METHOD FOR DETERMINING THE CALIBRATION DATA OF AN AIRBORNE GONIOMETRY APPARATUS

Abstract

The invention relates to a method (for determining calibration data for an airborne goniometry apparatus comprising an antenna array of several antennas, from several sets of calibration data measured in-flight by said goniometry apparatus, each associated with a measured angular position and comprising an amplitude datum and a phase datum measured by each antenna in said antenna array at said measured position.

The method comprises, for an estimated angular position, a phase (200) of calculating an estimated calibration data set and comprising the following steps: for each measured position, normalizing (204) the data set measured at said measured position, with respect to the phase data measured by each antenna, said normalizing providing as many normalized data sets as there are antennas for each measured position; for each antenna, calculating (206) a candidate data set by interpolating the measured data sets at said measured positions and previously normalized with respect to the phase measured by said antenna; selecting (210), as the estimated calibration data set, the candidate data set whose phase reference has the highest energy among said candidate data sets.

It also concerns a computer program and an apparatus implementing such a method, a calibration table obtained by such a method and a goniometry apparatus calibrated with such a method.

Claims

1. A computer-implemented method for determining calibration data, for an airborne goniometry apparatus comprising an antenna array of several antennas, from several calibration data sets measured in-flight by said airborne goniometry apparatus each associated with a measured angular position and comprising an amplitude datum and a phase datum measured by each antenna of said antenna array at said measured angular position, said computer-implemented method comprising: for an estimated angular position, a phase of calculating an estimated calibration data set comprising for each measured angular position, normalizing a calibration data set of said several calibration data sets that are measured at said measured angular position, with respect to the phase datum measured by each antenna of said antenna array, wherein said normalizing provides as many normalized data sets as there are antennas for said each measured angular position; for said each antenna, calculating a candidate data set by interpolating the several calibration data sets that are measured at said each measured angular position and previously normalized with respect to the phase datum measured by said each antenna; selecting, as the estimated calibration data set, the candidate data set whose phase reference has a highest energy among all candidate data sets of said candidate data set for each antenna.

2. The computer-implemented method according to claim 1, wherein the phase of calculating comprises calculating, for the estimated angular position, a covariance matrix, from the estimated calibration data set.

3. The computer-implemented method according to claim 1, wherein the phase of calculating comprises, for at least one candidate data set, calculating an energy of said at least one candidate data set, comprising calculating the energy associated with said each antenna in the at least one candidate data set; and calculating the energy of said at least one candidate data set based on the energy associated with said each antenna.

4. The computer-implemented method according to claim 1, wherein the phase of calculating is implemented for a plurality of estimated positions to build a calibration table comprising a multitude of angular positions according to a predetermined angular distribution.

5. The computer-implemented method according to claim 4, wherein the calibration table further comprises only the plurality of estimated positions, or a combination of the plurality of estimated positions and measured positions.

6. The computer-implemented method according to claim 4, wherein the calibration table further comprises the multitude of angular positions distributed following a constant angular pitch.

7. The computer-implemented method according to claim 1, wherein, for at least one estimated angular position, the interpolating takes into account only the calibration data sets that are measured for adjacent measured angular positions between which said estimated angular position lies.

8. The computer-implemented method according to claim 1, wherein, for at least one estimated position of each of said estimated angular position, the interpolating is performed by a GRIDDATA interpolation function.

9. The computer-implemented method according to claim 1, further comprising a phase of acquiring, in-flight, the several calibration data sets for a plurality of measured angular positions, for at least one frequency.

10. A computer program comprising computer instructions which when executed by a computer implement a computer-implemented method for determining calibration data, for an airborne goniometry apparatus comprising an antenna array of several antennas, from several calibration data sets measured in-flight by said airborne goniometry apparatus, each associated with a measured angular position and comprising an amplitude datum and a phase datum measured by each antenna of said antenna array at said measured angular position, said computer-implemented method comprising: for an estimated angular position, a phase of calculating an estimated calibration data set comprising for each measured angular position, normalizing a calibration data set of said several calibration data sets that are measured at said measured angular position, with respect to the phase datum measured by each antenna of said antenna array, wherein said normalizing provides as many normalized data sets as there are antennas for said each measured angular position; for said each antenna, calculating a candidate data set by interpolating the several calibration data sets that are measured at said each measured angular position and previously normalized with respect to the phase datum measured by said each antenna; selecting, as the estimated calibration data set, the candidate data set whose phase reference has a highest energy among all candidate data sets of said candidate data set for each antenna.

11. (canceled)

12. The computer-implemented method according to claim 1, wherein the phase of calculating is implemented by a processing device.

13. An airborne goniometry apparatus comprising: an antenna array of several antennas; wherein said airborne goniometry apparatus is calibrated by a computer-implemented method for determining calibration data from several calibration data sets measured in-flight by said airborne goniometry apparatus, wherein each calibration data set of said several calibration data sets is associated with a measured angular position and comprises an amplitude datum and a phase datum measured by each antenna of said antenna array at said measured angular position, wherein the computer-implemented method comprises for an estimated angular position, a phase of calculating an estimated calibration data set comprising for each measured angular position, normalizing a calibration data set of said several calibration data sets that are measured at said measured angular position, with respect to the phase datum measured by each antenna of said antenna array, wherein said normalizing provides as many normalized data sets as there are antennas for said each measured angular position; for said each antenna, calculating a candidate data set by interpolating the several calibration data sets that are measured at said each measured angular position and previously normalized with respect to the phase datum measured by said each antenna; selecting, as the estimated calibration data set, the candidate data set whose phase reference has a highest energy among all candidate data sets of said candidate data set for each antenna.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Other benefits and features shall become evident upon examining the detailed description of one or more embodiments, and from the enclosed drawings in which:

[0064] FIGS. 1a and 1b are schematic depictions of a non-limiting example of a configuration for calibrating an airborne goniometry apparatus which can be used in one or more embodiments of the invention;

[0065] FIG. 2 is a schematic depiction of a non-limiting example of a method for determining calibration data of an airborne goniometry apparatus according to one or more embodiments of the invention;

[0066] FIG. 3 is a schematic depiction of a non-limiting example of a method for determining a calibration table of an airborne goniometry apparatus according to one or more embodiments of the invention; and

[0067] FIGS. 4a and 4b are schematic depictions of calibration tables of a goniometry apparatus according to one or more embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0068] It is understood that the embodiments disclosed hereunder are by no means limiting. In particular, it is possible to imagine variants of the invention that comprise only a selection of the features disclosed hereinafter in isolation from the other features disclosed, if this selection of features is sufficient to confer a technical benefit or to differentiate the invention with respect to the prior state of the art. This selection comprises at least one preferably functional feature which lacks structural details, or only has a portion of the structural details if that portion is only sufficient to confer a technical benefit or to differentiate the invention with respect to the prior state of the art.

[0069] In the figures the same reference has been used for the features that are common to several FIGURES.

[0070] FIGS. 1a and 1b are schematic depictions of an example of a configuration for calibrating an airborne goniometry apparatus, seen from a side view and a top view, respectively, according to one or more embodiments of the invention.

[0071] FIGS. 1a and 1b schematically show a goniometry apparatus 102 transported by an aircraft 104, such as for example an airplane 104, and helicopter, an airship, a balloon, etc.

[0072] A calibration transmitter 106, of known position, is used to calibrate the airborne goniometry apparatus 102. The calibration transmitter 106 can be stationary or mobile. The calibration transmitter 106 can for example be disposed on the ground.

[0073] The calibration of the goniometry apparatus 102 is performed as follows. A calibration signal of known frequency is sent by the calibration transmitter 106 to the goniometry apparatus 102. The goniometry apparatus 102 measures the reception response of each antenna in the antenna array for the calibration signal, namely a complex vector whose real part is formed by an amplitude datum and whose imaginary part is formed by the phase datum. The measured data are stored associated with the known emission frequency, and the known angular position of the calibration transmitter 106 relative to the airborne goniometry apparatus 102.

[0074] For each angular position of the calibration transmitter 106, in one or more embodiments, with respect to the airborne goniometry apparatus 102, the calibration is repeated for different frequencies, or frequency bands, with a view to scanning an entire broad range of frequencies, in the context of a calibration sequence.

[0075] The position of the goniometry apparatus is changed to repeat the calibration at a new angular position so as to scan a range of angular positions, for example along a calibration path.

[0076] Thus, in one or more embodiments, at the end of calibration, a calibration data table is obtained. This table comprises for each pair {frequency, angular position} a set of complex vectors, each complex vector corresponding to the response of an antenna of the antenna array to a calibration signal.

[0077] According to a non-limiting example according to one or more embodiments of the invention, a reference magnitude can be calculated, for each frequency and each angular position, from the data set associated with said frequency at said angular position. For example, the calculated reference magnitude can be, in an entirely non-limiting manner, a covariance matrix indicating the reception differences between said receiving antennas, that is, between the complex vectors.

[0078] The angular position of the calibration transmitter 106 with respect to the goniometry apparatus 102 can be given by a combination of two angles, namely: [0079] a bearing angle, denoted G, depicted in FIG. 1b, which corresponds to the angle, in the horizontal plane, between on the one hand the direction connecting the aircraft 104 and the calibration transmitter 106, and on the other hand a reference direction, for example magnetic north; and [0080] an elevation angle, denoted S, depicted in FIG. 1a, corresponding to the angle formed between on the one hand the vertical direction between the goniometry apparatus 102 (and thus the aircraft 104) and the ground, and on the other hand the direction connecting the goniometry apparatus 102 (and thus the aircraft 104) and the calibration transmitter 106.

[0081] These angles can be provided or calculated from measured data, by sensors equipping the goniometry apparatus 102 or the aircraft 104, such as, for example: [0082] a GPS module indicating the position of the aircraft, [0083] an altimeter indicating the altitude of the aircraft, and [0084] optionally an inertial unit indicating the attitude, that is, the spatial orientation of the aircraft 104 with the goniometry apparatus 102 on board.

[0085] Each calibration signal emitted by the calibration generator 106 can be a signal burst.

[0086] As noted above, in one or more embodiments, the goniometry apparatus 102 comprises an antenna array formed by several antennas. During calibration, for each angular position and for each calibration signal, each antenna receives and measures a pair of data, namely an amplitude datum and a phase datum. Thus, for each angular position and for each calibration signal, the goniometry apparatus measures a data set formed by as many data pairs {Amplitude, Phase} as there are antennas in the antenna array. This set of measured data can then be used to calculate a calibration datum such as a covariance matrix, for example, between the measurements made by all the antennas of the antenna array.

[0087] When the calibration is performed in-flight, the measured angular positions do not necessarily correspond exactly to those desired. Thus, at the end of an acquisition phase, the measured angular positions are not distributed in the desired way, for example, with a desired angular pitch. In other words, in one or more embodiments, in-flight calibration does not allow obtaining a complete, even and balanced calibration table with a constant angular pitch because the trajectory of the air carrier with the goniometry apparatus on board is not totally predictable, particularly because of drifts related to winds for example.

[0088] At least one embodiment of the invention makes it possible to overcome this drawback by proposing a method for calculating calibration data for at least one unmeasured angular position, called “estimated angular position” or “estimated position”, by interpolating data measured in-flight for several measured angular positions, also called “measured position” herein.

[0089] FIG. 2 is a schematic depiction of a non-limiting example of a method according to one or more embodiments of the invention.

[0090] The method 200 of FIG. 2 allows calculating calibration data for an estimated angular position, for which no calibration data measured in-flight are available, from in-flight measured calibration data for several measured angular positions at the same frequency.

[0091] Preferably, in one or more embodiments, the measured angular positions whose calibration data are used to calculate the calibration data of the estimated position are angular positions close to, or around, or surrounding said estimated position.

[0092] The method 200 depicted in FIG. 2 is described for an estimated position. The method 200 can be repeated as many times as desired to calculate, individually, calibration data for several estimated positions.

[0093] The method 200 includes a step 202 in which measured data for several measured angular positions are extracted. These measured data will be used to calculate by interpolation calibration data for an estimated angular position. For example, in step 202, the measured data for M positions POSM.sup.1-POSM.sup.M are extracted. Considering that the antenna array comprises K antennas, the data measured for each position POSM.sup.m correspond to a set of K data pairs, each pair corresponding to one of the K antennas and comprising an amplitude datum and a phase datum measured by said antenna. Thus, in one or more embodiments, the measured data set for the position POSM.sup.m corresponds to:

POSM.sup.m={(AM.sub.1.sup.m,PH.sub.1.sup.m), . . . , (AM.sub.k.sup.m,PH.sub.k.sup.m), . . . , (AM.sub.K.sup.m,PH.sub.K.sup.m)}

wherein 1≤m≤M and 1≤k≤K

[0094] In step 204, the measured data set for each measured position POSM.sup.m is normalized with respect to the phase measured by each antenna in the antenna array. Normalization of the data set with respect to the phase of an antenna can be performed by dividing the phase data measured by all antennas by the phase data of said antenna. Thus, in this step 204, K data sets are obtained for each measured position POSM.sup.m. For example, for the data set POSM.sup.m, measured for the “m” measured position, the following K normalized data sets are obtained:

POSM.sup.m/1={(AM.sub.1.sup.m,1), . . . , (AM.sub.k.sup.m,PH.sub.k.sup.m/PH.sub.1.sup.m), . . . , (AM.sub.K.sup.m,PH.sub.K.sup.m/PH.sub.1.sup.m)},

normalized with respect to the antenna phase 1

POSM.sup.m/k={(AM.sub.1.sup.m,PH.sub.1.sup.m/PH.sub.k.sup.m), . . . , (AM.sub.k.sup.m,1), . . . , (AM.sub.K.sup.m,PH.sub.K.sup.m/PH.sub.k.sup.m)},

normalized with respect to the antenna phase k

POSM.sup.m/K={(AM.sub.1.sup.m,PH.sub.1.sup.m/PH.sub.K.sup.m), . . . , (AM.sub.k.sup.m,PH.sub.k.sup.m/PH.sub.K.sup.m), . . . , (AM.sub.K.sup.m,1)},

normalized with respect to the antenna phase K

[0095] In step 206, K candidate datasets are calculated by interpolation for the estimated position, considering the normalized antenna-by-antenna data.

[0096] In other words, in one or more embodiments, for all measured positions, the data normalized with respect to the antenna 1 are considered first: POSM.sup.1/1-POSM.sup.M/1. These normalized data POSM.sup.1/1-POSM.sup.M/1 are used to calculate by interpolation a candidate data set, denoted POSE.sup./1, for the estimated position POSE. The candidate data set POSE.sup./1 is thus obtained by taking antenna 1 as phase reference. The same operation is repeated with the data set normalized with respect to antenna 2: POSM.sup.1/2-POSM.sup.M/2 to calculate a candidate data set POSE.sup./2 obtained by taking as phase reference the antenna 2, and so forth so that step 206 provides K candidate data sets POSE.sup./1-POSE.sup./K, each obtained by taking as phase reference one of the antennas of the antenna array. Note that each candidate data set POSE.sup./k, wherein 1≤k≤K, provides as many data pairs (AM, PH) as there are antennas in the antenna array so that the obtained candidate data set with respect to antenna k can be:

POSE.sup./k={(AM.sub.1.sup./k,PH.sub.1.sup.k), . . . , (AM.sub.k.sup./k,PH.sub.k.sup./k), . . . , (AM.sub.K.sup./k,PH.sub.K.sup./k)}

wherein 1≤k≤K

[0097] In this step 206, each candidate data set is calculated using an interpolation function, such as for example the GRIDDATA function, in MATLAB.

[0098] In step 208, the energy of each candidate data set POSE.sup./k, wherein 1≤k≤K, is calculated. This energy can be calculated in different ways.

[0099] According to at least one embodiment, the energy of each candidate data set POSE/.sup.k is calculated as follows. First, in one or more embodiments, the energy of each pair of data (AM.sub.k.sup./k,PH.sub.k.sup./k) forming the candidate data set is calculated. For each pair of data, the energy can for example correspond to the modulus of the complex vector formed by this pair of data. Then, in one or more embodiments, the energy of the candidate data set POSE.sup./k can be calculated based on the energy of each data pair forming said candidate data set. For example, the energy of the candidate data set may correspond to the average of the energies of the data pairs forming said candidate data set.

[0100] The method 200 then includes a step 210, in which the candidate data set having the highest energy level is selected as the estimated data set for the POSE estimated position.

[0101] Optionally, in one or more embodiments, the method 200 may advantageously comprise a step 212 for calculating a calibration magnitude for the estimated position POSE. This calibration magnitude can be a covariance matrix obtained from the estimated data set selected in step 210. Thus, at the end of step 212, a calibration magnitude is obtained by interpolation for an estimated angular position and for a frequency for which no calibration data has been measured in-flight.

[0102] The method 200 can be repeated for as many estimated angular positions as desired to build a complete and balanced calibration table in terms of angular positions.

[0103] According to at least one embodiment of the invention, such a calibration table can be obtained without using one single phase reference for all estimated angular positions. Thus, for each estimated angular position, the phase reference used for interpolation can potentially be different. Furthermore, according to one or more embodiments of the invention, the phase reference used for each estimated position will always be the one with the highest energy level. Thus, one or more embodiments of the invention makes it possible to obtain, for (each) estimated angular position, estimated calibration data which are more accurate because they are less affected or tainted by noise which would be due to a phase reference whose energy is too low. Consequently, in one or more embodiments, the calibration data obtained by the method according to the invention are more accurate.

[0104] FIG. 3 is a schematic depiction of a non-limiting example of a method according to one or more embodiments of the invention for determining calibration data table of an airborne goniometry apparatus according to one or more embodiments of the invention.

[0105] The method 300 of FIG. 3 makes it possible to determine a calibration data table comprising, for a multitude of given angular positions and frequencies, calibration data for an airborne goniometry device.

[0106] The method comprises a phase 302 of measuring calibration data for a plurality of measured angular positions. This acquisition phase 302 is performed in the configuration shown in FIGS. 1a and 1b, that is, whereupon the goniometry apparatus is on board an airborne carrier. The acquisition phase provides data for a plurality of angular positions that are not unevenly distributed.

[0107] An example of the distribution of measured angular positions 402 is shown in FIG. 4a, according to one or more embodiments of the invention. Each measured angular position is represented by a rectangle. It can clearly be seen that the angular positions are not evenly distributed, that there are areas where there are no measured angular positions and areas comprising too many measured angular positions.

[0108] The method 300 next includes at least one iteration of a calibration data calculation for at least one estimated angular position, based on data measured in-flight during the acquisition phase. In particular, in one or more embodiments, the calculation of estimated calibration data for an estimated position may be performed according to the method 200 of FIG. 2.

[0109] Preferably, in one or more embodiments, the method 200 can be repeated for a plurality of angular positions so as to obtain a multitude of angular positions, for which calibration data are available, which are evenly distributed according to a desired constant or variable pitch.

[0110] An example of the angular position distribution thus obtained is shown in FIG. 4b, according to one or more embodiments of the invention. Each measured angular position 402 is represented by a rectangle and each estimated angular position 404 is represented by a circle. It can be clearly seen that in the calibration table in FIG. 4b, the angular positions are evenly distributed and according to a desired, and in particular constant, pitch. Furthermore, all areas are covered and there are no areas for which calibration data are not available.

[0111] In the method 300, in one or more embodiments, the acquisition phase is performed before the first iteration of the method 200. The first iteration of the method 200 may be performed after the acquisition phase is complete. Alternatively, in one or more embodiments, the first iteration of the method 200 can be performed/triggered without waiting for the completion of the acquisition phase.

[0112] Furthermore, in one or more embodiments, the method 300 can be repeated for each radio frequency involved in the calibration. In this case, the acquisition phase 302 can be common to several or even all radio frequencies.

[0113] At least one embodiment of the invention also relates to a data processing device configured to implement all the steps of the method according to one or more embodiments of the invention, and in particular the method 200 of FIG. 2. Such a device, not shown in the FIGURES, may be a computer, server, processor, programmable electronic chip, etc. configured to implement all the steps of the method, for example by virtue of a computer program.

[0114] The device according to one or more embodiments of the invention can be a physical machine or a virtual machine.

[0115] The invention in one or more embodiments also relates to a goniometry apparatus configured, or comprising a calibration table calculated, by a method according to one or more embodiments of the invention, and in particular by the method 200 of FIG. 2.

[0116] Of course, the invention is not limited to the examples detailed herein before given for purposes of illustration and the general scope of the invention is defined in the claims.