METHOD FOR CALIBRINGING AN AIR-BASED LOW-FREQUENCY SIGNATURE DEVICE

Description

[0001] The present invention relates to a method for calibrating an airborne direction-finding device, particularly for low frequencies. It also relates to an installation implementing such a method.

[0002] The field of the invention is the field of airborne direction-finding devices for the detection of radio frequency transmitters, in particular radio transmitters emitting low frequency signals. State of the art

[0003] An airborne direction finder typically consists of an antenna array and at least one receiver connected to that array. It is used to determine the angular position (azimuth and elevation angles) of radio frequency transmitters located at ground or sea level when mounted on an aircraft. To do this, the direction finder uses a calibration table that provides an angular position for each frequency and polarization, based on the receiver's response.

[0004] The calibration table of a direction finder can be obtained during a calibration phase in which a transmitter emits a calibration signal. The received response of the antenna array is measured and stored in association with the known characteristics of the calibration signal, namely its frequency, polarization, and angle of arrival (azimuth and elevation).

[0005] Calibration must be performed in the far field, at several tens of times the wavelength of the calibration signal. Furthermore, at low frequencies, i.e., below 500 MHz, the direction finder must be calibrated while mounted on the aircraft to account for the effect of the aircraft's metallic structure on the antenna array's reception response. For these reasons, low-frequency calibration of the airborne direction finder cannot be performed in an anechoic chamber, as no anechoic chamber large enough to meet all these requirements, particularly at low frequencies, does exist.

[0006] Furthermore, the currently known techniques for in-flight calibration of an airborne direction-finding device to determine a calibration table for each frequency, polarization, and angular position are very time-consuming and expensive.

[0007] One objective of the present invention is to remedy at least one of the aforementioned drawbacks.

[0008] Another objective of the present invention is to provide a less time-consuming and less expensive in-flight, low-frequency calibration solution for an airborne direction-finding device.

[0009] US2006238413A1 relates to a technique for calibrating a radio direction finder that can be installed on board an aircraft. The technique comprises: (a) setting a value for the time-dwell ranges of the communication frequency signal to a predetermined value; (b) synchronizing the time-dwell ranges between the calibration transmitter and calibration receiver using clock signals from a common synchronization source; (c) providing a hop-duration time interval within the time-dwell range for transmitting the frequency; and (d) generating the communication frequency signal corresponding to the frequency during the hop-duration interval. WO2007047119A2 relates to radio direction finding that correlates signals received from a remote mobile transmitter to determine the geographic position of that transmitter. The received signals are sampled, digitized, and stored in covariance matrices.They are then summed and normalized using an equation whose velocity terms are set to zero to minimize spurious correlation peaks, and a maximum correlation peak is developed. A conjugate gradient search routine is used to find the correlation peak of the summed data. The peak value is then analyzed to see if it is above or below a predetermined value. If the peak value is above the predetermined value, the transmitter is stationary and the localized correlation peak corresponds to the transmitter's location. If the peak value is below the predetermined value, the transmitter is moving and the peak does not indicate the transmitter's correct location. In this case, calibration of the system at different frequencies must be performed. JPH1084219A relates to a radio direction finding system comprising two orthogonal linearly polarized antennas. Description of the invention

[0010] The invention makes it possible to achieve at least one of these goals by means of a low-frequency, in-flight calibration method for a direction-finding device comprising an antenna array, carried by an aerial carrier, said method comprising, for a given angular reception position, a calibration step of said airborne direction-finding device at a given frequency, comprising the following operations: emission, by a calibration transmitter, at the said given frequency and in the direction of the said goniometry device, of at least two calibration signals, of orthogonal polarizations to each other, and measurement of a response of the said antenna array for each of the said signals;

[0011] Thus, the invention proposes to perform the calibration of the goniometry device when it is in flight, that is to say when it is carried by an air carrier, which makes it possible to avoid performing this calibration in an anechoic chamber, and therefore to avoid having to provide an anechoic chamber of very large dimensions, in particular for low frequencies.

[0012] Furthermore, the invention allows for more precise calibration of the goniometry device, particularly for low frequencies, because it allows for taking into account the influences of the metallic structure of said aerial carrier on which said goniometry device is located, particularly for low frequencies.

[0013] Furthermore, the invention enables faster, less time-consuming, and less expensive calibration of the airborne direction-finding device because it allows, in a single step performed for a given angular position and frequency, the determination of the device's calibration data for several, or even all, possible polarizations of electromagnetic waves. Indeed, from calibration data measured in flight for two mutually orthogonal polarizations, it is possible to deduce the calibration data for all possible polarizations of a radio frequency wave, since any polarization of a radio frequency wave can be decomposed onto the orthogonal basis formed by said orthogonal polarizations of the calibration signals.

[0014] In this application, "low frequency radio frequency signal" means a radio frequency signal with a frequency of 500MHz or less.

[0015] In this application, "aircraft" means any flying vehicle, such as an airplane, helicopter, airship, balloon, drone, etc.

[0016] In this application, angular position means a position defined by an azimuth angle and an elevation angle. The receiving angular position of the direction-finding device is the relative position of said device with respect to the calibration signal transmitter. In other words, the relative angular position between the airborne carrier and the calibration transmitter corresponds to the receiving angular position of each calibration signal.

[0017] According to advantageous embodiments, calibration signals can be emitted simultaneously.

[0018] This feature reduces the time required to perform the calibration step, thus making calibration less time-consuming and less expensive.

[0019] In this case, to differentiate the two calibration signals received by the direction finder, the calibration signals may have a frequency offset. This frequency offset may be minimal or negligible compared to the frequency of each calibration signal, so that the calibration signals are considered to have the same frequency while still being able to be distinguished upon reception.

[0020] For example, the frequency difference, denoted Δ, between the calibration signals can be 200kHz, but this value is by no means limiting and depends on the frequency of the calibration signals.

[0021] Following a non-limiting example of implementation, the frequencies F1 and F2 of the two calibration signals can be centered on the frequency Fm for which the calibration is performed, such that: F 1 − F 2 = Δ , And F m − F 1 = − Δ / 2 et F m − F 2 = + Δ / 2 .

[0022] Thus, each calibration signal has a frequency as close as possible to the frequency for which the calibration is performed, while still allowing them to be differentiated in reception.

[0023] According to advantageous embodiments, calibration signals can be emitted in turn.

[0024] This embodiment, although more time-consuming, allows the use of two calibration signals whose frequency is equal to the frequency Fm for which the calibration is performed.

[0025] According to an advantageous feature, the method according to the invention can include, for the same angular reception position, several iterations of the calibration step for different frequencies so as to perform a frequency sweep over a given frequency range.

[0026] Thus, when the airborne transmitter is in a receiving angular position relative to the transmitter, it is possible to scan a wide range of frequencies and measure calibration data for a multitude of frequencies across a broad frequency range. Consequently, the time and cost associated with calibrating the direction-finding device are reduced compared to current calibration techniques.

[0027] The frequency range can be scanned continuously. Preferably, the frequency range is scanned using a predetermined frequency step. The frequency step can be constant. Alternatively, the frequency step can be different for different frequency ranges within the frequency range. For example, a first frequency step can be used for one frequency range and a second frequency step can be used for a second frequency range. As yet another alternative, the frequency step can be calculated according to a predetermined relationship that takes into account the frequency for which the calibration is performed.

[0028] Depending on preferred embodiments, the different frequencies for which calibration is performed can be predefined and stored in a table.

[0029] In all cases, it is best to synchronize the calibration transmitter and the goniometry device so that the transmitter's transmission frequency is known by the goniometry device at all times.

[0030] Advantageously, the method according to the invention can include several iterations of the calibration step in different angular reception positions, in particular predefined, and in particular following a predetermined calibration trajectory.

[0031] Thus, the airborne carrier can be moved to different angular positions, and for each angular position, at least one iteration of the calibration step can be performed. Preferably, at each angular position, the calibration step can be repeated several times to cover a frequency range, as described above. When calibration measurements have been performed for the entire frequency range, the airborne carrier can be moved to a new receiving angular position. The receiving angular positions can be defined along a calibration trajectory.

[0032] The calibration trajectory can include a multitude of angular positions following a constant or variable angular step. The angular step can be a combination of an azimuth angular step and an elevation angular step, or only one of these steps.

[0033] The calibration trajectory can be defined to cover an azimuth angle range, for example from 0° to 360°.

[0034] Alternatively, or in addition, the calibration trajectory can be defined to cover a range of elevation angles, for example from 0° to 90°, with: 0° corresponds to the horizontal, meaning that the direction between the aircraft carrier and the calibration transmitter is approximately horizontal. This configuration can be achieved by placing the aircraft carrier far from the calibration transmitter; and 90° corresponds to the vertical, meaning that the direction between the aircraft carrier and the calibration transmitter is approximately vertical. This configuration can be achieved by placing the aircraft carrier directly above the calibration transmitter.

[0035] Depending on the embodiment, the calibration trajectory may include any combination of at least one of the following trajectories: at least one horizontal linear trajectory, at least one upward helical trajectory, and / or at least one downward helical trajectory.

[0036] Preferably, the calibration trajectory can be a combination of several of these trajectories to cover a maximum of azimuth and elevation angles in a minimum of time.

[0037] For example, the calibration trajectory may include: starting from a great distance from the transmitter and moving towards the transmitter: a horizontal trajectory followed by an upward helical trajectory until the direction finder is directly above the transmitter; and starting directly above the transmitter and moving away from the transmitter: a downward helical trajectory followed by a horizontal linear trajectory.

[0038] Such a calibration trajectory advantageously allows covering a maximum of angular reception positions in a minimum of time, whether in azimuth or elevation.

[0039] The method according to the invention may advantageously include, for an angular position and a frequency, at least one calculation step by interpolation of calibration data for at least one target polarization, different from orthogonal polarizations, from calibration data measured at said frequency and at said angular position, for said calibration signals.

[0040] Thus, it is possible to obtain calibration data even for polarizations other than those of the calibration signals. To do this, the target polarization is projected onto the orthogonal basis formed by the orthogonal polarizations of the calibration signals. Then, the calibration data measured for each of the orthogonal polarizations are used to calculate the calibration data corresponding to each component of the target polarization in the orthogonal basis. Finally, the calibration data obtained for each component of the target polarization are combined to obtain the calibration data for the target polarization.

[0041] The method according to the invention may further include, for a given angular position and polarization, at least one step of calculating calibration data for at least one target frequency not measured at said angular position and for said polarization, by interpolating calibration data measured for several frequencies at said angular position and for said polarization. Such interpolation is referred to as frequency interpolation in the remainder of the application and takes into account calibration data measured for several calibration frequencies at said angular position and for said polarization.

[0042] Thus, it is possible to obtain calibration data for frequencies for which no calibration data has been measured in flight, using frequency interpolation. Consequently, the calibration method according to the invention reduces flight time and the associated cost for calibrating the direction finder.

[0043] Frequency interpolation can be performed using any known function. For example, frequency interpolation can be performed using the GRIDDATA function in MATLAB.

[0044] Frequency interpolation can be performed during, or after, the in-flight calibration steps of the direction finder device.

[0045] Frequency interpolation can be performed at the goniometry device, at the calibration transmitter, or, preferably, on another device.

[0046] The method according to the invention may further comprise, for a given frequency and polarization, at least one step of calculating calibration data for at least one unmeasured target angular position, by interpolating calibration data measured for several angular positions at said frequency and for said polarization. Such interpolation is referred to as angular interpolation in the remainder of the application.

[0047] Thus, angular interpolation allows for the generation of calibration data for angular positions for which no calibration data has been measured. Consequently, this calibration process reduces the flight time and associated costs for calibrating the direction finder.

[0048] Angular interpolation can be performed using any known function. For example, angular interpolation can be performed using the GRIDDATA function in MATLAB.

[0049] Angular interpolation can be performed during, or after, the in-flight calibration steps of the direction finder device.

[0050] Angular interpolation can be performed at the level of the goniometry device, or at the calibration transmitter, or preferably by another device.

[0051] The goniometry device and the calibration transmitter can communicate with each other through a unidirectional or bidirectional communication channel.

[0052] Such communication can be used for example to synchronize the goniometer device and the calibration transmitter during calibration measurements, in particular to know and adjust the relative positions of the goniometer device and the transmitter, the frequencies of the calibration signals emitted by the transmitter, etc.

[0053] The polarization of each calibration signal can be any one of the following polarizations: vertical linear polarization, horizontal linear polarization, right circular polarization, left circular polarization, etc.

[0054] According to another aspect of the present invention, a calibration system for an airborne goniometry device is proposed, comprising means configured to implement the method according to the invention.

[0055] In particular, the system according to the invention may include: a goniometry device comprising an antenna array, intended to be carried by an aerial carrier, and at least one calibration transmitter, intended to emit at least two calibration signals of orthogonal polarizations in the direction of said goniometry device.

[0056] In particular, the system according to the invention may include, in terms of technical means, all the characteristics described above with reference to the process according to the invention and which are not repeated here in detail for the sake of brevity.

[0057] The calibration transmitter can be placed at ground level. In particular, the signal transmitter can be placed on the ground or on a vehicle.

[0058] Preferably, the calibration transmitter is located in a fixed geographical location.

[0059] In preferred embodiments, the calibration transmitter may comprise a single transmitting antenna with dual orthogonal polarizations, and in particular +45° / -45°. Thus, orthogonal polarization calibration signals can be transmitted with a single antenna.

[0060] Alternatively, the calibration transmitter may comprise two antennas configured, and in particular oriented, according to two orthogonal polarizations.

[0061] In all cases, the calibration transmitter may include at least one generator supplying the antenna(s) with electrical signals corresponding to the radio frequency signals to be emitted.

[0062] Advantageously, the system according to the invention may include a first positioner for modifying, or adjusting, the angular position of the transmitter, and in particular of the transmitting antenna(s).

[0063] The first positioner can be configured to modify or adjust the aiming direction of the transmitting antenna(s) in azimuth and / or elevation.

[0064] The first positioner can be a motorized positioner controlled by at least one control signal provided by a control unit, such as an electronic board, a computer, a calculator, etc. Such a control unit can, for example, be a calibration transmitter control unit.

[0065] Preferably, the first positioner can be commanded so that the aiming direction of the calibration transmitter's transmitting antenna(s) is always directed towards the flight direction finder.

[0066] Advantageously, the system according to the invention may include a second positioner to modify, or adjust, the angular position of the antenna array of the goniometry device.

[0067] The second positioner can be configured to modify or adjust the aiming direction of the antenna array in azimuth and / or elevation.

[0068] The second positioner can be a motorized positioner controlled by at least one control signal supplied by a control unit, such as an electronic board, a computer, a calculator, etc. Such a control unit can, for example, be a control unit for the direction-finding device.

[0069] Preferably, the second positioner can be controlled so that the aiming direction of the antenna array is always pointed by the calibration transmitter.

[0070] In addition, the system according to the invention may include a geolocation module, such as a GPS module, on the side of the goniometer device to locate the position of said goniometer device.

[0071] The position of the goniometer device can be used to ensure that, or to adjust if necessary, the goniometer device is in a receiving angular position for which a calibration data measurement is planned.

[0072] Alternatively or in addition, the position of the goniometry device may be communicated to the calibration transmitter, in order to adjust the orientation of said calibration transmitter, and in particular of the transmitting antenna or antenna of said calibration transmitter, in order to ensure that the aiming position of said calibration transmitter is in the direction of the goniometry device.

[0073] The geolocation module can be a geolocation module for the aircraft carrier. Alternatively, the geolocation module can be a geolocation module dedicated to the direction-finding device, and in particular, be part of the direction-finding device.

[0074] Advantageously, the system according to the invention can further include a module, disposed on the side of the goniometry device, to determine at least one inclination of said goniometry device, and / or of the aircraft carrier.

[0075] This at least one tilt can be used to correct, or adjust, the angular position of the received signal relative to the calibration transmitter. Indeed, the tilt or orientation of the direction-finding device, and especially of the aircraft carrier, can alter the received position of said direction-finding device relative to the calibration transmitter.

[0076] The tilt measurement module can be a module of the airframe. Alternatively, the tilt measurement module can be a module dedicated to the direction-finding device, and in particular, part of the direction-finding device.

[0077] The tilt measurement module can, for example, be an inertial measurement unit measuring one or more tilt angles, along one or more directions.

[0078] The system according to the invention may further include at least one computing unit provided for calculating, by interpolation, calibration data for at least one unmeasured polarization, or an unmeasured frequency or an unmeasured angular position.

[0079] Such a computing unit may be a computer, a calculator, a server, etc. and more generally any data processing device configured to perform such interpolation, for example by means of a computer program designed for this purpose and executed by said computing unit.

[0080] The calculation unit can be integrated into the goniometer. Alternatively, the calculation unit can be integrated into a device, dedicated or not, independent of said goniometer.

[0081] The computing unit can be a physical machine or a virtual machine.

[0082] In addition, the goniometry device and the calibration transmitter can be equipped with communication modules allowing them to communicate with each other.

[0083] Communication between the goniometry device and the calibration transmitter can be achieved through a unidirectional or bidirectional communication channel.

[0084] Communication can in particular be used to synchronize the transmitter and the goniometry device for calibration, especially with regard to their relative positions so as to obtain a given angular reception position, and / or with regard to the frequency of the calibration signals, and more generally to synchronize a calibration sequence. Description of the figures and methods of implementation

[0085] Other advantages and features will become apparent upon examination of the detailed description of a non-limiting embodiment, and the accompanying drawings on which: THE FIGURES 1a and 1b are schematic representations of a non-limiting example of a calibration configuration for an airborne direction-finding device; the FIGURE 2 is a schematic representation of a first, non-limiting embodiment of a calibration system for an airborne goniometry device according to the invention; the FIGURE 3 is a schematic representation of another non-limiting embodiment of a calibration system for an airborne direction-finding device according to the invention; the FIGURE 4 is a schematic representation of a non-limiting example of a calibration trajectory that can be implemented in the present invention; and the FIGURE 5 is a schematic representation of a non-limiting example embodiment of a method according to the invention.

[0086] It is understood that the embodiments described below are by no means exhaustive. In particular, variants of the invention may be conceived comprising only a selection of the features described below, isolated from the other features described, if this selection of features is sufficient to confer a technical advantage or to differentiate the invention from the prior art. This selection includes at least one preferably functional feature without structural details, or with only a portion of the structural details if this portion is sufficient solely to confer a technical advantage or to differentiate the invention from the prior art.

[0087] In the figures, elements common to several FIGURES retain the same reference.

[0088] THE FIGURES 1a and 1bare schematic representations of a non-limiting example of an in-flight calibration configuration of an airborne direction-finding device, respectively from a side view and from a top view.

[0089] THE FIGURES 1a and 1b Figures schematically depict a direction-finding device 102 carried by an aircraft, such as an airplane 104. Typically, the direction-finding device 102 comprises an antenna array (not shown) consisting of several antennas, each measuring, upon reception, an amplitude value and a phase value for a received radio frequency signal. The amplitude and phase values ​​form a complex vector, where the magnitude represents the amplitude value and the angle represents the phase value.

[0090] A calibration transmitter 106, whose position is known, is used to calibrate the airborne direction-finding device 102. The calibration transmitter 106 can be fixed or mobile. For example, the calibration transmitter 106 can be placed on the ground or on a vehicle. For calibration, the calibration transmitter 106 is constantly directed towards the direction-finding device 102.

[0091] The calibration of the direction-finding device 102 is performed as follows. Calibration signals of known frequencies and polarizations are emitted by the calibration transmitter 106 towards the direction-finding device 102 while it is in flight. For each calibration signal received, each antenna in the direction-finding device's antenna array measures a pair of data points {Amplitude, Phase}. This pair of data points measured by each antenna is stored in association with: The calibration signal frequency: this frequency is known; the calibration signal polarization: this polarization is also known; and the reception angle, that is, the relative angular position between the calibration transmitter 106 and the direction-finding device 102. This angular position is also known. Generally, this angular position is expressed as an elevation angle and an azimuth angle of the airborne direction-finding device 102 with respect to the calibration transmitter 106.

[0092] For each angular position of the airborne direction-finding device 102 relative to the calibration transmitter, the calibration step can be repeated for different frequencies, or frequency bands, in order to sweep a whole range of frequencies, as part of a calibration sequence.

[0093] Furthermore, the calibration step can be repeated in several angular reception positions, always within the framework of a calibration sequence.

[0094] Thus, at the end of the calibration process, a calibration table is obtained for a plurality of angular positions, with calibration data measured for a plurality of frequencies for each angular position. This calibration table includes, for each {frequency, angular position} pair, a calibration value for a given polarization.

[0095] As mentioned above, the 102 direction-finding device comprises an antenna array. In this case, the calibration value can, without limitation, be a covariance matrix indicating the differences in reception between the said receiving antennas, that is to say, the differences between the complex vectors measured for each receiving antenna.

[0096] The position of the calibration transmitter 106 relative to the goniometry device 102 can be given by a combination of two angles, namely: an elevation angle, also called a site angle, denoted EL, represented on the FIGURE 1a , corresponding to the angle formed between, on the one hand, the vertical direction 108 between the direction-finding device 102 (i.e., of the aircraft 104) and the ground, and on the other hand, the direction 110 connecting the direction-finding device 102 (i.e., of the aircraft 104) and the calibration transmitter 106; and an azimuth angle, also called bearing, denoted AZ, represented on the FIGURE 1b , which corresponds to the angle, in the horizontal plane, between on the one hand the direction 110 linking the aircraft 104 and the calibration transmitter 106, and on the other hand a reference direction 112, for example magnetic north.

[0097] These angles AZ and EL can be provided by sensors equipping the goniometry device 102 and / sensors equipping the aircraft 104.

[0098] Alternatively, these AZ and EL angles can be calculated from an altitude data and a geolocation data of the aircraft 104, respectively of the direction finder 102, provided by sensors equipping said aircraft 104 or said direction finder 102. Indeed, the geolocation of the calibration transmitter 106 being known, the azimuth and elevation angles can be calculated from the altitude and the geolocation of the direction finder 102 (or of the aircraft 104).

[0099] Each calibration signal emitted by the calibration transmitter 106 can be a burst of signals.

[0100] Thus, during a calibration phase, it is crucial that the direction finder 102 knows the frequency and polarization of each calibration signal emitted by the calibration transmitter 106 at the moment it receives that calibration signal. This preferably requires synchronizing the direction finder 102 and the calibration transmitter 106 so that when the calibration transmitter 106 emits a calibration signal, the direction finder 102 knows the frequency and polarization of said calibration signal in order to store the measured values ​​in association with said frequency and polarization.

[0101] There FIGURE 2 is a schematic representation of a first non-limiting example of a system according to the invention for calibrating an airborne goniometry device according to the invention.

[0102] The 200 system of the FIGURE 2includes the direction-finding device 102 carried by the aircraft carrier 104, and the calibration transmitter 106 in the configuration described with reference to FIGURES 1a and 1b .

[0103] In the system 200, the calibration transmitter 106 includes a dual-polarization antenna 202. Specifically, the dual-polarization antenna 202 consists of a first radiating element 204 and a second radiating element 206. Both radiating elements 204 and 206 are directed towards the direction-finding device 102 along the direction 110 and are perpendicular to each other. For example, the first radiating element 204 is inclined at an angle of -45° around the direction 110, and the second radiating element 206 is inclined at an angle of +45° around the direction 110. Thus, in operation, the radiating element 204 emits a calibration signal 208 with polarization POL1 perpendicular to the polarization POL2 of a calibration signal 210 emitted by the second element 206.

[0104] The calibration signals 208 and 210, with perpendicular polarizations POL1 and POL2, are received by the antenna array of the direction-finding device 102 carried by the aerial platform 104. For each calibration signal 208 and 210, each antenna in the array measures an amplitude value and a phase value forming a complex vector. Thus, for each calibration signal, the antenna array measures a dataset comprising, for each antenna, an amplitude value and a phase value forming a complex vector. This dataset is stored in association with: the known frequency of the calibration signal, the known polarization of the calibration signal, and the known angular position of the goniometry device 102 relative to the transmitter 106, this angular position being given by a pair of azimuth angle AZ and elevation angle EL and corresponding to the angle of reception of the calibration signal.

[0105] In system 200, the calibration transmitter 106 further includes a calibration signal generator 212 that feeds the dual-polarization antenna 202. For each calibration signal 208 and 210, the generator 212 produces an electrical signal representative of said calibration signal. This electrical signal is supplied to the dual-polarization antenna, specifically to the relevant radiating element of the dual-polarization antenna 202, which then emits the calibration signal 208 or 210.

[0106] According to one embodiment, the two calibration signals 208 and 210 can be emitted alternately. In this case, the two calibration signals 208 and 210 can be of the same frequency, namely the frequency at which the direction finder instrument is to be calibrated.

[0107] In another embodiment, the two calibration signals can be emitted simultaneously. In this case, it is necessary to discriminate between the two calibration signals upon reception. To do this, the two calibration signals 208 and 210 can have a slight frequency shift, denoted Δ, of negligible value compared to the frequency of these signals, allowing these signals to be discriminated upon reception by the direction-finding device. For example, if the direction-finding device is to be calibrated to a frequency F, one of the calibration signals can have a frequency of F+Δ / 2, and the other calibration signal can have a frequency of F-Δ / 2. In yet another embodiment, one of the calibration signals can have a frequency of F, and the other calibration signal can have a frequency of F-Δ, or F+Δ.The value of the frequency shift Δ can be equal to, and in particular must be at least equal to, the frequency separating power of the goniometry device.

[0108] Optionally, but particularly advantageously, the calibration transmitter 106 can also include a control module 214 allowing the frequency of the calibration signals 208 and 210 to be changed. Thus, for a relative angular position between the goniometry device 102 and the calibration transmitter 106, it is possible to perform a calibration for several frequencies, and preferably for a whole range of frequencies.

[0109] Optionally, but particularly advantageously, the calibration transmitter 106 may also include a positioner 216 for changing the orientation of the dual-emitting antenna 202 around at least one direction. Specifically, the positioner 216 can be configured to change / adjust the aiming direction of the dual-polarization antenna 202 so that it is always pointed towards the direction-finding device 102 during calibration. For example, the positioner 216 can be configured to change the aiming direction of the dual-polarization antenna 202 in the horizontal and vertical planes, i.e., the azimuth and elevation angles of the aiming direction of the dual-polarization antenna 202. The positioner 216 can be a motorized platform controllable by the control module 214.

[0110] Optionally, but particularly advantageously, the calibration transmitter 106 may also include a communication module (not shown) with the goniometry device 102, to synchronize the calibration transmitter 106 with said goniometry device 102 during calibration.

[0111] Optionally, but particularly advantageously, the system 200 can include a positioner 218 for modifying the orientation of the direction-finding device 102, and in particular its antenna array. Such a positioner 218 allows for the correction of orientation drifts due, for example, to the tilt of the aircraft carrier 104, ensuring the relative angular position of the direction-finding device 102 and the transmitter 106. The positioner 218 can be configured to modify / adjust the aiming direction of the direction-finding device 102's antenna array so that it is always pointed towards the calibration transmitter 106. For example, the positioner 218 can be configured to modify the aiming direction of the antenna array in the horizontal and vertical planes, i.e., the azimuth and elevation angles of the array's aiming direction. antennary.The positioner 218 can, for example, be a motorized platform. The positioner 218 can be controlled, for example, by a control module 220 of said direction-finding device 102 according to orientation data measured, for example, by an inertial measurement unit (not shown) associated with said direction-finding device 102 or with the aerial carrier 104.

[0112] Optionally, but particularly advantageously, the system 200 can also include a communication module enabling the goniometry device 102 to communicate with the calibration transmitter 106, in order to synchronize said calibration transmitter 106 with said goniometry device 102 during calibration.

[0113] Optionally, but with particular advantage, the 200 system can include a geolocation module (not shown) to detect the position of the direction finder 102 during calibration. Such a geolocation module can be associated with, or integrated into, the direction finder 102 or the aerial carrier 104.

[0114] Thus, the system according to the invention allows, for a given calibration frequency F, to measure, in a single step (or in a single pass through an angular position), calibration data for two vertical polarizations POL1 and POL2 at this frequency.

[0115] Once the calibration data have been measured for these orthogonal polarizations POL1 and POL2, it is then possible to determine, by calculation, calibration data for all possible polarizations of a radio frequency wave at the same frequency F, since all polarizations can be decomposed onto the orthogonal basis formed by the POL1 and POL2 polarizations. Ultimately, the system according to the invention makes it possible, for a given calibration frequency F, to determine, in a single step (or in a single pass through an angular position), calibration data for all possible polarizations of a radio frequency wave at the frequency F.

[0116] Optionally, but particularly advantageously, the system 200 can include a computing unit 222 to calculate the calibration data for a radio frequency wave of frequency F and polarization POL3, different from polarizations POL1 and POL2, from the calibration data measured at frequency F for polarizations POL1 and POL2. To do this, polarization POL3 is projected onto the orthogonal basis formed by the orthogonal polarizations POL1 and POL2. Then, the calibration data measured for each of the polarizations POL1 and POL2 are used to calculate, by interpolation, the calibration data corresponding to each component of the polarization POL3. Finally, the calibration data obtained for each component of the polarization POL3 are combined to obtain the calibration data for the polarization POL3 at frequency F.

[0117] Optionally, but with particular advantage, the 222 calculation unit can also be configured to calculate calibration data for at least one unmeasured frequency Fint at said POS angular position and POL polarization, for a given POL polarization and POS angular position. This calculation is performed by frequency interpolation of calibration data measured for several frequencies at said POS angular position and POL polarization. Frequency interpolation can be performed using any known function. For example, the GRIDDATA function can be used. This allows calibration data to be obtained even for frequencies for which no calibration data has been measured in flight.

[0118] Optionally, but with particular advantage, one of the 222 calculation units can be further configured to calculate, for a given frequency F and polarization POL, calibration data for at least one unmeasured angular position POS int, by angular interpolation of calibration data measured for several angular positions at said frequency F and polarization. Angular interpolation can be performed by any known function. For example, angular interpolation can be performed by the GRIDDATA function. Thus, it is possible to obtain, by calculation, calibration data even for angular positions for which calibration data have not been measured in flight.

[0119] In the example of the FIGURE 2A single computing unit 222 is used to perform all the interpolations described. Of course, it is possible to use a dedicated, individual computing unit for at least one, in particular each of the interpolations described.

[0120] The computing unit 222 can be located on the side of the direction-finding device 102, and in particular be integrated into the direction-finding device 102. Alternatively, the computing unit 222 can be located on the side of the calibration transmitter 106, and in particular be integrated into the calibration transmitter 106. According to yet another alternative, the computing unit 222 can be located at another site, and be in the form of a physical or virtual machine, integrated or not into another physical device.

[0121] The computing unit 222 could be a computer, a calculator, a server, a programmable chip, etc.

[0122] There FIGURE 3is a schematic representation of a second non-limiting example of a system according to the invention for in-flight calibration of an airborne direction-finding device according to the invention.

[0123] The 300 system of the FIGURE 3 includes all elements of the 200 system of the FIGURE 2 except for the following differences.

[0124] In the 300 system, the calibration transmitter comprises not a dual-polarization antenna but two separate antennas, 302 and 304, perpendicular to each other around the 110 direction. Each of the antennas 302 and 304 is fed by a calibration signal generator, respectively 306 and 308, individual and dedicated to said antenna.

[0125] According to other versions not shown of a system according to the invention, it is possible to provide a single signal generator common to both antennas 302 and 304.

[0126] There FIGURE 4is a schematic representation of a non-limiting example embodiment of a calibration trajectory that can be used in the present invention.

[0127] The goniometric device 102 on board the air carrier 104 can be moved to perform calibration measurements in several relative angular positions between said goniometric device 102 and the calibration transmitter 106, i.e. for several reception angles.

[0128] To achieve this, the 102 direction finder can be moved in flight along a calibration trajectory comprising a multitude of angular positions, using a constant or variable angular step. The angular step can be a combination of an azimuth angular step and an elevation angular step, or only one of these steps.

[0129] There FIGURE 4gives a non-limiting example of such a calibration trajectory. Calibration trajectory 402, shown on the FIGURE 4 For example, it allows you to cover a maximum of azimuth and elevation angles in a minimum amount of time.

[0130] The 402 calibration trajectory includes: starting from a great distance from the calibration emitter 106 and going towards said calibration emitter 106: a horizontal trajectory 404 followed by an upward helical trajectory 406 until it is directly above and above the calibration emitter 106; and starting directly above the calibration emitter 106 and moving away from the calibration emitter 106: a downward helical trajectory 408 followed by a horizontal linear trajectory 410.

[0131] Of course, this calibration trajectory is by no means limiting and other calibration trajectories may be used within the framework of the present invention.

[0132] There FIGURE 5 is a schematic representation of a non-limiting example embodiment of a method according to the invention for calibrating an airborne goniometry device.

[0133] The 500 process of the FIGURE 5 can be implemented by a system according to the invention, and in particular by any one of the systems 200 or 300 of the FIGURES 2 Or 3 .

[0134] The 500 procedure includes a step 502 for calibrating the direction-finding device when it is mounted on an aircraft. Step 502 is performed for a given angular position and frequency.

[0135] Step 502 includes a step 504 of emission of at least two calibration signals, of orthogonal polarizations, by a calibration transmitter directed towards the flight direction-finding device.

[0136] Step 502 then includes a step 506 of measurement, by each antenna of the antenna array of the goniometry device, of an amplitude data and a phase data, for each calibration signal.

[0137] In one embodiment, steps 504 and 506 are performed simultaneously because the calibration signals are emitted simultaneously. In another embodiment, steps 504 and 506 are performed first for one of the calibration signals, for example the POL1 polarization calibration signal 208, and then for the other calibration signal, for example the POL2 polarization calibration signal 210.

[0138] During a step 508, the data measured by each antenna of the antenna array are stored in association with the calibration signal frequency, the calibration signal polarization, and the relative angular position of the calibration transmitter with respect to the goniometry device.

[0139] In cases where calibration involves a frequency range, procedure 500 includes a step 510 of changing the frequency of the calibration signals, and a further iteration of calibration step 502 is performed. Steps 502 and 510 are repeated in this manner to perform a frequency sweep of the frequency range. The frequency sweep can, for example, be performed using a constant or variable frequency step, depending on the frequencies involved.

[0140] Once the entire range of calibration frequencies has been swept, the goniometry device is moved, in step 512, so that it is positioned in a new angular position, for example along a predetermined calibration trajectory, for example trajectory 402 of the FIGURE 4 In practice, the frequency range is swept very quickly so that the air carrier is not stopped in a given position and travels the calibration trajectory without stopping.

[0141] When the airborne instrument, and in particular the direction-finding device, is in a new angular position, steps 502-510 are repeated. The angular position of the direction-finding device is then changed again for a new iteration of steps 502-510, and so on to scan a range of angular positions, preferably along a calibration trajectory. The scanning of angular positions can, for example, be performed with a constant angular step, or with a variable angular step depending on the position of the direction-finding device relative to the calibration transmitter.

[0142] After scanning the range of angular positions, the direction-finding device no longer needs to be in flight. The aircraft can therefore land.

[0143] The process 500 may optionally include a calibration data interpolation step 514 comprising any combination of the following interpolation steps.

[0144] For example, interpolation step 514 may include a step 516 of interpolating calibration data for at least one unmeasured polarization POL int at an angular position POS and a frequency F, from calibration data previously measured for the polarizations of the calibration signals at that angular position POS and frequency F. To do this, the polarization POL int is projected onto the orthogonal basis formed by the orthogonal polarizations of the calibration signals, for example, POL1 and POL2. Then, the calibration data measured at that frequency F and angular position POS for each of the polarizations POL1 and POL2 are used to calculate the calibration data corresponding to each component of the polarization POL int.Finally, the calibration data obtained for each component of the POL int polarization are recomposed to obtain the calibration data of the POL int polarization, at the frequency F.

[0145] The interpolation step 514 may include a step 518 of interpolating calibration data for at least one unmeasured frequency Fint at a given angular position POS and polarization POL, from calibration data previously measured or calculated for other frequencies at that angular position POS and for the same polarization POL. This frequency interpolation step 518 may be performed by any interpolation function, for example GRIDDATA, taking as input the previously measured or calculated calibration data.

[0146] The interpolation step 514 may further include a step 520 of interpolating calibration data for at least one unmeasured angular position POS int at a frequency F and polarization POL, from calibration data previously measured or calculated for other angular positions at that frequency and for the same polarization POL. This angular interpolation step 520 may be performed by any interpolation function, for example GRIDDATA, taking as input the previously measured or calculated calibration data.

[0147] The process 500 may optionally include a step 522 for calculating a calibration quantity for at least one angular position POS, a frequency F, and a polarization POL, based on calibration data measured or calculated for each antenna in the antenna array. In one exemplary embodiment, this calibration quantity may be a covariance matrix between the measured / calculated receive data for each antenna in the antenna array for that POS position, frequency F, and polarization.

[0148] Of course, the invention is not limited to the detailed examples given above by way of illustration, and the general scope of the invention is defined in the claims.

Claims

1. A method (500) for calibrating at low frequency and in-flight a goniometry apparatus (102) comprising an antenna array, on board an air carrier (104), said method (500) comprising for an angular position of reception, a step (502) for calibrating said airborne goniometry apparatus (102) at a given frequency, comprising the following operations: - transmitting (504), by means of a calibration transmitter, at said given frequency and in the direction of said goniometry apparatus (102), at least two calibration signals (208, 210), with polarizations (POL1, POL2) orthogonal to each other, where low frequency radiofrequency signals have frequencies equal to, or lower than, 500 MHz, and - measuring (506) a response of said antenna array for each of said signals.

2. The method (500) according to the preceding claim, characterized in that the calibration signals (208, 210) are transmitted simultaneously and have a frequency shift therebetween that is negligible with respect to their frequency, and in particular of 200 kHz.

3. The method (500) according to any one of the preceding claims, characterized in that it comprises, for the same angular position of reception, several iterations of the calibration step (502) for different frequencies so as to perform a frequency scan over a given range of frequency.

4. The method (500) according to any one of the preceding claims, characterized in that it comprises several iterations of the calibration step (502) in different angular positions of reception, following a predetermined calibration path (402).

5. The method (500) according to claim 2, characterized in that the calibration path (402) comprises any combination of at least one of the following paths: - at least one horizontal linear path (404, 410), - at least one upward helical path (406), and / or - at least one downward helical path (408).

6. The method (500) according to any one of the preceding claims, characterized in that it comprises, for an angular position and a frequency, at least one step (516) for calculating by interpolation calibration data for at least one target polarization, which is different from orthogonal polarizations, based on calibration data measured at said frequency and at said angular position.

7. A system (200;300) for calibrating an airborne goniometry apparatus (102) comprising: - a goniometry apparatus (102) comprising an antenna array, intended to be on board an aerial carrier (104), and - at least one calibration transmitter (106), intended to transmit at least two calibration signals (208, 210) with orthogonal polarizations (POL1, POL2) in the direction of said goniometry apparatus (102); configured to implement all the steps of the method (100) according to any one of the preceding claims.

8. The system (200) according to the preceding claim, characterized in that the calibration transmitter (106) comprises a single dual orthogonal polarization transmission antenna (202), and in particular +45° / -45°.

9. The system (200; 300) according to any one of claims 7 or 8, characterized in that it further comprises a first positioner (216) for modifying the look direction of the calibration transmitter (106), in particular in azimuth and / or in elevation.

10. The system (200; 300) according to any one of claims 7 to 9, characterized in that it further comprises a second positioner (218) for modifying the look direction of the antenna array, in particular in azimuth and / or in elevation.

11. The system (200; 300) according to any one of claims 7 to 10, characterized in that it further comprises a geolocation module on the goniometry apparatus (102) side to locate the position of said goniometry apparatus (102).

12. The system (200; 300) according to any one of claims 7 to 11, characterized in that it further comprises a module, disposed on the goniometry apparatus side, to determine at least one tilt of said goniometry apparatus (102) and / or of the aerial carrier (104).

13. The system (200; 300) according to any one of claims 7 to 12, characterized in that it further comprises at least one calculation unit (222) intended to calculate, by interpolation, calibration data for at least one unmeasured polarization, or an unmeasured frequency or an unmeasured angular position.

14. The system (200; 300) according to any one of claims 7 to 13, characterized in that the goniometry apparatus (102) and the calibration apparatus (106) are equipped with communication modules enabling them to communicate with each other.