Method and device for aiding the navigation of an aircraft

Abstract

Method and device for assisting the navigation of an aircraft The present invention relates to a method (200) for assisting the navigation of an aircraft (100) comprising a GNSS receiver (110) and a barometric altimeter (120), comprising the provision (210) of a mesh comprising a plurality of meshes (M_i) of an area of ​​evolution of the aircraft comprising a current position (A(t)) of the aircraft at a current date (t); the calibration (220) of the mesh, comprising the allocation to each mesh of a calibration model (C_i(t)) on the basis of GNSS signals and a barometric measurement at the current date, and of a model for propagating uncertainties between the current position and the mesh on a geometric altitude (Z_geo, rec (A',t_est)) which would be determined at a position (A') of this mesh (M_i) from a barometric measurement;and for a planned future position of the aircraft (Aplan(tplan)), the estimation (230) of an uncertainty (DPV(Aplan,tplan)) on a geometric altitude (Z_geo,rec(Aplan,tplan)) which would be determined from a barometric measurement (Z_baro (Aplan,tplan)) at the future position and the calibration of the mesh retained. Figure for the abstract: 2;

Description

Title of the invention: Method and device for aiding the navigation of an aircraft

[0001] The present invention relates to a method for assisting the navigation of an aircraft comprising a GNSS signal receiver and an altimeter.

[0002] The present invention also relates to a corresponding device for assisting 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 the navigation of an aircraft.

[0003] 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.

[0004] The performance of such navigation aids is evaluated in particular by means of two indicators: precision and integrity.

[0005] Accuracy qualifies the position (or speed) error observed in nominal operation, i.e. in the absence of failure or signal loss.

[0006] Positioning integrity qualifies the degree of confidence that the user can place in the position (or speed) information delivered by the navigation aid system. Integrity includes the ability of the system to provide a loss of integrity alarm.

[0007] Integrity can be translated by the expression, for each calculated position or speed, of a confidence interval that the real 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 pre-accepted by the user. Integrity can also be translated 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.

[0008] The navigation aid system makes it possible in particular to establish an altitude in a given geometric reference point and to associate a geometric vertical protection level with it.

[0009] 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.

[0010] Depending on the type of aircraft, a more or less low level of vertical protection is imposed by the regulations that apply to the flight territory. For example, for unmanned aircraft such as drones, the required level of vertical protection is typically of the order of a few tens of meters.

[0011] Satellite systems are supplemented by so-called augmentation systems to guarantee vertical integrity to a few tens of 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 satellite data correction messages. The correction messages make it possible to increase the accuracy of the measurements and / or to guarantee the integrity of the navigation system at a particularly low level of autonomous failure risk Pr, typically of the order of 107 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 the 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 method 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 107 per hour and with vertical protection of a few tens of meters. In addition, this method does not 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 manner.

[0015] An aim of the invention is then to propose a navigation aid method making it possible to predict the evolution of the vertical protection level 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 invention relates to 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: - 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; - mesh calibration, including the allocation to each mesh of a calibration model based on: (i) GNSS signals received on the current date enabling a vertical protection level to be established in relation to a given risk and, where applicable, on at least one date prior to the current date, (ii) at least one barometric measurement taken on the current date and, where applicable, on the said date prior to the current date, and iii) a model for the 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 - 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 on the current date.

[0017] 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.

[0018] 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 where applicable.

[0019] Ultimately, it is therefore possible to predict, for a plurality of points of a portion of the trajectory of the upcoming aircraft, the minimum vertical protection limit which 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.

[0020] Thanks to this provision, the aircraft can be kept in flight as long as the vertical protection limit thus provided allows it, navigation being ensured from the barometric measurements as long as the integrity of the satellite signals is not restored.

[0021] It is therefore understood that the method according to the invention makes it possible to extend the flight time of the aircraft in numerous cases of loss of integrity of the satellite signals, while complying with 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.

[0022] According to other advantageous aspects of the invention, the method comprises one or more of the following characteristics, taken in isolation or in all technically possible combinations:

[0023] - the method comprises a step of deciding on the continuation of a mission to perform by the aircraft from the current position, the decision step including the evaluation of an integrity performance with the GNSS signals at the current date, and: I) if the evaluation is positive, the continuation of the mission, and otherwise, 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 (iii) depending on the remaining flight time, the provision of a landing instruction or a mission continuation instruction for a maximum duration calculated from the remaining flight time;

[0024] - the mesh calibration step includes: (a) the determination of a first geometric altitude of the current position from the GNSS signals received on the current date and of a first barometric altitude of the current position from the barometric altitude measurement carried out on the current date, (b) for each mesh, the construction of a candidate calibration model at the current date configured to estimate, at 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 at the estimation date, and c) for each mesh: (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 previous date, and if an uncertainty on a geometric altitude estimated at a reference point of the mesh with the previous calibration model is lower than that of the candidate calibration model, the assignment of the previous calibration model to the respective mesh, and otherwise, |3) the assignment of the candidate calibration model to this mesh;

[0025] - the evolution zone is subdivided into meshes in a vertical direction according to which the altitude is measured;

[0026] - the subdivision of the evolution zone into meshes is further carried out according to two horizontal directions perpendicular to the vertical direction;

[0027] - the calibration and estimation steps are repeated periodically;

[0028] - the mesh supply step is performed 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;

[0029] - the mesh supply step is performed before each iteration of the mesh supply step. calibration, 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;

[0030] - the calibration step comprises, after step c), a grouping step d), during which, for each of the outermost meshes of the mesh according to at least one predetermined direction, if the same calibration model has been assigned on the current date to said mesh and to a neighboring mesh according to the predetermined direction, only the calibration model of the neighboring mesh is retained;

[0031] - a number of mesh cells is fixed for all the iterations of the step of providing the mesh, and for an iteration of the providing step subsequent to a grouping step d), the volume of the evolution zone for said iteration of the providing step is equal to the volume of all the meshes retained at the end of said step d).

[0032] - at the calibration stage, the GNSS signals received on the current date allowing to establish a vertical level of protection in relation to a given risk and, where applicable, 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.

[0033] The invention also relates to a navigation aid device for an aircraft comprising: - at least one GNSS signal receiver, - 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, - a calibration module, configured to implement the mesh calibration step of the aircraft navigation aid method as described previously - a determination module, configured to estimate, for at least one future position of the planned 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.

[0034] According to other advantageous aspects of the invention, the device comprises the following characteristic

[0035] a decision module, configured to implement the decision step of the method for assisting navigation of an aircraft as described previously.

[0036] 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.

[0037] 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:

[0038] [Fig-1] [Fig.l] is a schematic representation of an aircraft comprising a navigation aid device according to the invention;

[0039] [Fig.2] [Fig.2] is a representation in the form of a flowchart of a mode of carrying out a process implemented by the device of [Fig.l];

[0040] [Fig.3] [Fig.3] is a representation of the method of [Fig.l], on which appear the input and output data of the different stages;

[0041] [Fig.4] [Fig.4] is a two-dimensional schematic representation of the evolution of a fixed mesh provided at the end of a step of providing the mesh of the process of [Fig.2] or [Fig.3], for an example of an aircraft trajectory between two times tl and t2;

[0042] [Fig.5] [Fig.5] is a two-dimensional schematic representation of the evolution of an adaptive mesh provided at the end of a step of providing the mesh of the process of [Fig.2] or [Fig.3], for an example of an aircraft trajectory between two times tl and t2;

[0043] [Fig.6] [Fig.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 process of [Fig.2] or [Fig.3].

[0044] The invention relates to a method for assisting the navigation of an aircraft 100.

[0045] The aircraft 100 is shown schematically in [Fig.l].

[0046] 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.

[0047] Alternatively, the aircraft 100 comprises several GNSS signal receivers 110 forming a navigation solution which combines the measurements of these receivers 110.

[0048] 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.

[0049] The processed signals can be emitted by one or more constellations among GPS, Galileo, GLONASS or even Beidou.

[0050] The satellite signals comprise navigation messages, the receiver 110 being configured to determine at least three geographic coordinates of the aircraft 100 from the navigation messages.

[0051] The receiver 110 may also be configured to receive augmented navigation messages from one or more satellite geolocation accuracy augmentation systems.

[0052] The precision augmentation system can be ground-based. It is then a LAAS type system or equivalently GBAS (in English, “Local Area Augmentation System” or “Ground Based Augmentation System”).

[0053] Alternatively or additionally, the precision augmentation system may comprise geostationary satellites. This is then a SBAS (Spatial Based Augmentation System) type system, such as the EGNOS, WAAS, MSAS, etc. systems.

[0054] In the following, the expression “GNSS signals” designates augmented or non-augmented signals.

[0055] Augmented navigation messages include, for example, information correction of geolocation measurements carried out by the receiver 110 and information characterizing the integrity of the GNSS system(s), where applicable.

[0056] 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.

[0057] Alternatively or in addition, the receiver 110, or where appropriate the navigation solution, can 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 propagation errors of the signals, so as to ensure vertical autonomous integrity (in English, “Vertical Advanced Receiver Autonomous Integrity Monitoring”).

[0058] The barometric altimeter 120 is configured to measure a pressure difference between the current position and a reference position.

[0059] The navigation aid device 130 is configured to receive data from the receiver 110 and the barometric altimeter 120.

[0060] The device 130 comprises means for implementing the navigation assistance method 200 described below.

[0061] The device 130 notably comprises a meshing module 130A, a calibration module 130B, a determination module 130C and a decision module 130D.

[0062] 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.

[0063] 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.

[0064] The navigation aid method 200 is described with reference to FIGS. 2 to 6.

[0065] This method is implemented when, for example, the aircraft 100 begins a mission. scheduled flight, 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 is carried out is respected.

[0066] 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 the position A(t).

[0067] The navigation assistance method 200 comprises a step 210 of providing a mesh of an area of ​​evolution of the aircraft comprising the current position A(t) of the aircraft 100.

[0068] The step 210 of providing the mesh is implemented by the mesh module 130A.

[0069] The step 210 of providing the mesh comprises providing a set of N meshes M_i, N being an integer greater than 1, preferably strictly greater than 1.

[0070] The set of N meshes paves a predetermined geographic volume V, in which the current position A(t) of the aircraft 100 is included.

[0071] 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.

[0072] In particular, at least a portion, advantageously the entirety, of a planned trajectory TR of the aircraft from the current position A(t) can be included in the geographical volume V.

[0073] Advantageously, each mesh M_i is parallelepipedal.

[0074] Advantageously, all the meshes M_i have the same volume V_i = V / N.

[0075] If the mesh is made up of equidistributed parallelepiped meshes M_i, each mesh M_i can be characterized by the geographic coordinates of a reference point of the mesh, for example the altitude Z_i, the latitude Lat_i, and the longitude Lon_i of this reference point.

[0076] The reference point is for example the center of the rectangular parallelepiped forming the mesh M_i.

[0077] 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 M_i of different altitudes Z_i according to a vertical direction Z relative to which the altitudes are defined.

[0078] 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 [Fig.4].

[0079] Advantageously, the meshes M_i are equidistributed along at least one of the horizontal directions X, Y, preferably along the two horizontal directions X and Y, as shown in [Fig.4].

[0080] The step 210 of providing the mesh may comprise: a) the provision by an operator to the meshing module 130A of the device 130 of the predetermined geographic volume V and / or of the planned trajectory TR and of the number N of meshes M_i, and b) the subdivision of the geographic volume V into N meshes by the mesh module 130A, the geographic volume V being either provided 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].

[0081] The mesh calibration step 220 follows the mesh supply step 210.

[0082] The calibration step 220 comprises the allocation to each mesh M_i of a calibration model C_i(t) at the current date t on which the method 100 is implemented.

[0083] The calibration step 220 is carried out by means of a calibration module 130B of the device 130.

[0084] 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 on the current date t.

[0085] 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).

[0086] 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.

[0087] The calibration step 220 comprises the construction of a candidate calibration model C_can(t) at the current date t.

[0088] 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.

[0089] 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: - the candidate calibration itself, which is the difference ô_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): ô_alt = Z_geo(t) - Z_baro(t); - a reference standard deviation o cantA.t) of the error of the candidate calibration at the current date t, which is equal to the vertical positioning standard deviation ov,GNss(À(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 = ov ,GNss(A(t)); and

[0090] - 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= Bv,GNss(A(t)); and

[0091] - the storage of the position A and the calibration time t.

[0092] The calibration module 130B implements an uncertainty propagation model between the current position A(t) and any mesh M_i, 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).

[0093] For each mesh M_i, 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.

[0094] To select the best calibration, the estimation date t_est can be chosen equal to the current date t.

[0095] For example, the model for the propagation of calibration uncertainties at any position A' and at the estimation date t_est can take into account one or more of the following terms, advantageously all of the following terms:

[0096] 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 can be evaluated according to the following equation:

[0097] (7H = kHdH ([Lat^j Lon^], [LatA^ Lon^]) (1)

[0098] where dH is the horizontal distance between position A' and the current position for calibration A(t) and kH a predetermined horizontal degradation rate. For example, kH may be equal to 2.7.10 4 m / m;

[0099] b) a degradation of temporal precision ot, 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: [oioo] a = (2) 'is he silent t \ /

[0101] where kt is a predetermined time degradation rate. For example, kt may be equal to 4.2.10 3 m / s;

[0102] 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:

[0103] (rv = kv\ZgeoA'-ZgeoA\ (3)

[0104] where kv is a predetermined vertical degradation rate. For example, kv may be equal to 1.066.10 1 m / m.

[0105] In the case where the three terms oH, ot >t_estet ov are calculated, the standard deviation ocan 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: ^can ( A'-' ^A^ ^is ) “ ^canAit)

[0107] Advantageously, in the case where the signals received by the receiver 110 include augmented navigation messages: - the total uncertainty associated with each candidate calibration model C_ can(t) includes uncertainty terms calculated from the augmented navigation messages, and / or - 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.

[0108] The vertical protection limit DPVican at the risk Pr of the barometric altitude at the reference point of the mesh M_i and at the instant t_est associated with the candidate calibration model C _can(t) of the mesh M_i will be provided by the following equation: P «Z> ^z> ^est ) ~ K-md&can(Lat^ Lont, (5)

[0110] where Kmd is the standardized threshold of missed detection at the risk level Pr, namely: [01111 (fi)

[0112] In the case where the mesh is only vertical, the calibration module 130B advantageously assigns to all the meshes M_i the latitudes and longitudes of the current position, so that Lat_i=Lat_geo(t) and Lon_i=Lon_geo(t) for all the meshes M_i.

[0113] For each mesh M_i, if a calibration model C_i(t') has been assigned to 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 mesh M_i at the date t_est associated with the candidate calibration model C_can(t) and that which is associated with the previous calibration model C_i(t').

[0114] For example, the calibration module 130B compares, for each mesh M_i, the vertical protection limit DPVi>can to the risk Pr associated with the candidate calibration model C_can(t) and that which is associated with the previous calibration model C_i(t').

[0115] If the uncertainty on the altitude Z_geo,rec(i, t_est) associated with the previous calibration model C_i(t') is strictly lower than that of the candidate calibration model C_ can(t), the previous calibration model C_i(t') is assigned to the mesh M_i at the current date t and becomes the current calibration model C_i(t).

[0116] 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 C_i(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 mesh M_i at the current date t and becomes the current calibration model C_i(t).

[0117] This step is represented schematically in [Fig.4] for which the geographical volume V containing the entire planned trajectory TR has been subdivided into eight parallelepiped cells of the same volume.

[0118] At a first current position A(ti) corresponding to a first date tb a first calibration step 220 was carried out. First calibration models C_i(ti) were assigned to each of the meshes Mi to M8.

[0119] At a second date t2 after the date tb the aircraft 100 is at a second current position A(t2) at which a second calibration step 220 is implemented while retaining the same mesh.

[0120] In this example, a previous calibration model is therefore available for each of the meshes, namely the calibration model assigned to the date tb

[0121] After comparing the candidate calibration model C_can(t2) with the previous calibration model C_i(ti) for each mesh M_i: - a new, more precise calibration model C_i(t2) is assigned to each of the M4, M5, M7 and M8 meshes thanks to the new satellite signals received on the second date t2; - on the other hand, based on the satellite signals received at the first date ti and the second date t2, it appears that the previous calibration model C_i(ti) is more accurate for the Mb meshes M2, M3 and M6 and must be kept as the calibration model at the second date t2.

[0122] Optionally, the calibration step 220 comprises a grouping step 220A, which is shown in [Fig.6].

[0123] 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 - for example, the vertical direction Z if the mesh is vertical.

[0124] 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.

[0125] If these calibration models are identical: - 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 [Fig.6]. - 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.

[0126] If the mesh is both vertical and horizontal, the grouping step 220A can be performed in at least two predetermined directions.

[0127] The grouping step 220A makes it possible to reduce the memory cost of the method.

[0128] 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 supply step 210, the grouping step 220A can be followed by a mesh refining step 220B, as shown in [Fig.6].

[0129] The mesh refining 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.

[0130] In the example of [Fig.6], the geographic volume V containing the planned trajectory TR has been subdivided vertically into five parallelepiped meshes Mi to M5 of the same volume at the end of a mesh supply step 210.

[0131] At the current position A(t) corresponding to the 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 M5.

[0132] During the grouping step 220A, it is observed that the mesh M5 which is at the edge of the volume V along the vertical direction Z has a calibration model C5 (t) identical to that of the neighboring mesh M4. The mesh M5 is therefore grouped with the mesh M4.

[0133] In the refining step 220B, the volume corresponding to the four meshes Mi to M4 for which a calibration model C_i(t) has been retained is subdivided into five new meshes M\ to M'5, with volumes smaller than that of the meshes Mi to M5. A new candidate calibration model is then calculated for each of these meshes M'i to M'5. Finally, a final calibration model C'i(t) to C'5(t) is assigned to the meshes M'i to M'5 according to the procedure described previously, the mesh M5 remaining ultimately linked to the mesh M'5.

[0134] This arrangement makes it possible to obtain a more precise calibration model across the entire geographic volume V at constant memory cost.

[0135] Once the mesh has been calibrated, the determination step 230 is implemented by the determination module 130C for at least one future position of the aircraft 100 planned Apian at a date tpian later than the current date t included in the evolution zone of the aircraft 100.

[0136] The determination step 230 comprises the estimation of an uncertainty on a reconstructed geometric altitude Z_geo,rec (Apian, tp kn) which would be determined from a barometric measurement Z_baro (Apian, tpian) at the future position Apian at the date tpian and from the calibration of the mesh retained at the current date t.

[0137] 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 Apian is located, or whose reference point is closest to the future position, may be associated with the reconstructed geometric altitude Z_geo,rec (Apian, tpian).

[0138] The difference between a reconstructed geometric altitude Z_geo,rec (Apian) of the Apian position (tpian>t) and its barometric altitude Z_baro (Apian, tpian) can be estimated according to the following equation:

[0139] Zgeoree(Ap]an J -Zbaro(Apjan) = 5(lltck(f) (?)

[0140] Alternatively, a vertical protection limit DPV(Apian, tpian) may be calculated by interpolation of the vertical protection limits DPV; the risk Pr associated with the calibration model C_i(t) retained at the current date t for a plurality of neighboring cells M_p, including the cell M_k in which the future position Apian (tpian) is located or the closest to this future position, may be associated with the reconstructed geometric altitude Z_geo,rec(Apian, tpian), according to the following equation, the set of neighboring cells M_p being designated by MV:

[0141] DP^A,*» t,, / „„)= interp^, ((LMp lmp Zr). BVp Ait?,J) + ((Latp Loup a2fta„(Lat„ Lon^ Zp, Xi / * J) (8)

[0142] the difference between the geometric altitude Z_geo(Apian) of the position A pian and its barometric altitude can be estimated according to the following equation: Zgmfec(Aplan)-Zbaro(Aplail) = imerpMV ( {Latp, Lonp, Zp), Salie#* A^^) ) (9)

[0143] For a vertical mesh, the neighboring meshes M_p can include the two consecutive meshes whose reference point altitudes frame the current estimated altitude.

[0144] For a vertical and horizontal mesh, the neighboring meshes M_p can include the eight meshes whose reference point positions are closest and surround the Apian position (tpian>t).

[0145] 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 (tpian> t) relative to the geographic volume V covered by the mesh.

[0146] 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.

[0147] Advantageously, the uncertainty on a reconstructed geometric altitude Z_geo, rec(Apian, tpian) is the largest uncertainty among all those calculated for passage dates included in an interval [tpian-Atpian, tphn-Atpian], Atpian being an uncertainty on the passage date q at the future position Apian.

[0148] 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.

[0149] 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, i.e. the vertical positioning based on the GNSS navigation solution alone, without taking into account the barometric measurements.

[0150] 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.

[0151] 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(Apian, tpian) of the aircraft 100 for all the planned future positions Apian from the current position A(t) during this remaining flight time t_vol remains lower than a second predetermined threshold.

[0152] Preferably, the first threshold and the second threshold are equal.

[0153] For this, the decision module 130D estimates the uncertainty on the reconstructed geometric altitude Z_geo, rec(Apian, tpian) of a plurality of planned future positions Apian included in the planned trajectory TR from the calibration models last assigned to the meshes M_i.

[0154] Preferably, if no candidate calibration model has been previously calculated for one or more given meshes M_i, an initial uncertainty model, such as a model provided by the manufacturer and based on an uncertainty on the barometric altitude determined during the manufacture of the altimeter 120, is assigned to each of these meshes.

[0155] The remaining flight time t_vol is then the time remaining before the planned date of passage to the last of the plurality of planned future positions for which the uncertainty on the reconstructed geometric altitude is less than a predetermined threshold.

[0156] 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.

[0157] 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.

[0158] 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.

[0159] The calibration 220, determination 230 and decision 240 steps are advantageously repeated later.

[0160] Advantageously, all of the calibration 220, determination 230 and decision 240 steps are repeated periodically.

[0161] In a particular embodiment, the step 210 of providing the mesh is also repeated.

[0162] For example, the step 210 of providing the mesh 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.

[0163] 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 V_i of the meshes M_i, the number of meshes can be reduced as the aircraft 100 advances.

[0164] 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.

[0165] This step is represented schematically in [Fig.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.

[0166] The volume V(ti) was subdivided in a first step 210 of providing the mesh into three parallelepiped meshes of the same predetermined volume Vmaine, so as to cover all the planned altitudes of the aircraft on the portion of the trajectory concerned.

[0167] At the first current calibration position A(ti), corresponding to the first date tb, 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.

[0168] At the second date t2 after the first date tb the aircraft 100 is at a second current calibration position A(t2).

[0169] 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.

[0170] A second calibration step 220 is then implemented.

[0171] 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 tb

[0172] 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.

[0173] Following the comparison of 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 mesh M3 using the new satellite signals received on date t2.

[0174] 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 precise for the Mb M2 meshes, and must be retained.

[0175] 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.

[0176] 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.

[0177] 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.

[0178] 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 the entirety of the planned trajectory TR.

[0179] This embodiment is therefore purely predictive.

[0180] Alternatively, the GNSS signals received at the current date t of the calibration 220 comprise 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.

[0181] In the case of the purely predictive embodiment, the decision step 240 on the continuation of the mission to be carried out may include, if the evaluation is negative, that is to say that the vertical protection level of the solution exceeds the first threshold predetermined during the planned trajectory portion TR considered, a sub-step of generating one or more alternative planned trajectories.

[0182] Each alternative planned trajectory has the same starting point and the same arrival 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.

[0183] 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.

[0184] If at least one alternative planned trajectory can be generated, the decision step 240 may comprise selecting an alternative planned trajectory and starting the mission.

[0185] It will be noted that the sub-step of generating one or more alternative planned trajectories can also be combined with the real-time embodiment.

Claims

Claims

1. Method (200) for aiding the navigation of an aircraft (100) comprising at least one receiver (110) of GNSS signals and a barometric altimeter (120), the method comprising the following steps of: - 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 (M_i);- calibration (220) of the mesh, comprising the allocation to each mesh (M_i) 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 (M_i) on a geometric altitude (Z_geo, rec (A',t_est)) which would be determined at any position (A') of this mesh (M_i) from a barometric measurement;and - for at least one planned future position of the aircraft (Apian(tpian )) included in the area of ​​evolution of the aircraft (100), estimation (230) of an uncertainty (DPV(Apian,tpian)) on a geometric altitude (Z_geo,rec(Apian,tpian)) which would be determined from a barometric measurement (Z_baro (Apian,tpian)) at the future position (Apian(tpian)) and the calibration of the mesh 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 evaluation is positive, the continuation of the mission, and otherwise, 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 below a threshold predetermined 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 (M_i), 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 M_i from a barometric measurement carried out at this position (A') at the estimation date (t_est), and c) for each mesh (M_i): 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 prior 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 prior calibration model is lower than that of the candidate calibration model (C_i_can(t)), the assignment of the prior calibration model to the respective mesh, and otherwise, |3) 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 (M_i) 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, wherein 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, 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 kept at the end of said step d).

11. Method (200) according to any one of claims 1 to 10, in which, 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 GNSS signal reception device 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 aiding navigation of an aircraft (100) comprising: - at least one receiver (110) of GNSS signals, - at least one barometric altimeter (120), - a meshing 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 aiding navigation of the aircraft (100) according to any one of claims 1 to 10, - a calibration module (130B), configured to implement the step of calibrating (220) the mesh of the method (200) for aiding 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 (Apian(tpian)) included in the aircraft evolution zone (100), an uncertainty (DPV(Apian,tpian)) on a geometric altitude (Z_geo,rec(Apian,tpian )) which would be determined from a barometric measurement (Z_baro (Apian,tpian)) at the future position (Apian(tpian)) 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. A computer program comprising instructions that 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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