to the first date ti, a first calibration was made. Calibration models C_i(ti) were therefore assigned to the meshes Mi to M3 during a first calibration step 220.

[0160] At the second date t2 after the first date ti, the aircraft 100 is at a second current calibration position A(t2).

[0161] The volume V(t2) is subdivided into four parallelepiped meshes in a second step 210 of providing the prior mesh of the same predetermined volume Vmaiiie, so as to cover all the planned altitudes of the aircraft on the new portion of the trajectory concerned.

[0162] A second calibration step 220 is then implemented

[0163] In this example, a previous calibration model is available for the meshes Mi, M2 and M3, namely the calibration model C_i(ti) assigned to the date ti.

[0164] No previous calibration model is available for the M4 mesh, which is therefore assigned the candidate calibration model C_can(t2) calculated for this mesh.

[0165] After comparing the candidate model C_can(t2) with the previous calibration models C_i(ti) available, a new calibration model C_3(t2) = C_can(t2) is assigned to the M3 mesh using the new satellite signals received on date t2.

[0166] On the other hand, based on the satellite signals received on date ti and date t2, it appears that the previous calibration model is more accurate for the meshes Mi, M2, and must be retained.

[0167] Advantageously, if the mesh supply step 210 is repeated before each calibration step 220, the calibration step 220 also comprises a grouping step 220A and a refining step 220B.

[0168] In a particular embodiment, the GNSS signals received are only signals received on one or more dates t' prior to the current calibration date t 220, calibration carried out before the start of the mission of the aircraft 100 and on the current date t.

[0169] These GNSS signals may then have been received directly and / or indirectly by the receiver 110. In particular, the GNSS signals may have been transmitted to the receiver 110 after reception by one or more other GNSS signal reception devices independent of the aircraft 100. In particular, it may only be almanac data associated with the satellite solution(s) implemented.

[0170] The uncertainty propagation model is then constructed solely from these signals prior to the current date t and it makes it possible to predict the performance of the GNSS signals which will potentially be received at the planned position, or even on a portion or all of the planned trajectory TR.

[0171] This embodiment is therefore purely predictive.

[0172] Alternatively, the GNSS signals received at the current date t of the calibration 220 include or consist of the GNSS signals actually and directly received by the receiver 110 at the current date t if such signals are available. In this case, the method allows a real-time update of the calibration model.

[0173] In the case of the purely predictive embodiment, the decision step 240 on the continuation of the mission to be carried out may comprise, if the evaluation is negative, that is to say that the vertical protection level of the solution exceeds the first predetermined threshold during the planned trajectory portion TR envisaged, a sub-step of generation of one or more alternative planned trajectories.

[0174] Each alternative planned trajectory has the same start point and end point as the planned trajectory and is generated under the constraint that the vertical protection level of the solution does not exceed the first predetermined threshold.

[0175] If no alternative planned trajectory can be generated, the decision step 240 may comprise a decision not to engage the mission or to provide one or more contingency landing zones, i.e. zones configured for an emergency landing, along the planned trajectory TR for each of the points of the planned trajectory TR for which the vertical protection level of the solution might not be sufficient in the event of loss of satellite signals.

[0176] If at least one alternative planned trajectory can be generated, the decision step 240 may include selecting an alternative planned trajectory and starting the mission.

[0177] It should be noted that the sub-step of generating one or more alternative planned trajectories can also be combined with the real-time implementation mode.

Claims

CLAIMS 1. Method (200) for assisting the navigation of an aircraft (100) comprising at least one GNSS signal receiver (110) and a barometric altimeter (120), the method comprising the following steps: - providing (210) a mesh of an area of ​​evolution of the aircraft comprising a current position (A(t)) of the aircraft (100) at a current date (t) for calibration, the mesh comprising a plurality of meshes (MJ); - calibration (220) of the mesh, comprising the allocation to each mesh (MJ) of a calibration model (C_i(t)) on the basis of: i) GNSS signals received on the current date (t) making it possible to establish a vertical protection level with respect to a given risk and, where applicable, at least one date (t') prior to the current date (t), ii) at least one barometric measurement carried out on the current date (t) and, where applicable, on said date prior to the current date, and iii) a model for the propagation of uncertainties between the current position (A(t)) and the mesh (MJ) on a geometric altitude (Z_geo, rec (A',t_est)) which would be determined at any position (A') of this mesh (MJ) from a barometric measurement; and - for at least one planned future aircraft position (A p ian(t p ian)) included in the area of ​​evolution of the aircraft (100), estimation (230) of an uncertainty (DPV(A p ian,t pian)) on a geometric altitude (Z_geo,rec(A pian ,t p ian)) which would be determined from a barometric measurement (Z_baro (A pian ,t p ian)) to the future position (A) pian (t p ian)) and the mesh calibration retained at the current date (t).

2. Method (200) according to the preceding claim, further comprising a decision step (240) on the continuation of a mission to be carried out by the aircraft (100) from the current position (A(t)), the decision step (240) comprising: - the evaluation of an integrity performance (IN) with the GNSS signals at the current date, and: I) if the assessment is positive, the continuation of the mission, and if not, II) the estimation of a remaining flight time (t_vol) under the assumption that the uncertainty on the barometric altitude of the aircraft (100) for all future positions planned from the current position during this remaining flight time (t_vol) remains lower than a predetermined threshold from the last calibration model available for each mesh, and III) depending on the remaining flight time (t_vol), the provision of a landing instruction or an instruction to continue the mission for a maximum duration calculated from the remaining flight time.

3. Method (200) according to any one of the preceding claims in which the step of calibrating (220) the mesh comprises: a) determining a first geometric altitude (Z_geo(t)) of the current position (A(t)) from the GNSS signals received at the current date (t) and a first barometric altitude (Z_baro(t)) of the current position (A(t)) from the barometric altitude measurement carried out at the current date (t), b) for each mesh (MJ), constructing a candidate calibration model (C_i_can(t)) at the current date (t) configured to estimate, at an estimation date (t_est) identical to or later than the current date (t), a geometric altitude (Z_geo, rec (A', t_est)) of the aircraft at any position (A') in the mesh MJ from a barometric measurement carried out at this position (A') at the estimation date (t_est), and c) for each mesh (MJ): a) if a previous calibration model,corresponding to a candidate calibration model at a date prior to the current date, has been assigned to the mesh at the earlier date, and if an uncertainty (DPV (A', t_est)) on a geometric altitude (Z_geo, rec) estimated at a reference point of the mesh with the earlier calibration model is lower than that of the candidate calibration model (C_i_can(t)), the assignment of the earlier calibration model to the respective mesh, and otherwise, P) the assignment of the candidate calibration model to this mesh.

4. Method (200) according to any one of the preceding claims, in which the evolution zone is subdivided into meshes (MJ) according to a vertical direction (Z) according to which the altitude is measured.

5. Method (200) according to claim 4, in which the subdivision of the evolution zone into meshes is further carried out according to two horizontal directions (X, Y) perpendicular to the vertical direction (Z).

6. Method (200) according to any one of the preceding claims, in which the calibration (220) and estimation (230) steps are repeated periodically.

7. Method (200) according to claim 6, in which the step of providing (210) the mesh is carried out before the first iteration of the calibration step (220), the evolution zone being chosen so that a predetermined trajectory (TR) to be followed by the aircraft (100) is entirely included in the evolution zone.

8. Method (200) according to claim 6, in which the step of providing (210) the mesh is carried out before each iteration of the calibration step (220), the evolution zone comprising for each step of providing at least a portion of a predetermined trajectory (TR) to be followed by the aircraft (100) from the current position (A(t)).

9. Method (200) according to claim 8 and claim 3, in which the calibration step (220) comprises, after step c), a grouping step d) (220A), during which, for each of the outermost meshes (M_b) of the mesh according to at least one predetermined direction, if the same calibration model has been assigned at the current date to said mesh and to a neighboring mesh (M_vois,b) according to the predetermined direction, only the calibration model (C_vois,b(t)) of the neighboring mesh is retained.

10. Method (200) according to claim 9, in which a number of meshes (N) of the mesh is fixed for all the iterations of the step of providing (210) the mesh, and in which for an iteration of the step of providing (210) subsequent to a step d) of grouping (220A), the volume of the evolution zone for said iteration (220B) of the step of providing is equal to the volume of all the meshes retained at the end of said step d).

11. Method (200) according to any one of claims 1 to 10, wherein, in the calibration step (220), the GNSS signals received on the current date (t) making it possible to establish a vertical protection level with respect to a given risk and, where appropriate, on at least one date (t') prior to the current date (t), are signals transmitted to the receiver (110) by at least one auxiliary reception device for GNSS signals independent of the aircraft (100), these signals transmitted to the receiver (110) having been received by the at least one auxiliary reception device prior to the current date (t), the method (200) then being implemented in a predictive mode.

12. Device (130) for assisting navigation of an aircraft (100) comprising: - at least one GNSS signal receiver (110), - at least one barometric altimeter (120), - a mesh module (130A), configured to implement the step of providing (210) a mesh of an area of ​​evolution of the aircraft (100) of the method (200) for assisting the navigation of the aircraft (100) according to any one of claims 1 to 10, - a calibration module (130B), configured to implement the calibration step (220) of the mesh of the method (200) for assisting navigation of the aircraft (100) according to any one of claims 1 to 10, and - a determination module (130C), configured to estimate, for at least one planned future position of the aircraft (A p ian(t p ian)) included in the aircraft evolution zone (100), an uncertainty (DPV(A p ian,t p ian)) on a geometric altitude (Z_geo,rec(A pian ,t p ian)) which would be determined from a barometric measurement (Z_baro (A pian ,t p ian)) to the future position (A) p ian(t pian)) and a calibration of the mesh retained at a current date (t) by the calibration module (130B).

13. Navigation aid device (130) according to claim 12, further comprising a decision module (130D), configured to implement the decision step (240) of the method (200) for assisting the navigation of an aircraft (100) according to claim 2.

14. Computer program comprising instructions which cause the device according to claim 12 or claim 13 to execute the navigation assistance method (200) according to any one of claims 1 to 11.

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