Aeronautical radio direction-finding pod and method

Abstract

The invention relates to an aeronautical radio-direction finding pod 1 comprising a frame 11, a frame support member 12 for attachment to an aircraft 50 along a main force axis, and an interferometer for receiving electromagnetic waves in a band selected from among one or more of the P, L, S, C, X, Ku, K and Ka radar bands, wherein the interferometer comprises at least two antennas supported by the frame 11 and having a radiation pattern with an opening at -3 dB of at least 30° centred on a line-of-sight axis, and, beyond 80° from the line-of-sight axis, a level at least 15 dB lower than that of the radiation maximum.

Description

Description Title of the invention: Aeronautical radiogoniometry pod and method for implementing the pod [1] |The invention relates to the field of airborne detection of radioelectric emitting sources, in particular radar. [2] The precise location of a radio wave transmitter is a need expressed in multiple circumstances, such as spectrum monitoring in an environment where the spectral resource is required by an increasing number of users and applications. [3] Carrying a radio direction finding device on an aircraft has significant advantages over ground installation: greater range, reduction of multi-path effects due to relief or surface area, i.e. vegetation and buildings, rapid movement allowing estimation of the distance of a stationary transmitter by a single device, where static triangulation would require two synchronized ground installations. [4] Phase direction finding or interferometry requires at least two antennas, whose phase difference of the received signals is linked to the angle between the direction of arrival of the wave and the line defined by the phase center of the two antennas. [5] Aircraft specializing in the collection of electromagnetic intelligence thus have multiple antenna arrays to cover the frequency bands and polarizations of interest, and whose installation is optimized to cover a wide angular sector, while being insensitive to the interactions of incident waves with the structure of the carrier, which would disturb the measurement of phase differences. Specialized aircraft have various protrusions, such as radomes protecting these antenna arrays. Such specialized aircraft are expensive to acquire and operate, and available in small numbers. [6] Such an optimized implementation is incompatible with a device that is easy to assemble and disassemble, portable on different types of aircraft depending on availability and the type of mission. [7] The Applicant identified a need for airborne detection equipment for radio-emitting sources, carried by an aircraft in a removable manner. [8] The invention improves the situation. [9] According to one aspect of the invention, an aeronautical radiogoniometry pod has a longitudinal axis or roll axis, a transverse axis or pitch axis and a depth axis or yaw axis corresponding to a line of sight. Said pod comprises a frame, a support member of the frame for attachment to an aircraft along a main force axis or depth axis by lifting, an interferometer for receiving electromagnetic waves of a band chosen from one or more of the radar bands P, L, S, C, X, Ku, K and Ka, the interferometer comprising at least two antennas, the antennas being supported by the frame, arranged under the frame and the support member of the frame, and having a radiation pattern with an aperture at -3 dB of at least 30° centered on the line of sight and directed away from the frame and the support member of the frame,and beyond 80° from the boresight a level at least 15 dB lower than that of the maximum radiation or a radiation pattern with an aperture at -3 dB of at least 60° centered on a boresight and beyond 80° from the boresight a level at least 10 dB lower than that of the maximum radiation, to allow orientation of the boresight of the antennas towards at least one area of interest.,

[0010] The pod can be carried by a light aircraft, a drone, a heavy aircraft or a helicopter. The pod can be removed at the end of the mission, making pod maintenance easier and independent of that of the aircraft. The aircraft follows a simple flight plan and the pod benefits from excellent operating conditions. The aircraft is inclined so that the useful observation area is in the main radiation lobe of the antennas and outside the areas of interaction with the carrier structure. An analysis of the evolution of the bearings angular during the turn associated with the inclination of the carrier is carried out in correlation with the evolution of its heading.

[0011] In a particularly economical embodiment, the antennas are arranged in pairs for a given frequency band.

[0012] In another embodiment, the antennas are arranged in triplet for a given frequency band. Ambiguities are reduced.

[0013] In one embodiment, the device comprises an S and C band electromagnetic wave receiving interferometer and an X and Ku band electromagnetic wave receiving interferometer. The cost is reduced.

[0014] In one embodiment, the device comprises an S-band electromagnetic wave receiving interferometer, a C-band electromagnetic wave receiving interferometer, an X-band electromagnetic wave receiving interferometer and, if applicable, a Ku-band electromagnetic wave receiving interferometer. Direction finding processing is facilitated.

[0015] In one embodiment, the device comprises an envelope transparent to said electromagnetic waves, the envelope surrounding the frame and the antennas. The antennas are arranged inside the envelope for their protection and to avoid degrading the drag.

[0016] In one embodiment, the antennas have a -3 dB aperture radiation pattern of at least 60° centered on a boresight axis and beyond 80° from the boresight axis a level at least 10 dB lower than that of the radiation maximum. The antennas provide a high quality signal. The phase center of the antennas remains stable in the angular sector of observation in which transmitters are detected.

[0017] In one embodiment, the antennas of an interferometer of a first frequency band are nested within the antennas of an interferometer of a second frequency band, the second frequency band being lower than the first frequency band. The pod is compact.

[0018] In one embodiment, the device comprises a signal receiver connected to said antennas of at least one interferometer, the receiver comprising a a device for separating the received signal into radar pulses, the pod further comprising a device for selecting radar pulses from the receiver, and for calculating a phase difference between the antennas of one of the at least one interferometer, an angular bearing calculator based on the evolution of the phase difference from the phase difference calculator and gyroscopic information. The angular bearing is precise.

[0019] In one embodiment, the device includes a gyroscopic unit configured to provide a georeferenced angular bearing.

[0020] In one embodiment, the device comprises an inertial unit configured to provide a geolocation of a source of a received transmission. The geolocation is precise.

[0021] In one embodiment, a method for implementing the above pod, during a flight of an aircraft carrying said pod, comprising tilting the aircraft to orient the line of sight of the antennas towards at least one area of interest, and eliminating angular ambiguities from the interferometry by comparing the variation in angular bearings during the turn associated with the tilt of the aircraft with the variation in gyroscopic information. Generally, waves from distant transmitters are contained in a plane close to the horizontal, and their interaction with the wing of the aircraft if it flies at a low tilt occurs and degrades the expected goniometric accuracy.Thus, in one embodiment, the aircraft, in order to carry out a goniometric measurement, enters a steep turn, which, with antennas with low side lobes in the plane of the wing, significantly reduces the interaction of the waves received with the wing and restores the goniometric precision.

[0022] In one embodiment, the pod is passive, particularly in the frequency bands of said antennas.

[0023] In one embodiment, the pod is devoid of an emissive member in the P, L, S, C, X, Ku, K and Ka radar bands.

[0024] In one embodiment, the pod is provided with a remote data link for transmitting telemetry.

[0025] In one embodiment, the pod is removable.

[0026] In one embodiment, the antennas of an interferometer are preferably mounted on a pod having standardized interfaces for fixing under the wing or under the fuselage, and where appropriate connectors allowing access to the on-board electrical network and exchanging information with the avionics and / or the crew by wire.

[0027] In one embodiment, the pod includes a gyroscopic power unit.

[0028] In one embodiment, the pod includes an inertial unit.

[0029] Other characteristics and advantages of the invention will appear on examining the detailed description below, and the appended drawings, in which:

[0030] [Fig.1] schematically illustrates in front view an aeronautical radio direction finding pod according to one aspect of the invention mounted under an aircraft in a steep left turn.

[0031] [Fig.2] schematically illustrates in front view an aeronautical radio direction finding pod according to one aspect of the invention mounted under an aircraft in a steep right turn.

[0032] [Fig.3] schematically illustrates in longitudinal section an aeronautical radiogoniometry pod according to one aspect of the invention.

[0033] [Fig.4] schematically illustrates in cross-section an aeronautical radio direction finding pod according to one aspect of the invention.

[0034] [Fig.5] is a block diagram of an aeronautical radio direction finding pod according to one aspect of the invention.

[0035] [Fig.6] is a block diagram of an aeronautical direction finding pod according to another aspect of the invention.

[0036] [Fig.7] represents curves illustrating the evolution of five angular bearings likely to correspond to the same phase differences of a signal coming from a transmitter at a long distance.

[0037] The attached drawings may not only serve to complete the invention, but also contribute to its definition, where appropriate.

[0038] The pod has a longitudinal axis that can correspond to the longitudinal axis of the aircraft or roll axis. The pod has a transverse axis that can correspond to the transverse axis of the aircraft or pitch axis, the wings of the aircraft being able to be oriented along the transverse axis or at an angle relative to the transverse axis. The pod has an elevator axis that can correspond to the elevator axis of the aircraft or yaw axis corresponding to a sight axis. The elevator axis can be vertical when the aircraft is flying at zero bank.

[0039] The invention relates to an aeronautical radio direction finding pod 1. A pod 1 is a device mechanically linked in a removable manner under an aircraft 50. The pod 1 is attached to the aircraft 50 by lifting, in particular along the depth axis. The aircraft 50 is capable of flying with or without the pod 1. The pod 1 can be attached to the aircraft 50, and conversely separated from the aircraft 50, on an airstrip or a terrain allowing takeoff. For this purpose, the aircraft 50 can be equipped with a load attachment device, for example of the type described in FR2968275, arranged under the fuselage or under a wing. The aircraft 50 can be piloted, autopiloted or remotely piloted. The aircraft 50 can be an airplane, for example a light propeller airplane, a drone or a helicopter.

[0040] The pod 1 comprises a frame 11, for example in the form of a beam elongated along the longitudinal axis. The pod 1 comprises a mechanical interface and an electrical interface with the aircraft 50 forming a support member 12 of the frame. The support member 12 of the frame is arranged above the frame. The mechanical interface may comprise at least one lifting ring and at least one support surface. The electrical interface may comprise a connector for supplying the pod 1 with electrical energy from the aircraft 50. The support member 12 of the frame may comprise a pneumatic interface for supplying the pod 1 with pneumatic energy from the aircraft 50. The support member 12 of the frame may comprise an electronic interface with the aircraft 50 for data communication between the aircraft 50 and the pod 1, for example by wire.

[0041] The pod 1 may comprise an envelope 13 transparent to electromagnetic waves. The envelope 13 surrounds the frame 11 and the other organs of the pod 1 to protect them and reduce aerodynamic drag. The envelope 13 is supported by the frame 11.

[0042] The pod 1 comprises at least one interferometer 2, 3, 4, 5 or group of antennas for receiving electromagnetic waves of a band chosen from one or more of the radar bands P, L, S, C, X, Ku, K and Ka. The group of antennas comprises at least two antennas. The antennas are supported by the frame 11. The antennas are arranged in the casing 13. The casing 13 surrounds the frame 11 and the antennas, as well as, where appropriate, other components. The antennas may be distributed along the longitudinal axis. Within the casing, the interferometers 2, 3, 4, 5 are arranged in the lower position, the frame 11 is arranged in the upper position. The support member 12 of the frame may project above the casing 13.

[0043] According to embodiments, the pod 1 comprises one, two or more groups of antennas. Each group of antennas is dedicated to one, two or three of the frequency bands. For example, the pod 1 comprises:

[0044] - a group of S-band antennas.

[0045] - a group of C-band antennas.

[0046] - a group of X-band antennas.

[0047] - a group of Ku band antennas.

[0048] - a group of Ka-band antennas.

[0049] - a group of S-band antennas and a group of C-band antennas.

[0050] - a group of X-band antennas and a group of Ku-band antennas.

[0051] - a group of P-band antennas and a group of L-band antennas.

[0052] - a group of S-band antennas, a group of C-band antennas and a group of X-band antennas.

[0053] - a group of S-band antennas, a group of C-band antennas, a group of X-band antennas and a group of Ku-band antennas as shown in Figure 3 which represents antenna positions constituting four interferometers 2, 3, 4, 5 with two antennas. The interferometers 2, 3, 4, 5 are nested while maintaining approximately the same length expressed in wavelengths at the central frequency of each of the bands. In other words, the distance between two antennas of each interferometer 2, 3, 4, 5 is equal to the product of the wavelength of the central frequency of the band of said interferometer 2, 3, 4, 5 and a constant coefficient for all the interferometers 2, 3, 4, 5 of the pod 1. Specifically, the antennas of the Ku-band interferometer are arranged between the antennas of the X-band interferometer. The antennas of the X-band interferometer are arranged between the antennas of the C-band interferometer. The antennas of the C-band interferometer are arranged between the antennas of the S-band interferometer. The antennas of the long-wavelength interferometer are arranged around the antennas of the shorter-wavelength interferometer.The goniometric measurement obtained by the antennas of an interferometer 2, 3, 4, 5 is all the more precise as the measurement of the differential phase between the signals received by the antennas is precise and the spacing between said antennas is high. However, increasing the spacing between said antennas creates angular ambiguities, several directions of incidence of the waves being likely to produce the same phase difference. The angular ambiguities can be eliminated by comparing the evolution of the possible directions of incidence of the waves during a turn, in particular for a stationary or quasi-stationary source.

[0054] Each antenna has a radiation pattern with an aperture at -3 dB of at least 30° centered on a boresight. The boresight is usually the elevator axis. The boresight is oriented downward. The boresight is oriented away from the support member 12 of the frame. The risk of electromagnetic interference with the support member 12 of the frame is reduced. The boresight is oriented away from the frame 11. The risk of electromagnetic interference with the frame 11 is reduced. Consequently, the risk of electromagnetic interference with the carrier aircraft is also reduced. The antennas of a group of antennas are identical. The antennas of a group of antennas have a common boresight. The radiation pattern may advantageously have an aperture at -3 dB greater than 60°. In the example illustrated in Figures 1 and 2, the aperture of the radiation pattern is 60°. A tilt of the aircraft 50 of 60° in roll makes it possible to scan an area between the horizon at 0° and -60° under the pod 1, i.e. between +30° and +90° relative to the vertical. The radiation pattern of each antenna has, beyond 80° from the line of sight, a level at least 10 dB lower than that of the maximum radiation. This level can advantageously be more than 15 dB lower than that of the maximum radiation. The risk of electromagnetic interference with the support member 12 of the frame and the frame 11, as well as with the carrier aircraft, is even more reduced.

[0055] The pod 1 comprises a receiver 14. The receiver 14 is arranged in the pod 1, inside the casing 13. The receiver 14 can be fixed to the frame 11. The receiver 14 receives the signals provided by the antennas of said at least one interferometer 2, 3, 4, 5. The antennas are positioned in the lower zone of the pod 1, in particular on the lower face of the casing 13. The receiver 14 is positioned in the upper zone of the pod 1. The receiver 14 is in contact with a lower surface of the frame 11. The receiver 14 can be arranged above the interferometers 2, 3, 4, 5.

[0056] In the embodiment illustrated in Figure 4, the antennas are connected by coaxial cables 15 to the receiver 14. The receiver 14 is multi-channel. The receiver 14 has a number of channels at least equal to the number of antennas in a group of antennas. These channels are coherent with each other, using in the embodiment illustrated in Figure 5 the same reference oscillator 16.

[0057] In the embodiment illustrated in Figure 5, the pod 1 comprises a multi-channel signal receiver 14. Each channel of the multi-channel signal receiver 14 is connected to an antenna of a group of antennas. The receiver 14 makes it possible to detect and analyze the emissions picked up by the antennas. The receiver 14 comprises a time and frequency separation device for the received signal into radar pulses. A computer selects the radar pulses of interest from the receiver 14. For the selected radar pulses, the computer calculates the phase differences between channels of the same group of antennas, then calculates the evolution of said phase differences. The computer translates said phase differences into angular bearings. snapshots in a reference frame. Pod 1 includes a gyroscopic unit 17 providing gyroscopic information. The gyroscopic information on the evolution of the reference frame during the turn of the aircraft 50 is used by the computer to carry out a tracking of the angular bearings which eliminates ambiguous bearings.

[0058] The 14-channel multi-channel receiver can be permanently connected to all the antennas, the reception band of each channel then being the radar band(s) of the corresponding group of antennas.

[0059] The 14-channel multi-channel receiver can also be connected via switches to the antennas of each antenna group. The 14-channel multi-channel receiver can then assign a time range to each antenna group. The number of channels in the 14-channel multi-channel receiver is then reduced, but each channel then has a reception band that covers the radar bands of said antenna groups.

[0060] The multi-channel receiver 14 is connected to a clock 16. The clock 16 ensures the synchronization of the channels of the multi-channel receiver. The receiver 14 comprises a device for separating the signal received into radar pulses. The signals amplified and filtered by the multi-channel receiver 14 are advantageously digitized. The downstream processing is carried out by a computer or by separate calculation modules. The downstream processing is described below in the simplest configuration of a group of two antennas. The downstream processing comprises a separation of the signals received into radar pulse trains 18, a phase difference calculation 19, a calculation of angular bearings 20, and a calculation of georeferenced angular bearing 21.

[0061] The separation of the received signals into radar pulse trains 18 is also called deinterleaving. Deinterleaving is usually based on a characterization in level, duration, central frequency, possible modulation, time of appearance of each pulse. These characteristics also allow, by comparison with a reference library, to make a selection of the radar emissions of interest. Said selection can be made by an algorithm, possibly supervised by a human operator.

[0062] For selected radar emissions, a phase difference calculation 19 is performed between pulses received at the same time on both antennas.

[0063] The interferometer formula well known to those skilled in the art which links the direction of origin of an emission and the phase difference between antennas is inverted by the following processing, namely the calculation of angular bearings 20. The calculation of angular bearings provides the different possible angular bearings.

[0064] The following processing is the calculation of georeferenced angular bearing 21. The calculation of georeferenced angular bearing 21 uses gyroscopic information from the gyroscopic unit 17. which makes it possible to carry out an angular tracking of the different possible bearings, in particular using Kalman filters, to identify a direction of origin in a georeferenced reference frame.

[0065] During a flight of the aircraft 50 equipped with the pod 1, a tilting of the aircraft 50 around a roll axis of the aircraft 50 orients the main lobe of the antenna groups towards at least one area of interest, see figure 7. The tilting contributes to producing a turn of the aircraft. The angular ambiguities of the interferometry are eliminated by the trajectory of the pod 1 which makes possible a comparison of the evolution of the possible directions during the turn.

[0066] Deinterlacing of the received signals can be carried out, for example, using the technique of Manon Mottier, Gilles Chardon, and Frédéric Pascal. Deinterlacing and classification of RADAR signals based on optimal transport distances. XXVIIIth Francophone Symposium on Signal and Image Processing GRETSI 2022, Sep 2022, Nancy, France, hal-03851279

[0067] Figure 7 represents, during the 30° turn of a pod 1 comprising an interferometer with 2 antennas separated by 4 wavelengths, after compensation by the gyrometric information associated with this turn, the evolution of the 5 angular bearings likely to correspond to the same phase differences produced by a transmitter at great distance. The true angular bearing corresponds to that whose compensated evolution remains identically zero, the other 4 correspond to ambiguities or ambiguous angular bearings. In in other words, an ambiguity exhibits a variation during the turn allowing it to be distinguished from the true angular bearing.

[0068] In addition, geolocation of a stationary or quasi-stationary transmitter can be performed. For this purpose, the aircraft 50 can perform two or more successive angular bearings for the purpose of triangulation, for which a geolocation member may usefully be an inertial unit. In the embodiment of FIG. 6, the pod comprises a gyroscopic unit 23 to enable the geolocation calculation 22. If the transmitter is at a short distance, said distance can be estimated during a single turn by a supplement to the analysis of the variation in the angular bearing during the turn, performed as above for the elimination of ambiguities.

[0069] The georeferenced angular bearing results from the georeferenced angular bearing calculation 21 can be stored in the pod 1, transmitted to the aircraft 50 by the electronic interface associated with the support member 12, and / or transmitted by radio. Alternatively, the geolocation calculation is carried out to provide coordinates of the emitting source.

Claims

Claims

1. Aeronautical radiogoniometry pod (1), having a longitudinal axis or roll axis, a transverse axis or pitch axis and a depth axis or yaw axis corresponding to a sighting axis, said pod comprising a frame (11), a support member (12) of the frame for attachment to an aircraft (50) along a main force axis by lifting, an interferometer for receiving electromagnetic waves of a band chosen from one or more of the radar bands P, L, S, C, X, Ku, K and Ka, the interferometer comprising at least two antennas, the antennas being supported by the frame (11), positioned in the lower zone of the pod (1),and having an aperture radiation pattern at -3 dB of at least 30° centered on the boresight and beyond 80° from the boresight a level at least 15 dB lower than that of the maximum radiation or an aperture radiation pattern at -3 dB of at least 60° centered on the boresight and beyond 80° from the boresight a level at least 10 dB lower than that of the maximum radiation, to allow orientation of the boresight of the antennas towards at least one area of interest.,

2. Pod (1) according to claim 1, comprising an interferometer for receiving electromagnetic waves in S and C bands and an interferometer for receiving electromagnetic waves in X and Ku bands.

3. Pod (1) according to claim 1, comprising an S-band electromagnetic wave reception interferometer, a C-band electromagnetic wave reception interferometer, an X-band electromagnetic wave reception interferometer and, where appropriate, a Ku-band electromagnetic wave reception interferometer.

4. Pod (1) according to one of the preceding claims, comprising an envelope (13) transparent to said electromagnetic waves, the envelope (13) surrounding the frame (11) and the antennas.

5. Pod (1) according to one of the preceding claims, in which the antennas of an interferometer of a first frequency band are nested in the antennas of an interferometer of a second frequency band, the second frequency band being lower than the first frequency band.

6. Pod (1) according to one of the preceding claims, comprising a signal receiver (14) connected to said antennas of at least one interferometer, the receiver (14) comprising a member for separating the received signal into radar pulses, the pod (1) further comprising a member for selecting radar pulses from the receiver (14), and for calculating a phase difference between the antennas of one of the at least one interferometer, an angular bearing calculator as a function of the evolution of the phase difference from the phase difference calculator and gyroscopic information,

7. Pod (1) according to claim 6, comprising a gyroscopic unit (17) configured to provide a georeferenced angular bearing.

8. Pod (1) according to claim 6, comprising an inertial unit (23) configured to provide geolocation of a source of a received transmission.

9. Method for implementing the pod (1) according to one of the preceding claims, during a flight of an aircraft (50) carrying said pod (1), comprising tilting the aircraft (50) to orient the line of sight of the antennas towards at least one area of interest, and eliminating angular ambiguities from the interferometry by comparing the variation in angular bearings during the turn associated with the tilt of the aircraft (50) with the variation in gyroscopic information.

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